Calibrating Draft Range Pressure Sensors with a Pressure Controller Calibra

Calibrating Draft Range Pressure Sensors with a Pressure Controller Calibra
Calibrating draft range
pressure sensors with
a pressure controller/
calibrator
Application Note
Draft range pressure sensors are used
to measure low gauge and differential
pressure in a wide variety of industries,
including hospitals, pharmaceutical
production facilities, semiconductor
and fiber optics manufacturing, as well
as in research and development. Such
low pressures are referred to as draft
range because they are pressures that
can be produced from a draft. Most of
the applications involve preventing an
environment or manufacturing process
from being contaminated. Because the
pressures measured by these types of
sensors are very low, calibrating them
accurately can be a challenge.
This application note discusses some
of the issues that must be addressed
when calibrating draft range pressure sensors. It also explains why the
7250LP Pressure Controller/Calibrator
can be an excellent solution for calibrating this type of workload.
Photo courtesy of NASA
Figure 1. Examples of typical
instruments that are used to monitor
DP in a clean room environment
(Fluke Calibration does not indorse
these products).
Two common types of differential pressure
Low gauge versus high gauge pressure
measurements are absolute and gauge. Absolute
calibration
Unstable reference pressure
All pressures are expressed as the difference
between two pressures. In other words, one pressure is always measured relative to another. (In
this sense, every pressure measurement is a differential measurement.) For example, the pressure
measured inside the tires of a car may be 220 kPa
(32 psi). This measurement is the difference
between the gas pressure inside the tire and the
surrounding ambient pressure outside the tire.
pressure is measured relative to the pressure that
would occur at absolute vacuum (note, absolute
vacuum is never achieved). Gauge pressure is measured as the difference in
pressure inside a system and
the surrounding atmosphere.
ATMOSPHERIC
Notice the reference points
PRESSURE
( ) for the vertical arrows
VACUUM
in figure 2. The reference for
ABSOLUTE
absolute mode pressure measurements is flat over time,
TIME
because the reference is a
vacuum. But in the gauge
mode graph, the reference
ATMOSPHERIC
changes over time because
PRESSURE
the reference is atmospheric
VACUUM
pressure, which is inherently
POSITIVE AND NEGATIVE GAUGE
unstable.
PRESSURE
Draft range pressure sensors make low gauge pressure measurements that are typically no greater
than ± 2.5 kPa (± 10 in H2O, ± 25 cm H2O). Most
are ± 1.25 kPa (5 in H2O, 12.5 cm H2O) or less. Very
minor levels of instability in the reference or test
pressure—that would be irrelevant in high gauge
pressure calibration—can cause critical errors in
very low gauge pressure measurements.
Figure 2. The stability of reference in two common
types of the differential pressure.
F r o m t h e F l u k e C a l i b r a t i o n D i g i t a l L i b r a r y @ w w w. f l u k e c a l . c o m / l i b r a r y
During a low gauge calibration, the measured
test pressure (pressurized gas) may be very stable,
but the reference (atmospheric pressure) may be
moving. When that happens, the pressure indicated on the device under test will be unstable.
This is particularly true when the differential pressure is very small and the reference is moving by a
large amount.
Unstable test pressure
Instability of the test pressure medium can
increase the challenges in a low pressure calibration. When a fluid such as gas is static, it does not
flow. It can only flow when there is differential
pressure acting on it. Therefore, if the gas in a
calibration circuit is not static, then there probably
is a differential pressure between the calibrator
and the device under test (DUT) which often leads
to errors.
The ideal gas law tells us of the relationship
between a pressure and the associated volume it is
contained in, the number of gas molecules in the
volume and the temperature of the pressurized gas
following the equation:
PV = nRT
Where P = pressure
V = volume
nR = the quantity of gas
T = temperature
Because they are all interrelated, we must consider what happens if volume, temperature, and/
or quantity of gas in the system changes as we
perform calibrations.
Most pressure systems have a considerable
amount of plumbing, including valves, fittings,
tubing, manifolds, adaptors, volumes, and so forth.
These items are frequently constructed from various different materials. The pressurized gas must
flow to fill all of the system components until the
pressure is uniform throughout the circuit. It takes
time for this to happen and stabilize within the
required tolerance. How much time depends on the
type of gas being used, total volume, restrictions
within the system, the measured pressure and the
uncertainty of the measurement.
