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900A
www.wpiinc.com
Micropressure System
INSTRUCTION MANUAL
Serial No._____________________
030813
World Precision Instruments
900A
CONTENTS
ABOUT THIS MANUAL ..................................................................................................................... 1
INTRODUCTION .................................................................................................................................. 2
Parts List ........................................................................................................................................... 3
Unpacking ....................................................................................................................................... 3
INSTRUMENT DESCRIPTION .......................................................................................................... 4
System Overview .......................................................................................................................... 4
Microelectrode Resistance ................................................................................................... 5
Using Sensitivity and Damping to Correct Oscillations.............................................. 5
Pressure and Vacuum Sources ........................................................................................... 6
Alarm ........................................................................................................................................... 6
Circuit Discussion .................................................................................................................... 6
Setup for Testing ............................................................................................................................ 8
Zeroing the Pressure Pod ..................................................................................................... 8
Re-zeroing after Pressure and Vacuum is Applied....................................................... 8
Testing the Microelectrode and Micropressure System ............................................. 9
OPERATING INSTRUCTIONS.........................................................................................................11
Zero Adjust ....................................................................................................................................11
Preparing the Microelectrode Holder and Pipette ...........................................................12
Adjusting Electrode Resistance...............................................................................................13
Operating the 900A ...................................................................................................................14
Moving the System ...............................................................................................................15
Setting Pressure Mode .........................................................................................................15
Measuring DC Potential ......................................................................................................16
MAINTENANCE..................................................................................................................................16
Cleaning .........................................................................................................................................16
Replacing the Fuse......................................................................................................................17
Service .............................................................................................................................................17
ACCESSORIES .....................................................................................................................................17
SPECIFICATIONS ................................................................................................................................17
DECLARATION OF CONFORMITY...............................................................................................18
WARRANTY ........................................................................................................................................19
Claims and Returns ....................................................................................................................19
Repairs ............................................................................................................................................19
Copyright © 2012 by World Precision Instruments, Inc. All rights reserved. No part of this publication
may be reproduced or translated into any language, in any form, without prior written permission of
World Precision Instruments, Inc.
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World Precision Instruments
900A
ABOUT THIS MANUAL
The following symbols are used in this guide:
This symbol indicates a CAUTION. Cautions warn against actions that can
cause damage to equipment. Please read these carefully.
This symbol indicates a WARNING. Warnings alert you to actions that can
cause personal injury or pose a physical threat. Please read these carefully.
NOTES and TIPS contain helpful information.
Fig. 1—The 900A is a micropressure system.
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INTRODUCTION
WPI’s model 900A micropressure system is designed to measure pressures from
–200 to +400mmHg in small blood vessels, cells and other electrolyte-filled microcavities. The micropressure system includes a main electronic Control Unit, probe
and an independent pressure pod or Pressure Pod, an amplifier, piezoelectric
valve and a pressure transducer. The Pressure Pod is small and lightweight, so it
can be mounted near the microelectrode to reduce dead space. Fluid-filled glass
micropipettes (supplied by the user) pulled to an outside diameter of 2–5μm are
paired with the 900A sensing probe. A pressure source supplying up to +500mmHg
of pressure and a vacuum source supplying up to –300mmHg vacuum is required,
but not supplied.
Notes and Warnings
NOTE: Use fresh micropipettes each time it is to be used.
NOTE: Be sure the air pressure source provides clean, dry air.
NOTE: Use micropipettes with tips pulled to 2 to 5μm in diameter.
NOTE: Use a short tube (12 ˝ or less) to connect the microelectrode holder to the
Pressure Pod tubing port.
CAUTION: Always pay attention to the tube between the holder and the
tubing port. If you find that solution drops have been sucked into the tube,
set to the Zero Set mode. Disconnect the tube, and use the air pressure
supply to blow the solution drops out. Leaving solution in the pressure tube may
damage the Pressure Pod.
!
CAUTION: It is important that the pressure and vacuum sources supply clean,
filtered, dry air. Air that is not clean and dry damages the piezovalve, and
damage of this sort is not covered under warranty.
CAUTION: If the microelectrode is connected to the Pressure Pod before the
pressure and vacuum sources are connected and turned on, fluid may be
pushed through the Fluid Trap and into the Pressure Pod causing damage to
the piezo valve. This damage is not covered under warranty.
!
CAUTION: Before moving the micropipette from one recording site to
another, set the Loop Status switch to Zero Set. This ensures that no
pressure is applied to the micropipette while it is being moved. If this is not
done, the pressure pod tries to compensate the increased tip resistance (caused by
the open circuit) by applying pressure to the micropipette. This may expel the filling
solution from the micropipette.
!
