Campbell Scientific 3WHB10K User Manual

3WHB10K 3-Wire Half Bridge

Terminal Input Module

Revision: 12/06

H

L

G

H

L

AG

H

L

AG

Copyright © 1996-2006

Campbell Scientific, Inc.

Warranty and Assistance

The 3WHB10K 3-WIRE HALF BRIDGE TERMINAL INPUT MODULE
is warranted by CAMPBELL SCIENTIFIC, INC. to be free from defects in
materials and workmanship under normal use and service for twelve (12)
months from date of shipment unless specified otherwise. Batteries have no
warranty. CAMPBELL SCIENTIFIC, INC.'s obligation under this warranty is
limited to repairing or replacing (at CAMPBELL SCIENTIFIC, INC.'s option)
defective products. The customer shall assume all costs of removing,
reinstalling, and shipping defective products to CAMPBELL SCIENTIFIC,
INC. CAMPBELL SCIENTIFIC, INC. will return such products by surface
carrier prepaid. This warranty shall not apply to any CAMPBELL
SCIENTIFIC, INC. products which have been subjected to modification,
misuse, neglect, accidents of nature, or shipping damage. This warranty is in
lieu of all other warranties, expressed or implied, including warranties of
merchantability or fitness for a particular purpose. CAMPBELL SCIENTIFIC,
INC. is not liable for special, indirect, incidental, or consequential damages.

Products may not be returned without prior authorization. The following
contact information is for US and International customers residing in countries
served by Campbell Scientific, Inc. directly. Affiliate companies handle
repairs for customers within their territories. Please visit
www.campbellsci.com to determine which Campbell Scientific company
serves your country. To obtain a Returned Materials Authorization (RMA),
contact CAMPBELL SCIENTIFIC, INC., phone [phone]. After an
applications engineer determines the nature of the problem, an RMA number
will be issued. Please write this number clearly on the outside of the shipping
container. CAMPBELL SCIENTIFIC's shipping address is:

CAMPBELL SCIENTIFIC, INC.

RMA#_____
815 West 1800 North
Logan, Utah 84321-1784

CAMPBELL SCIENTIFIC, INC. does not accept collect calls.

3WHB10K Table of Contents

PDF viewers note: These page numbers refer to the printed version of this document. Use
the Adobe Acrobat® bookmarks tab for links to specific sections.

1. Function........................................................................1

2. Specifications ..............................................................1

3. Wiring............................................................................2

4. Programming Examples..............................................2

4.1 CR10(X) ...................................................................................................3

4.2 21X...........................................................................................................4

4.3 CR7...........................................................................................................4

4.4 CR9000X..................................................................................................5

4.5 CR1000.....................................................................................................5

5. 100 Ohm PRT in 3 Wire Half Bridge ...........................5

Figures

Tables

5.1 Excitation Voltage....................................................................................6

5.2 Calibrating a PRT.....................................................................................6

5.3 Compensation for Wire Resistance...........................................................7

1-1. Terminal Input Module ...........................................................................1

2-1. Schematic................................................................................................ 1

3-1. 3-Wire Half Bridge Used to Measure PRT .............................................2

3-1. 3WHB10K Connections to Campbell Scientific Dataloggers.................2

4-1. Excitation Voltage for 100 Ohm PRT in 3WHB10K Based on

Maximum Temperature and Input Voltage Range...............................3

i

This is a blank page.

3WHB10K 3-Wire Half Bridge Terminal

H

L

AG

H

L

AG

H

L

G

Input Module

1. Function

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.

FIGURE 1-1. Terminal Input Module

2. Specifications

10 kOhm Completion Resistor

Tolerance @ 25°C
Temperature coefficient
0°-60°C

-55°-125°C
Power rating @ 70°C 0.25 W

Vx

H

L

or AG

FIGURE 2-1. Schematic

±0.01%

±4 ppm/°C
±8 ppm/°C

10 kΩ

H

L

G

1

3WHB10K 3-Wire Half Bridge Terminal Input Module

3. Wiring

Datalogger

Vx

10 kΩ

H

L

or AG

or G

FIGURE 3-1. 3-Wire Half Bridge Used to Measure PRT

TABLE 3-1. 3WHB10K Connections to

Function

Excitation Black Wire E1 EX1 Excitation 1
V1 Reference H SE1 SE1 1H
V2 Sense L SE2 SE2 1L
Ground G AG

Label/Lead

Sh ie ld

Campbell Scientific Dataloggers

CR10X,
CR510

H

L

G

CR23X,
CR1000,
CR800, CR850,
CR3000

A

B

21X, CR7,
CR9000X

4. Programming Examples

The following examples simply show the two instructions necessary to 1)
make the measurement and 2) calculate the temperature. The result of the 3­wire half bridge measurement as shown is R
PRT algorithm to calculate temperature.

All the examples are for a 100 Ohm PRT in the 3WHB10K. The excitation
voltages used were chosen with the assumption that the temperature would not
exceed 50°C. Table 4-1 lists 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 5.1.