Remember that draft range sensors
measure pressure ranges of ± 1.25 kPa
and less. Many of today’s instruments
0.35
ERROR % DUT FS
Why these unstable
conditions matter
claim uncertainties on low gauge measurements in
the range of ± 0.05 % to ± 0.15 % of span. Some
are as low as 0.015 % of span.
If the span is 500 Pa (2 in H2O) and the uncertainty specification is ± 0.1 % of span, the
maximum allowable error is ± 0.5 Pa. Maintaining a 4:1 test uncertainty ratio (TUR) means the
calibration uncertainty cannot be more than one
fourth of this (0.125 Pa). This, in turn, means that
the amount of uncertainty allowed by environmental instability should not exceed about 0.05 Pa
which represents about 0.5 ppm on atmospheric
pressure. An air conditioner turning on or off,
a door opening or closing, even a windy day or
someone walking past the calibration system can
have a larger than 0.05 Pa effect on the value of
the reference pressure.
Rapid temperature changes need to be avoided
wherever possible. A temperature change as small
as 0.5 °C in any size volume of gas near atmospheric pressure will change the pressure by about
171 Pa (0.7 in H2O). This is a big change compared
to the pressures that are typically being measured.
What might cause the temperature of the gas to
change? Common sources include air conditioners being turned on and off, and nearby electronic
equipment or computers with fans that cycle. Even
human hands touching the plumbing can change
the gas temperature significantly.
Gas flow inside the pressure circuit also must
be avoided or minimized to within the required
tolerance. Things that cause flow include leaks into
and out of the circuit, as well as localized temperature changes in one part of the circuit. These
effects are amplified by flow restrictions within the
circuit such as filters, small diameter tubing, and
needle valves. These conditions can create differential pressures that affect the calibration. This is
especially significant when you are attempting to
set zero gauge pressure, since all other pressures
are relative to zero. If zero is wrong, every pressure
indicated by the device will be wrong by the same
amount (see Figure 3).
0.3
0.25
0.2
0.15
0.1
0.05
0
0
20
40
Figure 3. Error caused by poor zero.
2 Fluke Calibration
Calibrating draft range pressure sensors with a pressure controller/calibrator
60
80
100
Pa
120
140
160
180
200
Simple, practical solutions to stabilize
the reference and test pressure
Stabilizing the reference pressure
There are two issues to consider:
• Fluctuations in atmospheric pressure
• Changes in ambient temperature
To eliminate the influence of a noisy reference
pressure, simply connect the reference ports of the
devices together, isolating them from the ambient
pressure. Doing this results in measuring all differential pressures relative to “sealed gauge.” This
is completely acceptable because the sealed gauge
reference pressure is within the normal range
of barometric pressure in that environment (see
Figure 4).
Moving ambient temperature affects the sealed
gauge pressure. As mentioned earlier, a change in
gas temperature of 0.5 °C will change the pressure
by 171 Pa. A temperature change of 3 °C—which
is not uncommon—results in a pressure change of
more than 1 kPa. This kind of change, by itself,
is not normally a concern, since the pressures set
and measured are differential and relative to the
sealed gauge pressure. However, how quickly they
change can be a concern.
The relationship between pressure and temperature is unaffected by volume. That is, a volume of
1 cc and a volume of 1000 cc at the same starting
pressure will each change by the same amount
if the temperatures of both change by 1 °C. But
a large volume will take longer to change than
will a small volume. So having a large thermally
protected volume is desirable, such as that provided by a dual volume unit (DVU). In addition, the
plumbing used in the test system should be thermally insulated. Since pressures are normally low,
thick walled plastic tubing is recommended for its
thermal properties.
Stabilizing the test pressure
Stabilizing the test pressure requires the elimination or control of the gas flow within the
pressurized circuit, or enough wait time to establish equilibrium. Two causes of flow can be
significant in low gauge pressure measurements:
• Localized temperature changes acting on parts of
the circuit
• Leaks into or out of the circuit
System design and moving any heat sources away
from the plumbing become critical to controlling
flow to levels that won’t affect the measurements
significantly.
It is also important to remove restrictions
between the volumes that exist within the system.