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900A
Parts List
After unpacking, verify that there is no visible damage to the sensor. Verify that all
items are included:
Each 900A system includes the following components:
(1) 900A Control Unit
(1) Power Cord
(1) 900A Probe
(1) 900A Pressure Pod
(8) Electrode Holders
(1) Fluid Trap
(2) Luer Fittings
(1) 1 ft. 3/32˝ plastic tubing
(1) 4 ft. 1/8˝ Tygon tubing
(1) Instruction Manual
Unpacking
Upon receipt of this instrument, make a thorough inspection of the contents and
check for possible damage. Missing cartons or obvious damage to cartons should be
noted on the delivery receipt before signing. Concealed damage should be reported
at once to the carrier and an inspection requested. Please read the section entitled
“Claims and Returns” on page 19 of this manual. Please contact WPI Customer
Service if any parts are missing at 941.371.1003 or [email protected]
Returns: Do not return any goods to WPI without obtaining prior approval (RMA
# required) and instructions from WPI’s Returns Department. Goods returned
(unauthorized) by collect freight may be refused. If a return shipment is necessary,
use the original container, if possible. If the original container is not available, use a
suitable substitute that is rigid and of adequate size. Wrap the instrument in paper
or plastic surrounded with at least 100mm (four inches) of shock absorbing material.
For further details, please read the section entitled “Claims and Returns” on page 19 of
this manual.
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INSTRUMENT DESCRIPTION
System Overview
Control Unit
Pressure
Pod
Probe
Trap
Fig. 2—The 900A system includes the control unit, pressure pod, probe and trap.
The 900A uses a salt concentration gradient at the tip of the sensing electrode
to measure pressure. The internal pressure of the microelectrode is continuously
adjusted to equal the pressure outside the tip, to keep the salt concentration
gradient in dynamic equilibrium. The amount of pressure required to maintain
equilibrium is digitally displayed on the front panel. The voltage proportional to this
reading can be sent to a recording device via the Pressure Signal output connector.
To equilibrate the salt gradient, the piezoelectric pressure pod actively regulates air
flow in and out of a small pressure chamber. A vacuum source connected to the
chamber outlet removes air to reduce pressure. A piezoelectric valve at the inlet
admits pressurized air to increase the pressure. This in turn regulates the pressure
inside the micropipette electrode so that it equals the pressures outside the tip.
The response rate of the piezoelectric valve (from fully closed to fully open) is 0.5ms.
The response rate of the overall system, when properly configured, is typically less
than 10ms. This figure is affected, however, by the pressure chamber’s residual
volume, which includes the micropipette, fluid trap, connecting tubing and the
pressure transducer piezoelectric valve outlet. Long lengths of tubing add dead
space, which slows overall system response. Keeping the residual volume low by
mounting the Pressure Pod close to the microelectrode and using short lengths of
small-bore tubing minimizes dead space and contributes to a rapid system response.
Microelectrode Resistance
When the value set by Electrode Resistance dial is greater than the intrinsic value,
the concentration gradient interface is shifted inside the pipette tip. (For example,
the value obtained by adjusting electrode resistance. See “Adjusting Electrode
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Resistance” on page 13.) This can affect the rate of response to an externally
applied, stepped pressure, but this is not significant for most applications. When the
interface is shifted inside the electrode, the tip opening contains a solution that is
more ionically compatible with the fluid outside the tip. This gives a longer diffusion
path from the pool of high concentration electrolyte to the outside, which may be
advantageous.
The shape of the microelectrode tip significantly affects the 900A’s response. Shorter
tipped microelectrodes that do not have an excessively long taper work best.
All electrode resistance and potential measurements are referenced to ground.
Ground should be established through an appropriate salt solution and reference
half cell such as WPI’s DriRef-2 or DriRef-5.
Using Sensitivity and Damping to Correct Oscillations
All feedback control systems tend to become unstable and oscillate under
certain conditions. The 900A provides two controls for combatting instability
and oscillation—Sensitivity and Damping. Decreasing Sensitivity or increasing
Damping often reduces or stops oscillations. The proper settings for both controls
depends on the particular application.
Keep the Sensitivity knob as high as possible to provide close matching and
following of the external pressure by the internal pressure in the micropipette. We
recommend starting at half of a full rotation, then adjusting in a counterclockwise
direction, as needed. Generally, to maintain proper response, Sensitivity should not
be set below one-third of the full rotation.
The Damping knob adjusts the amplitude of feedback. Turning the damping control
clockwise reduces the amplitude of the feedback. Turning the damping control
counter-clockwise increases the amplitude of the feedback. One-third to one-half
rotation typically produces a stable, quiet baseline.
Needle Deflected Left
Needle Deflected Right
Needle On Zero
0
Tip resistance is greater
than the pot reading
Tip resistance is less
than the pot reading
Tip resistance is equal
to the pot reading
Fig. 3—The Null Detector Meter deflection points to the actual value of the tip resistance.
