Campbell | User manual | CURS100 100 Ohm Current Shunt Terminal Input Module 1
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CURS100
100 Ohm Current Shunt
Terminal Input Module
Issued: 17.9.13
Copyright © 2000-2013 Campbell Scientific, Inc.
Printed under licence by Campbell Scientific Ltd.
CSL 338
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PLEASE READ FIRST
About this manual
Please note that this manual was originally produced by Campbell Scientific Inc. primarily for the
North American market. Some spellings, weights and measures may reflect this origin.
Some useful conversion factors:
Area: 1 in
2
(square inch) = 645 mm
Length: 1 in. (inch) = 25.4 mm
1 ft (foot) = 304.8 mm
1 yard = 0.914 m
1 mile = 1.609 km
2
Mass: 1 oz. (ounce) = 28.35 g
1 lb (pound weight) = 0.454 kg
Pressure: 1 psi (lb/in
2
) = 68.95 mb
Volume: 1 UK pint = 568.3 ml
1 US gallon = 3.785 litres
In addition, while most of the information in the manual is correct for all countries, certain information is specific to the North American market and so may not be applicable to European users.
Differences include the U.S standard external power supply details where some information (for example the AC transformer input voltage) will not be applicable for British/European use. Please note, however, that when a power supply adapter is ordered it will be suitable for use in your country.
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Some brackets, shields and enclosure options, including wiring, are not sold as standard items in the
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For further advice or support, please contact Campbell Scientific Ltd, or your local agent.
Campbell Scientific Ltd, Campbell Park, 80 Hathern Road, Shepshed, Loughborough, LE12 9GX, UK
Tel: +44 (0) 1509 601141 Fax: +44 (0) 1509 601091
Email: [email protected] www.campbellsci.co.uk
Contents
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Introduction ................................................................ 1
Specifications ............................................................ 1
Measurement Concepts ............................................ 2
Differential Measurement .................................................................... 3
Completing the Current Loop Circuit .................................................. 3
Transducer Wiring ..................................................... 4
Two-Wire Transducers......................................................................... 4
Possible Ground Loop Problems ................................................... 5
Minimum Supply Voltage ............................................................. 5
Three-Wire Transducers....................................................................... 6
Four-Wire Transducers ........................................................................ 6
Sensor and Programming Example .......................... 7
Calculating Multiplier and Offset—An Example ................................ 7
CR1000 Program Example .................................................................. 8
CR9000(X) Program Example ............................................................. 9
CR10(X) Program Example ................................................................. 9
CR23X Program Example.................................................................... 9
Figures
1-1. CURS100 terminal input module ......................................................... 1
4-1. 2-wire with datalogger power .............................................................. 4
4-2. 2-wire with external power .................................................................. 5
4-4. 3-wire with datalogger power .............................................................. 6
4-5. 3-wire with external power .................................................................. 6
4-6. 4-wire with datalogger power .............................................................. 6
4-7. 4-wire with external power .................................................................. 7
CURS100 100 Ohm Current Shunt
Terminal Input Module
1. Introduction
Terminal input modules connect directly to the datalogger's input terminals to provide completion resistors for resistive bridge measurements, voltage dividers, and precision current shunts. The CURS100 converts a current signal (for example, 4 to 20 mA) to a voltage that is measured by the datalogger. The 100 ohm resistor used for the current shunt allows currents up to 25 mA to be read on a ±2500 mV range (CR10, CR10X) and currents up to 50 mA to be read on a
±5000 mV range (CR800, CR850, CR1000, CR3000, CR5000, CR9000X,
CR9000, 21X, CR23X).
Figure 1-1. CURS100 terminal input module
2. Specifications
100 Ohm Shunt Resistor
Tolerance @ 25°C: ±0.01%
Temperature coefficient: ±0.8 ppm / ºC
Power rating: 0.25 W
1
CURS100 100 Ohm Current Shunt Terminal Input Module
Figure 2-1. CURS100 schematic
The CURS100 has three pins: high, low, and ground; these pins are the correct spacing to insert directly into the datalogger's high, low, and ground terminals
( on 21X, CR23X, CR800, CR850, CR1000, CR3000, CR5000, or CR9000(X) or AG on CR10(X)).
3. Measurement Concepts
Transducers that have current as an output signal consist of three parts: a sensor, a current transducer (quite often integrated with the sensor), and a power supply.
The power supply provides the required power to the sensor and the transducer.
The sensor signal changes with the phenomenon being measured. The current transducer converts the sensor signal into a current signal. The current output changes in a known way with the phenomenon being measured.
An advantage of current loop transducers over voltage output transducers is the current signal remains constant over long lead lengths.
Two disadvantages with current loop transducers are as follows. First, most transducers require constant current from the power supply, adding cost and size.