Volumes can be manifolds, filter housings even the
device under test (DUT). Restrictions include small
inside diameter tubing, needle valves and filters.
3 Fluke Calibration
Figure 4. 7250LP, DVU, and DUT connected.
Using a 7250LP to perform draft range
pressure calibrations
The 7250 Series Pressure Controller/Calibrator is
the latest generation of pressure transfer standards
to be developed by Fluke Calibration using the
Ruska Force Balanced, Quartz Bourdon Tube.
Tech Tips:
Manage the temperature. Keep the electronics away,
especially laptop computers that have fans that cycle.
When heat blows onto the plumbing, it heats up the gas
and it will move, creating flow. Divert air conditioning and
heating away from the system. Keep your hands off.
Don’t touch that tubing with your hands. Use plastics
such as PFA (Perfluoroalkoxy) tubing and other materials with thermal insulating properties.
Think BIG. Big tubing and ball valves both aid in stabilizing the pressure quickly. Avoid needle valves and
filters and small diameter tubing. Also avoid fittings and
adaptors with small diameter passages.
WAIT. Don’t rush. It takes time for the pressure to
equalize in a system using a viscous fluid. The lower the
differential pressure the slower things move. You have
to wait enough time.
Calibrating draft range pressure sensors with a pressure controller/calibrator
Figure 5. 7250LP.
The 7250LP is specially designed to perform
draft range pressure calibrations. It is available
in full scale values of 7.5 kPa (30 inH2O), 15 kPa
(60 inH2O), or 25 kPa (100 inH2O). The pressure
sensor is then dual-scaled to provide better accuracy and control performance at the low end of
each scale. The 7250LP includes a small, internal
volume which reduces the need for the DVU. For
some applications, where ambient conditions will
potentially fluctuate dramatically, the DVU will be
beneficial.
Do not use filters such as a Self Purging Liquid
Trap (SPLT) for this application unless it is
absolutely necessary. If it is, change the filter frequently—don’t just purge it.
Be sure to connect the REFERENCE port of the
7250LP to the low, or reference side of the DUT
and DVU and the TEST port to the high side of the
DUT and DVU (see Figure 4).
The 7250LP includes an internal bypass valve
that connects the test and reference ports. In
normal operation, this valve is closed. To ensure a
good zero differential pressure it is opened during
the 7250LP zeroing process and when in VENT
mode. It is recommended that when zeroing the
DUT or generating 0 differential pressure to place
the 7250LP in VENT mode.
Step through the calibration points, making sure
to wait long enough after the “Ready” indication
to stabilize the pressure between the DUT and the
controller/calibrator. Note that waiting an additional 30 to 60 seconds after ready is indicated is
common for draft range calibrations to ensure reliable measurements are made.
Tech Tip:
It is generally accepted practice to
average readings of the DUT and the
standard. But, averaging should only
be done if both can be averaged over
the same period of time. Avoid averaging one instrument and not the other
and avoid averaging one instrument and
then the other. Averaging should be:
Standard — DUT —
Standard — DUT —
Standard — DUT etc.
Averaging in this way will minimize or
eliminate the effect of drift.
For applications that allow for a less accurate controller/calibrator, Fluke Calibration offers the PPC4E
15K. The PPC4E 15K provides a transportable solution for pressure ranges from -15 to 15 kPa (-60 to
60 inH2O).
Fluke Calibration. Precision, performance, confidence.™
Fluke Calibration
PO Box 9090, Everett, WA 98206 U.S.A.
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
Modification of this document is not
permitted without written permission
from Fluke Calibration.
4 Fluke Calibration
Calibrating draft range pressure sensors with a pressure controller/calibrator
For more information call:
In the U.S.A. (800) 443-5853 or
Fax (425) 446-5116
In Europe/M-East/Africa +31 (0) 40 2675 200 or
Fax +31 (0) 40 2675 222
In Canada (800)-36-FLUKE or
Fax (905) 890-6866
From other countries +1 (425) 446-5500 or
Fax +1 (425) 446-5116
Web access: http://www.flukecal.com
©2009-2011 Fluke Calibration.
Specifications subject to change without notice.
Printed in U.S.A. 9/2011 3433793B A-EN-N
Pub-ID 11841-eng
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