When the Null Detector meter needle is deflected left, it indicates that the tip resistance
is greater than the resistance dial setting on the 900A front panel. If the needle
deflected right, the pipette tip resistance is less than the value shown on the 900A
resistance dial setting. When the null detector is at zero, the resistance of the pipette
is the value that appears on the 10-turn potentiometer.
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NOTE: Use an oscilloscope or recorder to monitor the pressure output signal to gain
better control of oscillations. The digital pressure meter on the 900A front panel
tends to average oscillations, especially those with higher frequencies. For this
reason, the digital display may give a constant reading, when in fact almost fullscale oscillations may be occurring.
Pressure and Vacuum Sources
The maximum pressure that should be applied to the pressure pod is +500mmHg.
The maximum recommended vacuum is –300mmHg. In the Auto mode, the system
can compensate for small fluctuations in the supply pressures, but large supply
pressure changes may cause a pressure reading error. If possible, therefore, both
pressure and vacuum sources should be stable.
If your experiment involves pressure measurements consistently lower than the
rated maximum, you can use lower supply pressure and vacuum. For example,
if pressure readings at the pressure measuring site are fairly consistent and slowchanging at approximately 150 to 200mmHg, a positive supply pressure of
+300mmHg would suffice.
To keep the pressure gradient high and ensure a quick, accurate response, plan to
provide positive/negative pressures at least 50mmHg greater that the maximum
positive/negative pressure to be measured. Pressure differentials of less than
50mmHg are inadequate to move air quickly, causing response times to increase
considerably.
!
CAUTION: It is important that the pressure and vacuum sources supply clean,
filtered, dry air. Air that is not clean and dry damages the piezo valve, and
damage of this sort is not covered under warranty.
Alarm
An alarm sounds when the pressure pod valve is completely open to indicate
that maximum pressure has been reached. The alarm also sounds when electrical
continuity is broken for any reason. For example, if the microelectrode comes out
of the solution, the filling solution level drops below the Ag/AgCl pellet, the ground
reference is disconnected, etc. or the tip is blocked.
Circuit Discussion
The 900A consists of an oscillator, head stage, phase detector, buffer amplifier,
pressure transducer and pressure pod.
OSCILLATOR: A Wein bridge oscillator generates a 1000Hz sinusoidal voltage. This
is amplitude stabilized by internal negative feedback transistors in the bridge circuit.
HEAD STAGE (or FOLLOWER): A 1000Hz carrier constant current is injected through
the microelectrode. The resulting voltage drop is compensated in the follower
circuit by advancing the Electrode Resistance dial so that balance null is achieved.
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The microelectrode resistance is then read directly in M from the dial. If the
microelectrode resistance changes, a 1000Hz imbalance signal is amplified and
detected by a phase detector.
PHASE DETECTOR: This is a synchronously switched detector in a single integrated
circuit package. The resulting detected signal is filtered and is available at the output
of the amplifier.
BUFFER AMPLIFIER: The buffer amplifier lies between the detector and the
pressure pod driver. It compensates the detector signal with output from the
pressure transducer in order to compensate for the inherent lag in propagating the
pressure changes transmitted to the micropipette tip. The lag would normally cause
the system to oscillate. The Damping knob sets the amplitude of the buffer amplifier.
Insufficient damping (For example, the Damping knob is turned too far counter
clockwise) causes the system to oscillate at higher frequencies. Too much damping
(For example, the Damping knob is turned too far clockwise) can result in a slow
rate of response and low frequency oscillations.
PRESSURE TRANSDUCER: The pressure transducer is a silicon strain gauge
resistance bridge device. Its volume and pressure displacement are small and
sensitivity is high. The bridge output is amplified, scaled and displayed on the digital
meter. The meter has a resolution of 0.1mmHg from +199.9 to –199.9mmHg.
Pressures outside this range can be viewed by toggling the Range switch to High.
The pressure output can also be monitored via the BNC connector marked Output
on the front panel. The output level of this signal is 10mV/mmHg.
PRESSURE POD: The pressure pod consists of a driver circuit to operate the
piezoelectric controller and an alarm circuit to warn when the maximum pressure of
the controller has been reached.
Setup for Testing
See Fig. 2 when setting up the 900A micropressure system. Be sure the setup is
complete before you turn on the power.
1.
Connect the seven-pin cable on the Pressure Pod to the connector labeled
Pressure Pod on the front panel of the Control Unit.
2.
Connect the 900A active probe to the connector labeled Probe on the front
panel of the Control Unit. Connect the tip of the 900A active probe to ground
using a banana to 2mm pin adapter (WPI #13776). Use the CKT and CHAS
ground connections, which should be electrcally connected with a metal strap.