The multiplier shown is for a 100 Ohm PRT. The multiplier for a 1000 Ohm
PRT is 10.

2

, the input required for the

s/Ro

3WHB10K 3-Wire Half Bridge Terminal Input Module

TABLE 4-1. Excitation Voltage for 100 Ohm PRT in

3WHB10K Based on Maximum Temperature and Input

Voltage Range

Excitation Voltage, mV

Max.

Temp

°C

50 119.4 2119 4237
100 138.5 1830 3660
150 157.31 1614 3228
200 175.84 1447 2893
250 194.07 1313 2626
300 212.02 1204 2408
350 229.67 1113 2227
400 247.04 1037 2074
450 264.11 971 1943
500 280.9 915 1830
550 297.39 866 1731
600 313.59 822 1644
650 329.51 784 1567
700 345.13 749 1499
750 360.47 718 1437
800 375.51 691 1381
850 390.26 666 1331

PRT

Resistance

Ohms

±25 mV Input

Range, CR10(X),

CR800, CR850,

CR1000,

±50 mV Range,

21X, CR7,

CR3000,

CR9000X

4.1 CR10(X)

1: 3W Half Bridge (P7)
1: 1 Reps
2: 23 ± 25 mV 60 Hz Rejection Range
3: 1 SE Channel
4: 1 Excite all reps w/Exchan 1
5: 2100 mV Excitation
6: 1 Loc [ Rs_R0 ]
7: 100 Mult
8: 0 Offset

2: Temperature RTD (P16)
1: 1 Reps
2: 1 R/RO Loc [ Rs_R0 ]
3: 2 Loc [ Temp_C ]
4: 1.0 Mult
5: 0.0 Offset

3

3WHB10K 3-Wire Half Bridge Terminal Input Module

4.2 21X

1: 3W Half Bridge (P7)
1: 1 Reps
2: 3 ± 50 mV Slow Range
3: 1 SE Channel
4: 1 Excite all reps w/Exchan 1
5: 4200 mV Excitation
6: 1 Loc [ Rs_R0 ]
7: 100 Mult
8: 0 Offset

2: Temperature RTD (P16)
1: 1 Reps
2: 1 R/RO Loc [ Rs_R0 ]
3: 2 Loc [ Temp_C ]
4: 1.0 Mult
5: 0 Offset

4.3 CR7

1: 3-Wire Half Bridge (P7)
1: 1 Reps
2: 4 ± 50 mV Slow Range
3: 1 In Card
4: 1 SE Channel
5: 1 Ex Card
6: 1 Ex Channel
7: 1 Meas/Ex
8: 4200 mV Excitation
9: 1 Loc [ Rs_R0 ]
10: 100 Mult
11: 0 Offset

2: Temperature RTD (P16)
1: 1 Reps
2: 1 R/RO Loc [ Rs_R0 ]
3: 2 Loc [ Temp_C ]
4: 1 Mult
5: 0 Offset

4

3WHB10K 3-Wire Half Bridge Terminal Input Module

4.4 CR9000X

'CR9000X Datalogger

Public RS_Ro, Temp_F

DataTable (Temp_F,1,-1)
DataInterval (0,0,0.10)
Sample (1,Temp_F,FP2)
EndTable

BeginProg
Scan (1,mSec,0,0)
BrHalf3W (Rs_Ro,1,V50,5,1,6,1,1,4200,True,30,40,100,0)
PRT (Temp_F,1,Rs_Ro,1.8,32)
CallTable Temp_F
NextScan
EndProg

4.5 CR1000

'CR1000 Series Datalogger

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,mV25,1,Vx1,1,2100,True ,0,250,100,0)
PRT (Temp_C,1,Rs_R0,1.0,0)
CallTable Hourly
NextScan
EndProg

5. 100 Ohm PRT in 3 Wire Half Bridge

The advantages of the 3-wire half bridge over other measurements that correct
for lead wire resistance such as a 4-wire half bridge, are that it only requires 3
lead wires going to the sensor and takes 2 single-ended input channels,
whereas the 4-wire half bridge requires 4 wires and 2 differential channels.

5

3WHB10K 3-Wire Half Bridge Terminal Input Module

The result of the 3-wire half bridge instruction is equivalent to the ratio of the
PRT resistance, R

to the resistance of the 10 k fixed resistor, Rf.

s

The RTD Instruction (16) computes the temperature (°C) for a DIN 43760
standard PRT from the ratio of the PRT resistance at the temperature being
measured (R

) to its resistance at 0°C (R0). Thus, a multiplier of Rf/R0 is used

s

with the 3-wire half bridge instruction to obtain th e desired intermediate, R
= (R
R

x Rf/Ro). When Rf = 10,000 and R0 = 100, the multiplier is 100; when

s/Rf

is 1000 the multiplier is 10.