Secondly, the conditioned output quality may not be as good as a similar unconditioned sensor being measured directly by a datalogger.
The output of the transducer is wired so the current must flow through the 100 ohm resistor in the CURS100.
Ohm's law describes how a voltage (V) is generated by the signal current (I) through a completion resistor (R):
V = I (R).
This voltage is measured by the datalogger.
2
User Manual
3.1 Differential Measurement
The voltage across the completion resistor is measured with the differential voltage measurement. Use VoltDiff() for the CRBasic dataloggers (for example,
CR1000, CR5000, or CR9000(X)). Use Instruction 2 for Edlog dataloggers (for example, CR10X). The differential voltage measurement measures the difference in voltage between the low and high terminals. The CURS100 connects the resistor between the high and the low terminals.
3.2 Completing the Current Loop Circuit
As shown in Figure 2-1, the 100 Ω sense resistor in the CURS100 is not connected
to the adjacent ground pin that connects into the datalogger signal ground ( or
AG). Hence, an additional connection must be made in order to complete the loop. Which is commonly done by connecting the CURS100 L terminal to a
datalogger G (power ground) terminal with a jumper wire (Figure 3-1).
Connecting the L terminal to the adjacent ground ( or G) terminal on the
CURS100 will result in unwanted return currents flowing into the datalogger signal ground ( or AG) which could induce undesirable offset errors in lowlevel, single-ended measurements. The ground ( or G) terminal on the
CURS100 can be used to connect cable shields to ground.
Completing the loop by connecting voltages other than ground is possible as long as the datalogger voltage input limits are not exceeded. These input limits specify the voltage range, relative to datalogger ground, which both H and L input voltages must be within in order to be processed correctly by the datalogger. The input limits are ±2.5 V for the CR10(X) and ±5 V for the CR800, CR850,
CR1000, CR3000, CR5000, and CR9000(X). Hence, when measuring currents up to 50 mA with the CURS100, a connection to datalogger ground is necessary in order for the resulting (50 mA)
(100 Ω) = 5 V signal to comply with the ±5 V input limits for the CR800, CR850, CR1000, CR3000, CR5000, and CR9000(X) dataloggers.
3
CURS100 100 Ohm Current Shunt Terminal Input Module
Figure 3-1. CURS100 L terminal connected to a datalogger G terminal using a jumper wire.
NOTE Normally the L terminal on the CURS100 should be connected to a
datalogger G terminal (power ground) with a jumper wire (Figure
3-1). Connecting the L terminal to the adjacent ground (
or G) terminal on the CURS100 can result in unwanted return currents on the datalogger signal ground, which could induce undesirable offset errors in low-level, single-ended measurements. The G terminal on the CURS100 can be used to connect cable shields to ground.
4. Transducer Wiring
Current transducers differ mainly in how they are powered and in the relative isolation of the current output. In this section, the transducers are grouped by the total number of wires the transducer uses to obtain power and output the current.
4.1 Two-Wire Transducers
In a two-wire transducer, the power supply is in series within the current loop.
The transducer regulates the amount of current that flows; the current drawn from the battery is exactly the current used as a signal.
Figure 4-1. 2-wire with datalogger power
4
User Manual
Figure 4-2.
2-wire with external power
4.1.1 Possible Ground Loop Problems
The resistor must be grounded at the datalogger to ensure that measurements are within common mode range. The signal (or low) output on the transducer is higher than the datalogger ground by the voltage drop across the resistor. A ground-loop error may occur if the signal output is not electrically isolated but is connected to the sensor's case. If such a sensor is in contact with earth ground (for example, a pressure transducer in a well or stream), an alternative path for current flow is established through earth ground to the datalogger earth ground. This path is in parallel with the path from the signal output through the resistor; hence, not all the current will pass through the resistor and the measured voltage will be too low.
4.1.2 Minimum Supply Voltage
When the power supply is in the current loop, as is the case in a 2-wire transducer, it is necessary to consider the effect of voltage drop across the resistor on the voltage applied to the transducer.
For example, suppose a 4 to 20 mA transducer requires at least 9 volts to operate correctly and the system is powered by a 12 volt battery. The voltage the transducer sees is the battery voltage minus the voltage drop in the rest of the current loop. At 20 mA output, the voltage drop across the 100 ohm resistor is 2 volts. When the battery is at 12 volts, this leaves 10 volts for the transducer and everything is fine. However, if the battery voltage drops to 11 volts, a 20 mA current will leave just 9 volts for the transducer. In this case, when the battery drops below 11 volts, the output of the transducer may be in error.
Figure 4-3. 2-wire supply voltage
5
CURS100 100 Ohm Current Shunt Terminal Input Module
4.2 Three-Wire Transducers
A three-wire current loop transducer has the power supply connected directly to the transducer. The voltage of the power supply is the voltage applied to the transducer. The current output returns to power ground. Datalogger ground is connected to sensor ground and the current output by the sensor must pass through the resistor before going to ground.