3.
Connect the power cord to the receptacle on the rear panel of the Control Unit.
Plug into the wall socket.
4.
Connect the reference electrode from the CAL900A or the experiment to the
circuit ground terminal marked CKT on the front panel of the Control Unit.
5.
Place the Loop Status in the Zero Set position. This maintains a pressure of
0mmHg in the pressure chamber.
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6.
Set the Damping and Sensitivity controls to the 12:00 position.
7.
Set the Electrode Resistance dial fully counterclockwise to zero.
8.
Set the Offset switch to the Off position.
9.
Set the Pressure switch to Off position.
6.
Turn the power on and let the system warm up for 15 minutes.
Zeroing the Pressure Pod
1.
Complete the setup procedure described on above. Verify that the Loop Status
is set to Zero Set and the system has warmed up for 15 minutes.
4.
In the Zero Set mode, applying no pressure or vacuum to the pressure pod,
insert a screwdriver into the recess on the pressure pod marked XDCR Zero and
turn clockwise or counterclockwise until the digital meter on the 900A reads
0.00mmHg.
Re-zeroing after Pressure and Vacuum is Applied
1.
Remain in Zero Set mode.
2.
Regulate the source of pressure and vacuum for the 900A pressure pod to
approximately +350mmHg for the pressure, and –150mmHg for the vacuum.
If you are using a pressure manometer (WPI #PM015) to monitor the pressure,
use port B on the meter for measurement.
NOTE: The pressure and vacuum sources may be regulated to pressures other
than those suggested here, however, IT IS CRUCIAL THAT THE PRESSURE
SIDE ALWAYS BE OF A GREATER ABSOLUTE VALUE THAN THE VACUUM SIDE,
AND THAT THERE IS A MINIMUM OF 75mmHg DIFFERENCE BETWEEN THE
PRESSURE AND VACUUM SOURCES.
3.
Connect these regulated pressure and vacuum sources to their respective input
ports on the pressure pod.
4.
Block the To Micropipette port on the pressure pod. A finger can be used for
this operation if it’s held firmly against the port opening.
5.
Keeping the To Micropipette port blocked, insert a screwdriver into the
recess marked Pressure on the 900A front panel, and turn clockwise or
counterclockwise until the digital meter reads 0.00mmHg. If the unit has been
properly re-zeroed, very little change occurs when the tubing port is opened.
Testing the Microelectrode and Micropressure System
1.
2.
3.
8
Setup the system as describe on page 7. Zero the Pressure Pod as described
above. Then, prepare two stock solutions: 1M KCl and 0.1M KCL.
Put 0.1M KCl into the CAL900A Pressure Calibration Chamber. The level should
be enough to cover the Ag/AgCl pellet. This accessory is sold separately
Connect the blue ground (Ag/AgCl pellet) connection of the CAL900A test
chamber to the ground connection on the 900A using a banana to 2mm pin
adapter (WPI #13776).
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900A
4.
5.
6.
7.
8.
9.
Prepare a micropipette holder and glass capillary. Using Microfil, fill up the glass
capillary with 1M KCl. It helps to soak the tip of the capillary in KCl before filling
the rest of the capillary. Separately fill the micropipette holder with 1M KCl, and
then install the capillary in the holder. NO air bubbles may exist in the pathway
between the tip of the glass capillary and the Ag/AgCl pellet in the holder.
Install the holder/capillary assembly into the CAL900A cap, and secure the cap
by screwing it onto the base of the CAL900A for a good pressure seal.
NOTE: Be careful not to break the pipette tip when inserting it into the cap or in
the CAL900A. Be mindful that the tip of the micropipette can strike the ground
electrode or the CAL900A chamber wall.
Connect the micropipette holder to the liquid trap and the liquid trap to the
900A Pressure pod.
Remove the 900A probe from the ground location and connect it to the
micropipette holder.
The Null Detector needle goes to a hard left position.
Begin turning the Electrode Resistance dial clockwise. The needle of the Null
Detector display eventually moves from the left to right. Adjust the dial until the
Null Detector needle is in the zero position.
Take note of the resistance reading on the 10–turn resistance adjustment
potentiometer. This is the actual resistance of the pipette tip in proper
equilibrium. Each full turn is 1000K of resistance. The correct electrode
resistance should be between 250K to 400K.
TIP: If the electrode resistance is more than 400K, it is likely that there is an
air bubble somewhere in the pipette between the tip and the Ag/AgCl pellet. If
the resistance is less than 250K, it is likely that the pipette tip inside diameter
is greater than 5μm. This may happen if the tip is broken.