0

The fixed resistor must be thermally stable. Over the -55° to 85°C extended
temperature range for the datalogger, the ±4 ppm/°C temperature coefficient
would result in a maximum error of ±0.04°C at 60°C. The
±8 ppm/°C temperature coefficient would result in a maximum error of
±0.13°C at -55°C.

5.1 Excitation Voltage

R

s

R

f

s/R0

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, R
measure a temperature in the range of -10 to 40°C, the maximum voltage drop
will be at 40°C when R
voltage that can be used when the measurement range is ±25 mV, we assume
V2 equal to 25 mV and use Ohm's Law to solve for the resulting current, I.

V

is equal to I multiplied by the total resistance:

x

If the actual resistances were the nominal values, the 25 mV range would not
be exceeded with V
resistances and to leave a little room for higher temperatures, set V

2.1 volts.

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.

, and is greatest when Rs is greatest. For example, if it is desired to

s

=115.54 Ohms. To find the maximum excitation

s

I = 25 mV/R

= 25 mV/115.54 Ohms

s

= 0.216 mA

V

= I(Rs+Rf) = 2.18 V

x

= 2.18 V. To allow for the tolerances in the actual

x

equal to

x

6

With the PRT at 0°C, R
reciprocal of the multiplier required to calculate temperature, R

. Thus, the above result becomes R0/Rf, the

s=R0

f/R0

making a measurement with the PRT in an ice bath, errors in both R
can be accounted for.

. By

and R0.

s

3WHB10K 3-Wire Half Bridge Terminal Input Module

To perform the calibration, connect the PRT to the datalogger and program the
datalogger to measure the PRT with the 3-wire half bridge as shown in the
example section. For a 100 Ohm PRT use a multiplier of 100; for a 1000 Ohm
PRT use a multiplier of 10. Place the PRT in an ice bath (@ 0°C; R
Read the result of the bridge measurement. The reading is R
equal to R

since Rs=R0. The correct value of the multiplier, Rf/R0, is the

0/Rf

multiplier used divided by this reading. For example, if, with a 100 Ohm PRT,
the initial reading is 0.9890, the correct multiplier is: R

101.11.

5.3 Compensation for Wire Resistance

The 3-wire half bridge compensates for lead wire resistance by assuming that
the resistance of wire A is the same as the resistance of wire B (Figure 3-1).
The maximum difference expected in wire resistance is 2%, but is more likely
to be on the order of 1%. The resistance of R
actually R

For example, assume that a 100 Ohm PRT is separated from the datalogger by
500 feet of 22 awg wires. The average resistance of 22 AWG wire is 16.5 Ohms
per 1000 feet, which would give each 500 foot lead wire a nominal resistance of

8.3 Ohms. Two percent of 8.3 Ohms is 0.17 Ohms. Assuming that the greater
resistance is in wire B, the resistance measured for the PRT (R
the ice bath would be 100.17 Ohms, and the resistance at 40°C would be 115.71.
The measured ratio R
temperature computed by Instruction 16 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.

plus the difference in resistance of wires A and B.

s

is 1.1551; the actual ratio is 115.54/100 = 1.1554. The

s/R0

).

s=R0

, which is

s/Rf

= 100/0.9890 =

f/R0

calculated with Instruction 7, is

s

= 100 Ohms) in

0

7

3WHB10K 3-Wire Half Bridge Terminal Input Module

This is a blank page.

8

This is a blank page.

Campbell Scientific Companies

Campbell Scientific, Inc. (CSI)

815 West 1800 North

Logan, Utah 84321

UNITED STATES
www.campbellsci.com
[e-mail]

Campbell Scientific Africa Pty. Ltd. (CSAf)

PO Box 2450

Somerset West 7129

SOUTH AFRICA

www.csafrica.co.za

[e-mail]

Campbell Scientific Australia Pty. Ltd. (CSA)

PO Box 444

Thuringowa Central
QLD 4812 AUSTRALIA
www.campbellsci.com.au

[e-mail]

Campbell Scientific do Brazil Ltda. (CSB)

Rua Luisa Crapsi Orsi, 15 Butantã

CEP: 005543-000 São Paulo SP BRAZIL

www.campbellsci.com.br

[e-mail]

Campbell Scientific Canada Corp. (CSC)

11564 - 149th Street NW

Edmonton, Alberta T5M 1W7

CANADA

www.campbellsci.ca

[e-mail]

Campbell Scientific Ltd. (CSL)

Campbell Park

80 Hathern Road

Shepshed, Loughborough LE12 9GX

UNITED KINGDOM

www.campbellsci.co.uk

[e-mail]

Campbell Scientific Ltd. (France)

Miniparc du Verger - Bat. H

1, rue de Terre Neuve - Les Ulis

91967 COURTABOEUF CEDEX

FRANCE

www.campbellsci.fr

[e-mail]

Campbell Scientific Spain, S. L.

Psg. Font 14, local 8

08013 Barcelona

SPAIN

www.campbellsci.es

[e-mail]

Please visit www.campbellsci.com to obtain contact information for your local US or International representative.