4WPB100 and 4WPB1K PRT Bridge Terminal Input Modules

4WPB100 and 4WPB1K
PRT Bridge
Terminal Input Modules
User Guide
6.2.06
Copyright © 2000-2005 Campbell Scientific Inc.
Printed under Licence by Campbell Scientific Ltd.
CSL 340
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Table of Contents
1. Specifications ..............................................................1
2. Wiring............................................................................2
3. PRT in 4-Wire Half Bridge ...........................................2
3.1 Excitation Voltage ....................................................................................3
3.2 Calibrating a PRT .....................................................................................3
4. Programming Examples..............................................4
4.1
4.2
4.3
4.4
4.5
4.6
4.7
CR10 or CR10X .......................................................................................6
21X ...........................................................................................................6
CR7...........................................................................................................6
CR23X ......................................................................................................7
CR1000.....................................................................................................7
CR3000.....................................................................................................7
CR9000.....................................................................................................7
Figures
1. Terminal Input Module...............................................................................1
2. Circuit Schematic........................................................................................2
3. Wiring for Example Programs....................................................................2
Tables
1. Excitation Voltage or 100 Ohm PRT in 4WPB100 Based on
Maximum Temperature and Input Voltage Range .....................................5
2. Excitation Voltage for 1000 Ohm PRT in 4WPB1000 Based on
Maximum Temperature and Input Voltage Range .....................................5
This is a blank page.
i
4WPB100 and 4WPB1K PRT Bridge
Terminal Input Modules
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
4WPB100 and 4WPB1K are used to provide completion resistors
for 4-wire half bridge measurements of 100Ω and 1kΩ Platinum
Resistance Thermometers (PRTs), respectively.
H
L
G
H
L
AG
H
L
AG
Figure 1 Terminal Input Module
NOTE
If you have a silver CR10 Wiring Panel, you can use a maximum
of two Terminal Input Modules at a time. This is because only
differential channels 3 and 6 have a ground terminal next to the
low side of the differential channel. You may need to bend the
TIM leads slightly to fit the TIM into a silver wiring panel.
1. Specifications
Current limiting 10k Ohm Resistor
Tolerance @ 25°C:
Power rating:
±5%
0.25W
Tolerance @ 25°C:
Temperature coefficient:
0 to +60°C
-55 to +125°C
Power rating:
±0.01%
Completion Resistor
4 ppm/°C
8 ppm/°C
0.25W
1
4WPB100, 4WPB1K PRT Bridge Terminal Input Modules
Figure 2 Circuit Schematic
2. Wiring
The Terminal Input Module is connected to the appropriate channel. The dashed
lines in Figure 2 indicate the sensor wiring. When making 4-wire half bridge
measurements, the 4WPB is connected to a differential channel, and the sense
leads from the PRT to the next differential channel. The black excitation wire is
connected to the excitation channel. In the following examples the 4WPB is
connected to differential channel 1 and the PRT to differential channel 2; the
excitation wire is connected to excitation channel 1 (Figure 3).
Datalogger
Ex1
4WPB100
1H
1L
H
L
PRT
G
AG or
2H
2L
Figure 3 Wiring for Example Programs
3. PRT in 4-Wire Half Bridge
A 4-wire half bridge is the best choice for accuracy where the Platinum
Resistance Thermometer (PRT) is separated from other bridge completion
resistors by a lead length having more than a few thousandths of an ohm
resistance. Four wires to the sensor allow one set of wires to carry the excitation
current and a separate set of sense wires that allow the voltage across the PRT to
be measured without the effect of any voltage drop in the excitation leads.
Figure 2 shows the circuit used to measure the PRT. The 10kΩ resistor allows
the use of a high excitation voltage and low voltage ranges on the measurements.
This ensures that noise in the excitation does not have an effect on signal noise
and that self heating of the PRT due to excitation is kept to a minimum. Because
the fixed resistor (Rf) and the PRT (Rs) have approximately the same resistance,
the differential measurement of the voltage drop across the PRT can be made on
the same range as the differential measurement of the voltage drop across Rf.
2
User Guide
The result of the 4-wire half bridge Instruction is:
V2
V1
the voltage drop is equal to the current (I), times the resistance thus:
V2 I x R s R s
=
=
V1 I x R f
Rf
The RTD Instruction (Instruction 16) computes the temperature (°C) for a DIN
43760 standard PRT from the ratio of the PRT resistance at the temperature
being measured (Rs) to its resistance at 0°C (R0). Thus, a multiplier of Rf/R0 is
used with the 4-wire half bridge instruction to obtain the desired intermediate,
Rs/R0 = (Rs/Rf x Rf/R0). If Rf and R0 are equal, the multiplier is 1.