NOTE: If the needle of the Null Detector meter is deflected to the left of zero,
then the pipette tip resistance is in fact greater than the value indicated on the
Electrode Resistance dial. Conversely, if the Null Detector needle is deflected
to the right of zero, the pipette tip resistance is less than that indicated on the
Electrode Resistance dial.
10. Turn the Electrode Resistance potentiometer clockwise gradually. The Null
Detector meter moves to the right. Adjust the potentiometer until the reading is
about 75.
11. Switch the Loop Status switch to Auto. The needle moves back to approximately
the zero position, but should be slightly negative (about –1 to –3mmHg).
12. View the signal continuously on an oscilloscope or data acquisition system. The
pressure signal oscillates. Use the Damping and Sensitivity controls to reduce
oscillation to lowest level possible. Generally, the Sensitivity control should
be adjusted first, and it should be adjusted counter-clockwise to decrease the
sensitivity. As the sensitivity is reduced, the oscillation should become smaller.
TIP: If the sensitivity is continuously reduced, at some point the signal becomes
unstable. This results in the signal going off-scale in the positive or negative
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direction. When this happens, the PLL amplifier will lose its “lock” and the
carrier signal will be lost, causing the control signal to the Pressure Pod to go
to its extreme limit. If this happens, quickly turn the sensitivity control back in a
CLOCKWISE direction to regain control of the signal and re-establish stability. By
experimenting with this process a few times, an adjustment for minimum level
of oscillation can be achieved while still maintaining control of the signal. See
“Using Sensitivity and Damping to Correct Oscillations” on page 5.
During this process a vacuum may be applied to the Pressure Pod, which could
draw liquid into the tube going to the Pressure Pod.
NOTE: A liquid trap is provided with the 900A and should be installed between
the micropipette holder and the 900A Pressure Pod to prevent intrusion of liquid
into the pressure pod.
CAUTION: Unfortunately, the control valve within the Pressure Pod is very
sensitive to salt water. If salt water gets into the valve, there is a high
probability of irreparably damage. This is the most expensive component in
the Pressure Pod. Make sure the liquid trap is in place, and that it is positioned
upright. It’s a good idea during the setup process to pay attention to the possibility
for migration of liquid into the pressure pod! Damage caused by water intrusion is
not covered under warranty.
!
13. Once the signal is stabilized, switch on the Offset control and adjust the dial to
remove any offset voltage displayed on the digital pressure meter. The offset
control affects the meter only, not the pressure signal. If you are recording the
signal, as most people are, the meter offset is inconsequential.
!
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CAUTION: When measurements are not being made, switch the Loop Status
control to Zero Set mode. This is the safest way to leave the system when
you must step away.
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900A
OPERATING INSTRUCTIONS
Zero Adjust
1.
Disconnect the vacuum and pressure sources and the tubing to the
microelectrode from the Pressure Pod, so that the Pressure Pod is open to
atmosphere. Turn the Control Unit on, and let it warm up for 15 minutes.
2.
The digital meter should read 0mmHg. If the reading is incorrect, insert a small
screwdriver into the recess marked XDCR Zero on the front of the Pressure Pod,
and turn clockwise or counterclockwise to bring the digital meter reading to
0mmHg.
Fig. 4—XDCR zero adjust is pointed out in this image.
3.
Connect the pressure and vacuum sources to the Pressure Pod. Make sure
that the pressure source supplies clean, dry air. Turn the pressure and vacuum
sources on.
4.
Block the micropipette output on the Pressure Pod by placing one finger
over the tubing port. The digital meter should read 0mmHg. If the reading is
incorrect, insert a screwdriver into the Pressure recess on the front panel of the
900A and turn clockwise or counterclockwise to bring the digital reading to
0mmHg.
NOTE: If you are unable to get a 0 reading, turn off and disconnect the pressure
and vacuum supplies and repeat the previous step.
5.
Now open the pipette output by removing your finger. The panel pressure meter
should remain at 0mmHg.
If your work requires highly accurate readings, re-calibrate the instrument:
1.
Connect the pressure and vacuum sources to the Pressure Pod.
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2.
Using a “T” fitting, connect the pressure source (supplying clean, dry air) to the
microelectrode outlet port and to a WPI PM Series pressure manometer or
other accurate pressure-measuring device. This pressure source must supply
pressures of the same order of magnitude as the pressures to be measured in
your application.
3.
Turn the pressure source on. The pressure reading on the 900A should match
that given on the reference meter.
4.
If the reading is not correct, insert a screwdriver into the recess on the back
of the Pressure Pod, and turn clockwise or counterclockwise until the correct
reading is displayed on the 900A.
Since the system gain has now been changed it is necessary to repeat the Zero
Adjust procedure, possibly several times, until all zero adjusts are correct and the
pressure input is measured accurately.
