Campbell 3WHB10K User manual

Revision:03/2020
Copyright © 1996 – 2020
Campbell Scientific, Inc.

Table of contents
1. Function 1
2. Specifications 2
3. Wiring 2
4. Programming examples 4
4.1 GRANITE 9/10 program example 5
4.2 GRANITE 6/CR6 program example 6
4.3 CR1000X program example 6
4.4 CR1000 program example 7
4.5 CR9000X program example 7
5. 100 Ohm PRT in 3-wire half bridge 8
5.1 Excitation voltage 8
5.2 Calibrating a PRT 9
5.3 Compensation for wire resistance 9
Table of Contents - i

1. Function
A terminal input module (TIM) connects directly to a data logger or GRANITE analog input
module. It provides completion resistors for resistive bridge measurements, voltage dividers, and
precision current shunts. The 3WHB10k is used in 3-wire half bridge measurements and includes
a 10 kΩ completion resistor.
NOTE:
The GRANITE 6 and CR6 include the fixed resistor and current excitation required to complete
the half-bridge circuit without a terminal input module. However, the GRANITE 6 and CR6
are still compatible with a terminal input module and may be used with one, should the
application require it.
FIGURE 1-1. Terminal input module
3WHB10K 3-Wire Half Bridge Terminal Input Module 1

2. Specifications
10 kΩ completion resistor
Tolerance @ 25 °C: ±0.01%
Maximum temperature
coefficient
±0.8 ppm/°C
Power rating @ 70 °C: 0.25 W
EU certificate of conformity: https://s.campbellsci.com/documents/us/compliance/eudoc_
terminal-input-modules.pdf
FIGURE 2-1. Schematic
3. Wiring
The terminal input module is connected to adjacent Hand Lterminals. When making 3-wire half
bridge measurements, the 3WHB10K performs two single-ended measurements. The sense wires
from the platinum resistance thermometer (PRT) attach to the 3WHB10K. The black excitation
wire is connected to an excitation terminal. In the following example, the 3WHB10K is connected
to the 1H and 1L terminals, using them for two single-ended measurements. The excitation wire is
connected to excitation terminal VX1 (FIGURE 3-1 (p. 3)).
3WHB10K 3-Wire Half Bridge Terminal Input Module 2

Often, sensors only have two wires. Use the 3WHB10K as a fixed, known resistor to measure them.
Wire a jumper between the Hand Lterminals on the 3WHB10K to allow the sensor to receive
excitation voltage. Without the jumper, the sensor will not function. See FIGURE 3-1 (p. 3).
When using a 2-wire sensor with the 3WHB10K, use the BrHalf() (or CDM_BrHalf())
instruction in place of the BrHalf3W() (or CDM_BrHalf3W()) instruction. The parameters of
the BrHalf3W() and BrHalf() instructions are identical; so, only the instruction name
needs to be changed in the Programming examples (p. 4) example programs.
FIGURE 3-1. 3-wire half bridge used to measure PRT
The terminal spacing of the GRANITE 6 and GRANITE analog input module is different than
other data loggers. The terminal input modules are still compatible, but the pins must be gently
bent inward a small amount to allow the terminal input module to be mounted.
It is still possible to use terminal input modules in adjacent terminal blocks if the modules are
staggered. This is done by gently bending one module forward and the next terminal backward
enough to clear the first module. Continue to alternate the modules as needed.
3WHB10K 3-Wire Half Bridge Terminal Input Module 3

Table 3-1: 3WHB10K connections to Campbell Scientific data loggers
Function Label/Wire
GRANITE
analog input
module
GRANITE6,
CR6
CR3000,
CR1000X,
CR800, CR850,
CR1000
CR9000X
Excitation Black wire X1 U5 VX1 Excitation 1
V1 high
reference H 1H U1 1H 1H
V1 low sense L 1L U2 1L 1L
Ground G ⏚ ⏚ ⏚ ⏚
1The GRANITE 9 and GRANITE 10 do not directly make analog measurements. Instead, they use analog input
modules such as the VOLT 108 or VOLT 116. When making a half-bridge measurement, the terminal input module is
connected to the analog input module, which is then connected to the GRANITE 9 or GRANITE 10.
4. Programming examples
The following examples show the two instructions necessary to 1) make the measurement and 2)
calculate the temperature. The result of the half bridge measurement as shown is Rs/R0, the input
required for the PRT algorithm to calculate temperature.
If using a calibrated sensor, the exact measurement of R0will be known. Use this value to
increase the accuracy of the PRTCalc() instruction by inserting the following equation
between the BrHalf4W() and PRTCalc() instructions in the example programs.
Rs_R0 = Rs_R0*100/R0
where R0 is the sensor resistance at 0 °C
The following examples are based on a maximum sensor resistance of 100 Ω at 0 °C. The
excitation voltages used were chosen with the assumption that the temperature would not
exceed 50 °C. Calculation of optimum excitation voltage is discussed in Excitation voltage (p. 8).
3WHB10K 3-Wire Half Bridge Terminal Input Module 4