The fixed resistor must be thermally stable. The 4 ppm/°C temperature
coefficient would result in a maximum error of 0.05°C at 60°C. The 8ppm/°C
temperature coefficient would result in a maximum error of 0.33°C at 125°C.
Because the measurement is ratiometric (Rs/Rf) and does not rely on the absolute
values of either Rs or Rf, the properties of the 10kΩ resistor do not affect the
result.
3.1 Excitation Voltage
The best resolution is obtained when the excitation voltage is large enough to
cause the signal voltage to fill the measurement voltage range. The voltage drop
across the PRT is equal to the current, I, multiplied by the resistance of the PRT,
Rs, and is greatest when Rs is greatest. For example, if it is desired to measure a
temperature in the range of -10 to 40°C, the maximum voltage drop will be at
40°C when Rs=115.54 ohms. To find the maximum excitation voltage that can
be used when the measurement range is ±25mV, we assume V2 equal to 25mV
and use Ohm’s Law to solve for the resulting current, I.
I = 25mV/Rs = 25mV/115.54 ohms
= 0.216mA
Vx is equal to I multiplied by the total resistance:
Vx = I(R1+Rs+Rf) = 2.21V
If the actual resistances were the nominal values, the 25mV range would not be
exceeded with Vx = 2.2V. To allow for the tolerances in the actual resistances, it
is decided to set Vx equal to 2.1V (e.g. if the 10kΩ resistor is 5% low, then
Rs/(R1+Rs+Rf)=115.54/9715.54, and Vx must be 2.102V to keep Vs less than
25mV).
3.2 Calibrating a PRT
The greatest source of error in a PRT is likely to be that the resistance at 0°C
deviates from the nominal value. Calibrating the PRT in an ice bath can correct
this offset and any offset in the fixed resistor in the Terminal Input Module.
The result of the 4-wire half bridge is:
V2 I x R s R s
=
=
V1 I x R f R f
3
4WPB100, 4WPB1K PRT Bridge Terminal Input Modules
With the PRT at 0°C, Rs=R0. Thus, the above result becomes R0/Rf, the
reciprocal of the multiplier required to calculate temperature, Rf/R0. By making
a measurement with the PRT in an ice bath, errors in both Rs and R0. can be
accounted for.
To perform the calibration, connect the PRT to the datalogger and program the
datalogger to measure the PRT with the 4-wire half bridge as shown in the
example section (multiplier = 1). Place the PRT in an ice bath (at 0°C; Rs=R0).
Read the result of the bridge measurement. The reading is Rs/Rf, which is equal
to R0/Rf since Rs=R0. The correct value of the multiplier, Rf/R0, is the reciprocal
of this reading. For example, if the initial reading is 0.9890, the correct
multiplier is: Rf/R0 = 1/0.9890 = 1.0111.
4. Programming Examples
The following examples simply show the two instructions necessary to make the
measurement and calculate the temperature. The result of the 4-wire half bridge
measurement as shown is Rs/R0, the input required for the PRT algorithm to
calculate temperature.
NOTE
‘Full Bridge’ is shown as the name for measurement Instruction 9
(used with the CR10/10X, 21X, and CR7). When Instruction 9 is
used with the first measurement range not set to the maximum
input range, it becomes a 4-wire half bridge measurement.
All the examples are for a 100 ohm PRT in the 4WPB100. The excitation
voltages used were chosen with the assumption that the temperature would not
exceed 50°C. Tables 1 and 2 list excitation voltage as a function of maximum
temperature and the input voltage ranges used with the different dataloggers.
Calculation of optimum excitation voltage is discussed in Section 3.1.
4
User Guide
Table 1 Excitation Voltage for 100 Ohm PRT in 4WPB100 Based on
Maximum Temperature and Input Voltage Range
Max.
Temp ºC
PRT
Resistance
Ohms
50
100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
850
119.4
138.5
157.31
175.84
194.07
212.02
229.67
247.04
264.11
280.9
297.39
313.59
329.51
345.13
360.47
375.51
390.26
Excitation Voltage, mV
±25mV
±50mV
Input Range
Input Range
(CR10/10X
(21X,CR7,CR3000,CR9000)
& CR1000)
2035
4070
1758
3516
1551
3101
1390
2780
1262
2523
1157
2314
1070
2140
997
1993
934
1867
879
1759
832
1664
790
1581
753
1507
720
1441
691
1382
664
1328
640
1280
Table 2 Excitation Voltage for 1000 Ohm PRT in 4WPB1000 Based on
Maximum Temperature and Input Voltage Range
Max.