Preparing the Microelectrode Holder and Pipette
Use a micropipette pulled to a tip diameter of 2–5μm. Prepare a KCl or NaCl
filling solution with a concentration of 1M (molar) or higher. A high concentration
is important in order to maintain a large concentration gradient between the
microelectrode’s internal salt solution and the solution outside the tip.
1.
Immerse the pipette tip into the 1M solution and fill the tip by allowing the
solution to wick up.
2.
Backfill the shank of the micropipette. (MicroFil™, WPI’s flexible syringe needle,
works particularly well for this application.) Do not allow air bubbles to remain
in the pipette.
3.
Fill the electrode holder, making sure that no air bubbles remain in the holder,
and that the filling solution covers the Ag/AgCl pellet.
4.
Insert the pipette into the holder and tighten the cap.
5.
Check again to be sure that no air bubbles remain in the pipette or the holder.
If fluid leaks from the microelectrode tip or if it becomes necessary to purge the
tip, make sure to avoid an open circuit, that the level of the filling solution is
high enough to maintain contact with the Ag/AgCl pellet.
6.
Insert the holder onto the probe tip. Connect a 12-inch or shorter (30cm or
shorter) length of small-bore tubing to the pressure port on the electrode holder.
7.
Do not connect this tube to the Pressure Pod yet! First connect the pressure and
vacuum supply tubes to the Pressure Pod, and turn them on.
CAUTION: If the microelectrode is connected to the Pressure Pod before the
pressure and vacuum sources are connected and turned on, fluid may be
pushed through the Fluid Trap and into the Pressure Pod causing damage to
the piezo valve. This damage is not covered under warranty.
!
8.
12
Referring to Fig. 5, connect the pipette holder to the fluid trap.
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900A
pressure pod
trap
tubing
pipette holder
tubing
Fig. 5—The pipette assembly is attached to pressure pod.
9.
Check that the pressure and vacuum sources are connected to the Pressure Pod
and that both are turned on. Connect the microelectrode/holder assembly to
the micropipette outlet port.
Adjusting Electrode Resistance
Fig. 6—Rotate the electrode resistance knob clockwise or counterclockwise until the null
detector meter reads 0μA DC.
1.
If everything is correct, the 900A adjusts the pressure in the tubing to maintain
0mmHg at the microelectrode tip. Turn the Damping and the Sensitivity knobs
to about 1/2 of a full rotation.
2.
Immerse the pipette tip into a dilute electrolyte solution — for example, 0.1M
concentration of the filling solution.
Place a ground reference electrode into the solution.
Connect the ground reference electrode to the circuit ground on the front panel,
marked CKT. Use WPI’s Dri-Ref2 or Dri-Ref5 or another Ag/AgCl wire reference
electrode.
3.
4.
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5.
Rotate the Electrode Resistance knob on the 900A front panel until the Null
Detector meter reads 0μA DC. (0 is at the center of the scale.) The dial setting
of the Electrode Resistance knob, which ranges from 0 to 10M (one full turn
per M), now directly indicates microelectrode resistance. Electrode resistance
typically ranges from 100K to 1M, depending on tip diameter and filling
solution concentration. The ideal tip diameter, from 2 to 5μm, produces
resistance of 150 to 250K.
Operating the 900A
1.
2.
Set the Loop Status switch at Zero Set.
Advance the Electrode Resistance knob to the right until the Null Detector
reads from 50 to 100μA. The resistance value will be about twice that of the
actual resistance of the microelectrode as obtained above.
Turn the Loop Status switch to Auto. In this operating mode, the electronic
feedback system automatically adjusts the microelectrode tip resistance to
the higher value of resistance selected by drawing some of the external dilute
solution into the tip. When the proper resistance is reached, the Null Detector
needle returns to 0μA.
The instrument is now ready for use. The pressure reading should be slightly
negative. This is because the height of the filling solution exerts pressure on the
electrode tip.
3.
Fig. 7—(Left) Loop status switch
Fig. 8—(Right) Pressure Pod range switch
4. To compensate this slight negative pressure, turn on the Pressure Offset toggle
switch and adjust the Pressure Offset knob until the meter reads 0mmHg.
NOTE: Adjusting the Pressure Offset adjusts only the meter reading and does
not affect the recorder output value.
To view pressures greater than ±199.9mmHg, toggle the Range switch above
the Pressure Pod connector to High.
!
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CAUTION: The pressure transducer in the Pressure Pod has a maximum
pressure rating of 500mmHg. Great care should be taken not to apply
pressures greater than 500mmHg to avoid damaging the transducer.
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900A
Moving the System
CAUTION: Before moving the micropipette from one recording site to
another, set the Loop Status switch to Zero Set. This ensures that no
pressure is applied to the micropipette while it is being moved. If this is not
done, the pressure pod tries to compensate the increased tip resistance (caused by
the open circuit) by applying pressure to the micropipette. This may expel the filling
solution from the micropipette.