4.1 GRANITE 9/10 program example
The GRANITE9 and GRANITE10 require the use of an analog input module, such as the
VOLT108, when making a half-bridge measurement.
CRBasic Example 1: GRANITE9/10 3-wire half bridge example
'3-wire half bridge example
'GRANITE 9 and GRANITE 10 data loggers (with a VOLT 108)
'Declare Variables and Units
Public Temp_C_3wire
Public Rs_R0
'Define Data Tables
DataTable(Hourly,True,-1)
DataInterval(0,60,Min,10)
Average(1,Temp_C_3wire,IEEE4,False)
EndTable
'Main Program
BeginProg
'Configure the VOLT 108 Module and assign it CPI address 'CPI_BUSA+1'
CPIAddModule(VOLT108,10," ",CPI_BUSA+1)
'Main Scan
Scan(5,Sec,1,0)
'Half Bridge, 3-wire measurements on the VOLT 108
CDM_BrHalf3W(VOLT108,CPI_BUSA+1,Rs_R0,1,mV1000,1,1,1,3200,True,500,60,1,0)
'PRT temperature calculation
PRTCalc (Temp_C_3wire,1,Rs_R0,0,1.0,0)
'Call Data Tables and Store Data
CallTable Hourly
NextScan
EndProg
3WHB10K 3-Wire Half Bridge Terminal Input Module 5

4.2 GRANITE 6/CR6 program example
The GRANITE 6 and CR6 include the fixed resistor and current excitation required to complete
the half-bridge circuit without a terminal input module. However, the GRANITE 6 and CR6 are
still compatible with a terminal input module and may be used with one, should the application
require it.
CRBasic Example 2: GRANITE 6/CR6 3-wire half bridge example
'GRANITE 6/CR6 data logger 3-wire half bridge using a terminal input module
Public Rs_R0, Temp_C
DataTable (Hourly,True,-1)
DataInterval (0,60,Min,0)
Average (1,Temp_C,IEEE4,0)
EndTable
BeginProg
Scan (1,Sec,0,0)
BrHalf3W (Rs_R0,1,mV1000,U1,U4,1,2500,True ,0,250,100,0)
PRTCalc (Temp_C,1,Rs_R0,0,1,0)
CallTable Hourly
NextScan
EndProg
4.3 CR1000X program example
CRBasic Example 3: CR1000X 3-wire half bridge example
'CR1000X-series data logger 3-wire half bridge
Public Rs_R0, Temp_C
DataTable (Hourly,True,-1)
DataInterval (0,60,Min,0)
Average (1,Temp_C,IEEE4,0)
EndTable
BeginProg
Scan (1,Sec,0,0)
BrHalf3W (Rs_R0,1,mV1000,1,Vx1,1,3200,True ,0,250,100,0)
PRTCalc (Temp_C,1,Rs_R0,0,1,0)
CallTable Hourly
NextScan
EndProg
3WHB10K 3-Wire Half Bridge Terminal Input Module 6

4.4 CR1000 program example
CRBasic Example 4: CR1000 3-wire half bridge example
'CR1000-series data logger 3-wire half bridge
Public Rs_R0, Temp_C
DataTable (Hourly,True,-1)
DataInterval (0,60,Min,0)
Average (1,Temp_C,IEEE4,0)
EndTable
BeginProg
Scan (1,Sec,0,0)
BrHalf3W (Rs_R0,1,mV2500,1,Vx1,1,2500,True,0,250,100,0)
PRTCalc (Temp_C,1,Rs_R0,0,1,0)
CallTable Hourly
NextScan
EndProg
4.5 CR9000X program example
CRBasic Example 5: CR9000X 3-wire half bridge example
'CR9000X data logger 3-wire half bridge
Public Rs_Ro, Temp_F
DataTable (Temp_F,1,-1)
DataInterval (0,60,Min,0)
Sample (1,Temp_F,FP2)
EndTable
BeginProg
Scan (1,mSec,0,0)
BrHalf3W (RS_Ro,1,mV1000,5,1,6,1,1,3200,True,30,40,100,0)
PRTCalc (Temp_F,1,Rs_Ro,0,1.8,32)
CallTable Temp_F
NextScan
EndProg
3WHB10K 3-Wire Half Bridge Terminal Input Module 7