Figure 4-4. 3-wire with datalogger power
6
Figure 4-5. 3-wire with external power
4.3 Four-Wire Transducers
A four-wire transducer has separate wires for power input and ground and for signal output and ground. The signal ground may or may not be internally tied to the power ground. Some transducers have completely isolated outputs.
Figure 4-6.
4-wire with datalogger power
User Manual
Figure 4-7. 4-wire with external power
5. Sensor and Programming Example
In this example, the input voltage range, and the multiplier and offset values are calculated for a 4 to 20 mA output pressure transducer. Examples showing the differential measurement made on Channel 1 are then given for the CR1000,
CR9000(X), CR10(X), and 21X dataloggers; programming for the CR800,
CR850, CR3000, and CR5000 is virtually identical to the CR1000.
5.1 Voltage Range
The voltage range on which to make the measurement should be the smallest range that will accommodate the maximum signal the sensor will output. Using the smallest possible range will give the best resolution.
The voltage across the resistor, V, is equal to the resistance (100 ohms) multiplied by the current, I.
V = 100 I
The maximum voltage occurs at the maximum current. Thus, a 4 to 20 mA transducer will output its maximum voltage at 20 mA.
V = 100 ohms
0.02 A = 2 V
An output of 2 volts is measured on the ±2500 mV range on the CR800, CR850,
CR1000, and CR10(X), or on the ±5000 mV range on the 21X, CR3000, CR5000, or CR9000(X).
5.2 Calculating Multiplier and Offset —An Example
The multiplier and the offset are the slope and y-intercept of a line and are computed with Ohm’s law and a linear fit.
For example, measure a current loop transducer that detects pressure where the sensor specifications are as follows:
Transducer range — 200 to 700 psi
Transducer output range — 4 to 20 mA
7
CURS100 100 Ohm Current Shunt Terminal Input Module
The transducer will output 4 mA at 200 psi and 20 mA at 700 psi. Using Ohm's law, the voltage across the resistor at 200 psi is:
V = I
R
V = 0.004
100
V = 0.4 V or 400 mV and at 700 psi is:
V = 0.020
100
V = 2.0 V or 2000 mV
Since the datalogger measures in mV, the multiplier (or slope) must be in units of psi/mV. Therefore, the y values have the units psi and the x values mV.
The equation of a line is:
(y ─ y1) = m (x ─ x1)
Solve the equation for m that is the slope of the line (or multiplier). m
700 psi
200 psi
2000 mV
400 mV
psi mV
Now replace the known values to determine the intercept (or offset). Where y = m(x) + b
200 b
psi
200
0 .
3125
0 .
3125 psi
mV
400
400
75 mV psi
b m = multiplier (slope) = 0.3125 and b = the offset (intercept) = 75.0.
5.3 CR1000 Program Example
'CR1000 program example for sensor with 4-20mA output.
'Assuming a flow meter that outputs a 4-20mA signal representing 0 - 100 gal/min,
'the voltage across the resistor at 0 gal/min = 4mA * 100 ohms = 400mV,
'and at 100 gal/min is 20mA * 100 ohms = 2000mV. The change in mV is
'2000mV - 400mV = 1600mV for 0 - 100 gal/min flow rate.
'The measurement result (X) for the VoltDiff instruction is mV. The
'multiplier to convert mV to gal/min is: mV * 100gal/min / 1600mV = 0.0625,
'the offset is 400mV * .0625 = -25.0.
8
Public Measure
DataTable (Hourly,True,-1)
DataInterval (0,60,Min,0)
Average (1,Measure,IEEE4,0)
EndTable
BeginProg
Scan (1,Sec,1,0)
'Generic 4-20 mA Input measurement Measure:
VoltDiff (Measure,1,mV2500,1,True,0,_60Hz,0.0625,-25.0)
CallTable (Hourly)
NextScan
EndProg
5.4 CR9000(X) Program Example
VoltDiff(Press_psi, 1, mV5000, 5, 1, 1, 0, 0, 0.3125, 75)
5.5 CR10(X) Program Example
1: Volt (Diff) (P2)
1: 1
2: 25
3:
4:
5:
6:
1
1
75
Reps
± 2500 mV 60 Hz Rejection Range
DIFF Channel
Loc [ Press_psi ]
0.3125 Mult
Offset
5.6 CR23X Program Example
1: Volt (Diff) (P2)
1:
2:
3:
4:
5:
6:
1
25
1
1
0.3125
75
Reps
± 5000 mV, 60 Hz Reject, Fast Range
DIFF Channel
Loc [ Press_psi ]
Mult
Offset
User Manual
9
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