!
Once the micropipette is placed in the desired recording site, check the electrode
resistance.
1.
If the Null Detector needle is deflected completely to the left, the tip resistance
has increased. Turn the Electrode Resistance knob clockwise until the needle
begins to move to the right. The needle does not need to reach 0, but must only
move from the maximum left position in order to confirm that the open circuit
has been closed.
2.
Turn the Loop Status switch to Auto. The 900A controls the pressure in the
micropipette to generate the tip resistance preset on the Electrode Resistance
dial. The Null Detector should return to 0.
• If the Null Detector reading or the pressure reading is unsteady, adjust the
DAMPING and/or SENSITIVITY. If this does not correct the instability, there
may be air bubbles in the micropipette/holder system.
• If the Null Detector pointer fails to move when the Electrode Resistance dial
is adjusted, one of two situations may have occurred:
• The external pressure applied to the micropipette was large enough
to force an excessive amount of dilute solution into the tip, resulting
in a resistance greater than 10M. To correct this, turn the Electrode
Resistance dial down to the original setting, as described above. Turn
the Loop Status switch to Auto. The 900A will then adjust the pressure
inside the micropipette to expel the dilute electrolyte and decrease the tip
resistance to its original value. Another method of purging the micropipette tip is to follow the procedure described below in “Set pressure.”
• The microelectrode is not making contact with the electrolyte solution,
or the reference electrode is not connected. In either case, the problem
is an open circuit. Adjusting the Electrode Resistance dial or changing
the loop status switch to auto has no effect. Check the placement of the
micropipette, the reference electrode, the liquid level in the micropipette,
as well as all connections. Take care not to leave the Loop Status switch
on Auto, to avoid discharging the contents of the microelectrode.
Setting Pressure Mode
Set Pressure mode allows you to preset the internal pressure of the micropipette.
This is useful for:
•
applying positive pressure to flush the tip
•
applying negative pressure to draw solution into the tip.
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CAUTION: Extreme care must be taken when using negative pressures not
to force fluid through the fluid trap and into the Pressure Pod. Disconnect
the microelectrode holder, and attach the pressure tubing to a manometer to
check the calibration of the 900A pressure transducer against an external standard.
To enter the Set Pressure mode:
1.
Toggle the ± switch to the center or off position, and rotate the Set knob to 0.
2.
Switch the Loop Status control to Set.
3.
Select + for positive or – for negative pressure, using the toggle switch. A
selected pressure can be applied to the inside of the micropipette.
4.
Turn the Set knob to select the magnitude of the pressure. The selected internal
micropipette pressure is displayed in mmHg on the digital meter.
Set Pressure mode can be used to purge the micropipette tip when too much dilute
solution has diffused into the microelectrode. Applying a few mmHg of positive
pressure gently expels filling solution from the shank of the micropipette into the tip.
The tip resistance can be monitored by watching the Null Detector.
•
If the tip resistance is greater than the dial setting, the needle points left.
•
If the tip resistance is less than the dial setting, the needle points right.
Measuring DC Potential
Although the primary function of the 900A is to measure pressures, it can also be
used to measure DC potential at the microelectrode site.
1.
Connect a recording device to the follower Output.
2.
To offset background current from the amplifier, insert a small screwdriver into
the recess labeled Follower Zero.
3.
Turn clockwise or counterclockwise until the recording device reads 0V.
Excursion in this mode is limited. You can, of course, provide additional offset
correction externally, for example, at the reference electrode or the recording
device.
MAINTENANCE
Cleaning
Carefully wash the holder, beaker and syringes with distilled water before and after
use.
CAUTION: Always pay attention to the tube between the holder and the
tubing port. If you find that solution drops have been sucked into the tube,
set to the Zero Set mode. Disconnect the tube, and use the air pressure
supply to blow the solution drops out. Leaving solution in the pressure tube may
damage the Pressure Pod.
16
World Precision Instruments
900A
Replacing the Fuse
You can easily replace the fuse in Control Unit. The part numbers for replacement
fuses are:
#3822 for 110V line power
#6408 for 220 V line power
Service
Contact World Precision Instruments, Inc. at 941.371.1003 or [email protected]
wpiinc.com for all service needs.
ACCESSORIES
Table 1: Accessories
Part Number
2851
2933
13776
MEH6RF*
MEH6SF*
CAL900A
Description
BNC-to-BNC cable
Rack mounting hardware
Banana to 2mm pin adapter
Electrode Holders
Electrode Holders
Pressure Calibration Chamber
*Electrode holders may be ordered to accommodate any of the following glass sizes:
1.0, 1.2, 1.5 and 2.0mm diameter.
