Temperature Measurement From Rtd

Temperature Measurement INDUSTRIAL INSTRUMENTATION

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RESISTANCE THERMOMETER

(Change in electrical property)

Variation of Resistance in metallic conductors with change in

temperature – principle of Resistance Temperature Detector

(RTD)

The relation between electrical resistance of a metal and the

corresponding temperature T is generally given as

Resistance at Temperature T = 0˚C

Resistance at Temperature ‘T’

Constants

Although this is a nonlinear relationship, it can be seen from

figure that the curve is nearly linear for copper and platinum

over a fairly long range. However, copper being easily

susceptible to chemical reactions such as oxidation, sulphate

formation, etc., platinum is chosen for RTDs. The Platinum

Resistance Thermometers are also referred to as PRTs.


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PLATINUM RESISTANCE THERMOMETER

The ends of this wire are joined to terminals A and B on the top

of the instrument. Another exactly similar lead, with its lower

end shorted to B is connected to terminal C to compensate for

the resistance of the leads.

The Pt-100 RTD has a sensitivity of 0.385 /˚C.

Range: -40˚C to 1200˚C.

Accuracy: 0.2% to 1.2% at different ranges.

Errors:

Self-Heating: E.g. 1 ma current through 100Ω RTD

generates 100µW power. Error will be 1˚C/mW in free air.

It can be reduced to 0.1˚C/mW in air flowing at 1m/s.

Reduction of this effect:

1) Pulses can be given instead of continuous supply.

2) Circuit is designed such that very current flows in it.

Lead Wire Resistance: E.g. 1Ω connected to 100Ω PRT

causes 1% measurement error

MEASUREMENT WITH RTDs

Two Wire Connection:


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The simplest resistance measurement configuration uses two

wires to connect the thermometer to a Wheatstone bridge.

In this configuration, the resistance of the connecting wires is

always included with that of the sensor leading to errors in the

signal. So, it is mainly used when high accuracy is not required.

Using this configuration, about 10 m of cable can be used.

Three Wire Connection:

In order to minimize the effects of lead resistances, a three wire

configuration can be used.

Here, the two leads to the sensor are on the adjoining arms.

There is a lead resistance in each arm of the bridge


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They cancel out as can be seen from the following analysis:

High quality connection cables should be used for this type of

configuration because we have assumed that the two lead

resistances are equal.

This configuration allows for up to 600m of cable.

Four Wire Compensation:

See note for diagram and analysis

The four-wire resistance thermometer configuration even further

increases the accuracy and reliability of the measurement of

resistance.

It provides full cancellation of spurious effects and cable

resistance of up to 15Ω can be handled, though in principle, the

resistance error due to lead wire resistance is zero in four-wire

measurements.


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TEMPERATURE COMPUTATION:

From Callendar-Van Deuson Relation:

With the advent of computers, the temperature corresponding to

a measured resistance can be found by the method of iteration

from the Callendar-Van Deuson Relations.

From Callendar and Griffiths’ method:

Callander and Griffiths’ observed that the following simple

relation gives true readings up to 630˚C:

where, c is the mean temperature coefficient of Resistance

between 0˚C and 100˚C, and is given by,

The difference between the true temperature and that obtained

from the above equation is given by

where, is a constant for a particular specimen of wire and its

value varies between 1.488 and 1.498. So, the procedure is

given as follows:


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Step 1: Find Platinum temperature from Equation (1), by

measuring . The last two quantities are to be

determined only once and for all.

Step 2: Substitute this value for on the left and for T on the

right-hand side of equation (3) to obtain a revised value of T.

Step 3: Substitute the value of T obtained from Step 2 in the

right hand side of Equation (3) to obtain a more accurate value

of T

Step 4: Repeat Step 3 until the value of T converges. This

iterative procedure is also called the successive approximation

method.

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Temperature Measurement From Rtd