5. 100 Ohm PRT in 3-wire half
bridge
The advantages of the 3-wire half bridge over other measurements that correct for wire resistance
such as a 4-wire half bridge, are that it only requires 3 wires going to the sensor and takes 2
single-ended terminals, whereas the 4-wire half bridge requires 4 wires and 2 differential
channels.
The result of the 3-wire half bridge instruction is equivalent to the ratio of the PRT resistance, Rs
to the resistance of the 10k fixed resistor, Rf.
The PRTCalc() instruction 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/R0is used with the 3-wire half bridge instruction to obtain the
desired intermediate, Rs/R0= (Rs/Rfx Rf/R0). If Rfand R0are equal, the multiplier is 1.
The fixed resistor must be thermally stable. The 0.8ppm/°C temperature coefficient would result
in a maximum error of 0.035°C at 125°C. This measurement is ratiometric (Rs/Rf) and does not
rely on the absolute values of either Rsor Rf.
The properties of the 10 kΩ resistor do not affect the result. The purpose of this resistor in the
circuit is to limit current.
5.1 Excitation voltage
When determining the excitation voltage, it is important to consider the maximum excitation
current the sensor can experience without self-heating. This is typically less than 35 mA. Refer to
the manufacturer's data sheet for the sensor for the specific value.
Once the maximum excitation current is known, the excitation voltage is then calculated.
Vx= Ix(Rf+ RSmax)
Where:
Rf= PRT completion resistor value
3WHB10K 3-Wire Half Bridge Terminal Input Module 8

RSmax = Maximum sensor resistance based on the maximum expected temperature to be
measured
Using the typical 35 mA maximum excitation current, the maximum excitation voltage for the
sensor when RSmax is 100 Ω is:
Vx= 0.35 mA (10 kΩ + 100 Ω) = 3.54 V
When RSmax is 1.25 kΩ, this changes to:
Vx= 0.35 mA (10 kΩ + 1.25 kΩ) = 3.94 V
5.2 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 3-wire half bridge is:
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 Rsand R0can be accounted for.
To perform the calibration, connect the PRT to the data logger and program the data logger to
measure the PRT with the 3-wire half bridge as shown in the example section (multiplier = 1).
Place the PRT in an ice bath (@ 0°C; Rs=R0). Read the result of the bridge measurement. The
reading is Rs/Rf, which is equal to R0/Rfsince Rs=R0. The correct value of the multiplier, Rf/R0, is
the multiplier used divided by this reading. For example, if, with a 100 Ω PRT, the initial reading is
0.9890, the correct multiplier is: Rf/R0= 100/0.9890 = 101.11.
5.3 Compensation for wire resistance
The 3-wire half bridge compensates for wire resistance by assuming that the resistance of wire A
is the same as the resistance of wire B (FIGURE 3-1 (p. 3)). The maximum difference expected in
wire resistance is 2%, but is more likely to be on the order of 1%. The resistance of Rscalculated
with BrHalf3W(), is actually Rsplus the difference in resistance of wires A and B.
For example, assume that a 100 Ω PRT is separated from the data logger by 500 feet of 22 AWG
wires. The average resistance of 22AWG wire is 16.5 Ω per 1000 feet, which would give each 500-
foot wire a nominal resistance of 8.3Ω. Two percent of 8.3Ω is 0.17Ω. Assuming that the greater
resistance is in wire B, the resistance measured for the PRT (R0= 100 Ω) in the ice bath would be
3WHB10K 3-Wire Half Bridge Terminal Input Module 9

100.17Ω, and the resistance at 40°C would be 115.71. The measured ratio Rs/R0is 1.1551; the
actual ratio is 115.54/100 = 1.1554. The temperature computed by PRTCalc() from the
measured ratio would be about 0.1°C lower than the actual temperature of the PRT. This source
of error does not exist in a 4-wire half bridge where a differential measurement is used to directly
measure the voltage across the PRT.
3WHB10K 3-Wire Half Bridge Terminal Input Module 10

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