SPECIFICATIONS
This unit conforms to the following specifications:
Pressure Range ...............................................................................................+500 to –300mmHg
Linearity ............................................................................................. < ±0.5% from a straight line
Stability ..................................................................................+/- 0.1mmHg up to 1 hour or more
Accuracy................................................................................................................. ±0.5% of full scale
Risetime .......................................................>10ms (10-90%), depending on residual volume
Output (“Pressure Signal”) ........................................................................................... 10mV/mmHg
Amplifier Probe ..................................................... Input Resistance >1010, Voltage Gain 1.0
Dimensions
Main Frame ...................................................................... 17x5.25x10 in. (43.2x13.3x25.4cm)
Pressure Pod ..............................................................................3.7x1x2.25 in. (9.4x2.5x5.7cm)
Power .......................................................................................................................110 VAC/220 VAC
World Precision Instruments
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DECLARATION OF CONFORMITY
18
World Precision Instruments
900A
WARRANTY
WPI (World Precision Instruments, Inc.) warrants to the original purchaser that this equipment,
including its components and parts, shall be free from defects in material and workmanship for a
period of 30 days* from the date of receipt. WPI’s obligation under this warranty shall be limited
to repair or replacement, at WPI’s option, of the equipment or defective components or parts upon
receipt thereof f.o.b. WPI, Sarasota, Florida U.S.A. Return of a repaired instrument shall be f.o.b.
Sarasota.
The above warranty is contingent upon normal usage and does not cover products which have been
modified without WPI’s approval or which have been subjected to unusual physical or electrical stress
or on which the original identification marks have been removed or altered. The above warranty will
not apply if adjustment, repair or parts replacement is required because of accident, neglect, misuse,
failure of electric power, air conditioning, humidity control, or causes other than normal and ordinary
usage.
To the extent that any of its equipment is furnished by a manufacturer other than WPI, the foregoing
warranty shall be applicable only to the extent of the warranty furnished by such other manufacturer.
This warranty will not apply to appearance terms, such as knobs, handles, dials or the like.
WPI makes no warranty of any kind, express or implied or statutory, including without limitation any
warranties of merchantability and/or fitness for a particular purpose. WPI shall not be liable for any
damages, whether direct, indirect, special or consequential arising from a failure of this product to
operate in the manner desired by the user. WPI shall not be liable for any damage to data or property
that may be caused directly or indirectly by use of this product.
Claims and Returns
Inspect all shipments upon receipt. Missing cartons or obvious damage to cartons should be noted
on the delivery receipt before signing. Concealed loss or damage should be reported at once to the
carrier and an inspection requested. All claims for shortage or damage must be made within ten
(10) days after receipt of shipment. Claims for lost shipments must be made within thirty (30) days
of receipt of invoice or other notification of shipment. Please save damaged or pilfered cartons until
claim is settled. In some instances, photographic documentation may be required. Some items are
time-sensitive; WPI assumes no extended warranty or any liability for use beyond the date specified
on the container
Do not return any goods to us without obtaining prior approval and instructions from our Returns
Department. Goods returned (unauthorized) by collect freight may be refused. Goods accepted for
restocking will be exchanged or credited to your WPI account. Goods returned which were ordered
by customers in error are subject to a 25% restocking charge. Equipment which was built as a special
order cannot be returned.
Repairs
Contact our Customer Service Department for assistance in the repair of apparatus. Do not return
goods until instructions have been received. Returned items must be securely packed to prevent
further damage in transit. The Customer is responsible for paying shipping expenses, including
adequate insurance on all items returned for repairs. Identification of the item(s) by model number,
name, as well as complete description of the difficulties experienced should be written on the repair
purchase order and on a tag attached to the item.
* Electrodes, batteries and other consumable parts are warranted for 30 days only from the date on which the
customer receives these items.
World Precision Instruments
19
World Precision Instruments, Inc.
USA
International Trade Center, 175 Sarasota Center Blvd., Sarasota FL 34240-9258
Tel: 941-371-1003 • Fax: 941-377-5428 • E-mail: [email protected]
UK
1 Hunting Gate, Hitchin, Hertfordshire SG4 0TJ
Tel: 44 (0)1462 424700 • Fax: 44 (0)1462 424701 • E-mail: [email protected]
Germany
Zossener Str. 55, 10961 Berlin
Tel: 030-6188845 • Fax: 030-6188670 • E-mail: [email protected]
China & Hong Kong
WPI Shanghai Trading Co., Ltd.
Rm 20a, No8 Dong Fang Rd., Lu Jia Zui Financial District, Shanghai PRC
Tel: +86 688 85517 • E-mail:[email protected]
Internet
www.wpiinc.com • store.wpiinc.com • www.wpichemistry.com
www.wpi-europe.com • www.wpiinc.cn
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