A Pt100 Class A RTD is recommended to use a 4-wire connection, why is it so?
How do 2, 3, or a 4-wire connection differ?
How to choose the alpha (TCR) curve?
The following article covers the frequently asked questions regarding RTD in various aspects.
A Pt100 Class A RTD is recommended to use a 4-wire connection, why is it so?
A 4-wire based connection is able to compensate entirely for the effect of the wire resistance of the cable circuit. In a connection using a 2-wire or even a 3-wire it becomes necessary to program in a manual way the value of the cable resistance that’s measured as a parameter to correct the displayed readout.
But despite this, it is observed that the parameter will hold good only for the temperature that was prevalent at the moment the cable’s resistance was measured. With change in the ambient temperature, the accuracy of the measurement circuit is lost.
In comparison, a 4-wire connection is capable of compensating for changes in ambient temperature.
Since Pt100 RTDs of type Class A are specifically meant to measure temperature accurately, it becomes really important that the implementation of the wire connection be made as a 4-wire connection.
How do 2, 3, or a 4-wire connection differ?
It is a norm for RTDs to be made available in a 2, 3, or 4-wire connection configuration.
Choosing the most suitable configuration that will hold good for an application is dependent on numerous factors, but it is equally important that there’s a match between instrumentation and sensor configuration, else the effect of the circuitry that cancels out the wire resistance will have no bearing.
Factors that cannot be ignored:
- Installation cost – use of higher number of wires will incur a substantial cost
- Space consumption – use of fairly large number of wires consumes that much more space
- Accuracy related – 2-wire connection configurations are reliable only when used with elements having high resistance.
1. Construction of 2-wire connection RTD
2-wire connection RTD’s temperature reading is artificially on a higher side as other than its element resistance, the lead wires resistance also gets added along.
2. Construction of 3-wire connection RTD
The result displayed by an RTD that’s on a 3-wire connection configuration is accurate only if the lead wire resistance error is negated, and this can happen only if the instrumentation is able to measure the true resistance of the 3-wire configuration.
There is a cancellation of the lead wire resistance error only when all of the lead wires are of similar resistance. When the 3 wires in use are of the same length, composition, and AWG, the result is that the lead-wire resistance will have a match of around 5%.
3. Construction of 4-wire connection RTD
It depends on the ability of the instrumentation in being able to determine the true resistance of the 4-wire connection for the resistance to be cancelled out from the RTD’s result.
The lead wire resistance error will be cancelled out from a 4-wire connection RTD’s resistance measurement when the 4 wires in use are of the same length, composition, and AWG.
4. Are the 2, 3, and 4-wire connection RTD configuration interchangeable?
- A 4-wire connection RTD can be used as a 3-wire connection RTD by not utilising one the leads, or in other words by tying it off.
- Likewise, 4-wire connection RTDs can be used as a 2-lead RTD after shorting the leads that are in common.
It needs to be noted that when common leads are shorted, it eliminates the benefit that a 4-wire connection brings in by cancelling out the lead wire resistance.
How to choose the alpha (TCR) curve?
TCR or Temperature Coefficient of Resistance is the average change in resistance that the sensing element will undergo over a particular temperature range like between 0℃ to 100℃.
The value is considered to be a characteristic of the material the element is made of and is not dependent on the base resistance. It’s unit of measurement is Ohms/ohm/C
Example:
A probe can read 100 ohms at 0°C, but at a different temperature suppose at 100°C, the .00385 probe will show a reading of 138.5 ohms and the .00392 probe will show a reading of 139.20 ohms.
- Resistance curve mainly depends on instrumentation. Refer to your instrumentation manual for acceptable Resistance Temperature Detector (RTD) input types.
- Common TCRs include:
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