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5. 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

 

wire

 

length

 

having

 

more

 

than

 

a

 

few

 

thousandths

 

of

 

an

 

Ohm

 

resistance.

 

Four

 

wires

 

to

 

the

 

sensor

 

allows

 

one

 

set

 

of

 

wires

 

to

 

carry

 

the

 

excitation

 

current

 

with

 

a

 

separate

 

set

 

of

 

sense

 

wires

 

allowing

 

the

 

voltage

 

across

 

the

 

PRT

 

to

 

be

 

measured

 

without

 

the

 

effect

 

of

 

any

 

voltage

 

drop

 

in

 

the

 

excitation

 

wires.

 

This

 

arrangement

 

cancels

 

out

 

both

 

the

 

effect

 

of

 

the

 

wire

 

length

 

and

 

differences

 

in

 

resistance

 

of

 

the

 

excitation

 

wires

 

going

 

out

 

to

 

and

 

returning

 

from

 

the

 

sensor.

FIGURE

 

2-1

 

(p.

 

2)

 

shows

 

the

 

circuit

 

used

 

to

 

measure

 

the

 

PRT.

 

The

 

10

 

kΩ

 

resistor

 

allows

 

the

 

use

 

of

 

a

 

high

 

excitation

 

voltage

 

and

 

low

 

voltage

 

ranges

 

on

 

the

 

measurements.

 

This

 e

nsures

 

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

 

(R

f

)

 

and

 

the

 

PRT

 

(R

s

)

 

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

R

f

.

The

 

result

 

of

 

the

 

four

 

wire

 

half

 

bridge

 

Instruction

 

is:

the voltage drop is equal to the current (I), times the resistance thus:

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 (R

s

) to its resistance at 0 °C

(R

0

). Thus, a multiplier of R

f

/R

0

is used with the 4-wire half bridge instruction to obtain the

desired intermediate, R

s

/R

0

= (R

s

/R

f

x R

f

/R

0

). If R

f

and R

0

are 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 (R

s

/R

f

) and does not

rely on the absolute values of either R

s

or R

f

.

The properties of the 10 kΩ resistor do not affect the result. The purpose of this resistor in the
circuit is to limit current.

4WPB100, 4WPB1K PRT Bridge Terminal Input Modules

8

Содержание 4WPB100

Страница 1: ...Revision 05 2020 Copyright 1996 2020 Campbell Scientific CSL I D 340 ...

Страница 2: ......

Страница 3: ...s under guarantee or not Please state the faults as clearly as possible and if the product is out of the guarantee period it should be accompanied by a purchase order Quotations for repairs can be given on request It is the policy of Campbell Scientific to protect the health of its employees and provide a safe working environment in support of this policy a Declaration of Hazardous Material and De...

Страница 4: ......

Страница 5: ...s digital cell phones and aerials may also not be applicable according to your locality Some brackets shields and enclosure options including wiring are not sold as standard items in the European market in some cases alternatives are offered Details of the alternatives will be covered in separate manuals Part numbers prefixed with a symbol are special order parts for use with non EU variants or fo...

Страница 6: ......

Страница 7: ...e beginning work Wear a hardhat and eye protection and take other appropriate safety precautions while working on or around tripods and towers Do not climb tripods or towers at any time and prohibit climbing by other persons Take reasonable precautions to secure tripod and tower sites from trespassers Use only manufacturer recommended parts materials and tools Utility and Electrical You can be kil...

Страница 8: ......

Страница 9: ...ing 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 PRT in 4 wire half bridge 8 5 1 Excitation voltage 9 5 2 Calibrating a PRT 9 Table of Contents i ...

Страница 10: ......

Страница 11: ...rs for 4 wire half bridge measurements of 100 Ω and 1 kΩ platinum resistance thermometer PRT respectively 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 F...

Страница 12: ...etion 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 ceu documents eu compliance eudoc_ terminal input modules pdf FIGURE 2 1 Circuit schematic 4WPB100 4WPB1K PRT Bridge Terminal Input Modules 2 ...

Страница 13: ...erminals The excitation wire is connected to the VX1 terminal FIGURE 3 1 Wiring for example program The terminal spacing of the GRANITE 6 and GRANITE analogue 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 in...

Страница 14: ...ctions 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 R0 will be known Use this value to increase the accuracy of the PRTCalc instruction by inserting the following equation between the BrHalf4W and ...

Страница 15: ... Data Tables DataTable Hourly True 1 DataInterval 0 60 Min 10 Average 1 Temp_C_4wire 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 4 wire measurements on the VOLT 108 CDM_BrHalf4W VOLT108 CPI_BUSA 1 Rs_R0 1 mV1000 mV1000 1 1 1 400 _ True True 500 60 1 0 PRT tempe...

Страница 16: ...Hourly True 1 DataInterval 0 60 Min 0 Average 1 Temp_C IEEE4 0 EndTable BeginProg Scan 1 Sec 0 0 BrHalf4W Rs_R0 1 mV1000 mV1000 U1 U5 1 400 True True 0 250 1 0 0 PRTCalc Temp_C 1 Rs_R0 0 1 0 CallTable Hourly NextScan EndProg 4 3 CR1000X program example CRBasic Example 3 CR1000X 4 wire half bridge example CR1000X series data logger 4 wire half bridge Public Rs_R0 Temp_C DataTable Hourly True 1 Data...

Страница 17: ...rue 0 250 1 0 0 PRTCalc Temp_C 1 Rs_R0 0 1 0 CallTable Hourly NextScan EndProg 4 5 CR9000X program example CRBasic Example 5 CR9000X 4 wire half bridge example CR9000X data logger 4 wire half bridge 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 BrHalf4W Rs_Ro 1 mV1000 mV1000 4 1 5 7 1 400 True True 30 40 1 0 0 PRTCalc Temp_F 1...

Страница 18: ...t 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 The result of the four wire half bridge Instruction is the voltage drop is equal to the current I times the resistance thus The PRTCalc instructi...

Страница 19: ...00 Ω 125 Ω 100 Ω 3579 mV 4WPB1K Vx 0 35 mA 10 000 Ω 1250 Ω 1000 Ω 4290 mV Small variations in sensor resistance do not cause significant differences in the calculated maximum excitation voltage For example changing the sensor resistance to 84 Ω when used with the 4WPB100 reduces the maximum excitation from 3579 mV to 3564 mV 5 2 Calibrating a PRT The greatest source of error in a PRT is likely to ...

Страница 20: ...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 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 4WPB100 4WPB1K PRT Bridge Terminal Input Modules 10 ...

Страница 21: ......

Страница 22: ...info campbellsci fr www campbellsci fr Germany Location Phone Email Website Bremen Germany 49 0 421 460974 0 info campbellsci de www campbellsci de India Location Phone Email Website New Delhi DL India 91 11 46500481 482 info campbellsci in www campbellsci in South Africa Location Phone Email Website Stellenbosch South Africa 27 21 8809960 sales campbellsci co za www campbellsci co za Spain Locati...

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