Temp. ºC
PRT
Resistance
Ohms
50
100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
850
1194.
1385.
1573.1
1758.4
1940.7
2120.2
2296.7
2470.4
2641.1
2809.
2973.9
3135.9
3295.1
3451.3
3604.7
3755.1
3902.6
Excitation Voltage, mV
±200mV
±250mV
±500mV
Input Range Input Range Input Range
(CR9000
(CR10/10X
(21X, CR7)
& CR3000) & CR1000)
1959
2448
4897
1716
2145
4291
1535
1919
3837
1394
1743
3486
1282
1603
3205
1190
1488
2976
1114
1393
2786
1050
1313
2625
995
1244
2488
948
1184
2369
906
1133
2265
870
1087
2174
837
1047
2093
808
1011
2021
783
978
1956
759
949
1898
738
923
1845
5
4WPB100, 4WPB1K PRT Bridge Terminal Input Modules
4.1 CR10 or CR10X
01:
1:
2:
3:
4:
5:
6:
7:
8:
9:
Full Bridge w/mv Excit (P9)
1
Reps
33
± 25 mV 50 Hz Rejection Ex Range
33
± 25 mV 50 Hz Rejection Br Range
1
DIFF Channel
1
Excite all reps w/Exchan 1
2035
mV Excitation
1
Loc [ Rs_R0
]
1.0
Mult
0
Offset
1:
2:
3:
4:
5:
Temperature RTD (P16)
1
Reps
1
R/R0 Loc [ Rs_R0
2
Loc [ Temp_C
]
1
Mult
0
Offset
02:
]
4.2 21X
01:
1:
2:
3:
4:
5:
6:
7:
8:
9:
02:
1:
2:
3:
4:
5:
Full Bridge w/mv Excit (P9)
1
Reps
3
± 50 mV Slow Ex Range
3
± 50 mV Slow Br Range
1
DIFF Channel
1
Excite all reps w/Exchan 1
4070
mV Excitation
1
Loc [ Rs_R0
]
1.0
Mult
0.0
Offset
Temperature RTD (P16)
1
Reps
1
R/R0 Loc [ Rs_R0
]
2
Loc [ Temp_C
]
1.0
Mult
0.0
Offset
4.3 CR7
01:
1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
6
Full Bridge w/mv Excit (P9)
1
Reps
4
± 50 mV Slow Range
4
± 50 mV Slow Range
1
In Card
1
DIFF Channel
1
Ex Card
1
Ex Channel
1
Meas/Ex
4070
mV Excitation
1
Loc [ Rs_R0
]
1.0
Mult
0.0
Offset
User Guide
02:
1:
2:
3:
4:
5:
Temperature RTD (P16)
1
Reps
1
R/R0 Loc [ Rs_R0
2
Loc [ Temp_C
]
1.0
Mult
0.0
Offset
1:
2:
3:
4:
5:
6:
7:
8:
9:
Full Bridge w/mv Excit (P9)
1
Reps
32
± 50 mV 50 Hz Rejection Ex Range
32
± 50 mV 50 Hz Rejection Br Range
1
DIFF Channel
1
Excite all reps w/Exchan 1
4070
mV Excitation
1
Loc [ Rs_R0
]
1.0
Mult
0
Offset
1:
2:
3:
4:
5:
Temperature RTD (P16)
1
Reps
1
R/R0 Loc [ Rs_R0
2
Loc [ Temp_C
]
1
Mult
0
Offset
]
4.4 CR23X
01:
02:
]
4.5 CR1000
BrHalf4W(Rs_R0,1,mV25,mV25,1,Vx1,1,2035,1,1,0,_50Hz,1.0,0)
PRT(Temp_C,1,Rs_R0,1.0,0)
4.6 CR3000
BrHalf4W (Rs_R0,1,mV50,mV50,1,Vx1,1,4070,1,1,0,_50Hz,1.0,0)
PRT(Temp_C,1,Rs_R0,1.0,0)
4.7 CR9000
BrHalf4W(Rs_R0,1,mV50,mV50,1,1,2,1,1,4070,1,1,0,100,1,0)
PRT(Temp_C,1,Rs_R0)
7