Platinum resistance thermometers
02 October 2008
Temperature affects all processes but is always difficult to measure. In the first section, of a two-part article, Bill Earlie of electrical test and measurement specialist, Cropico, looks at platinum resistance thermometers and the ways in which they can be used.
Platinum resistance thermometers are much more accurate than thermocouples but they have some drawbacks, notably a more limited temperature range, a higher thermal mass and are usually more expensive. These resistance thermometers have a linear and repeatable resistance against temperature. The two common types in use are Pt100, which has a resistance of 100Ω at 0°C, and Pt25, which has a resistance of 25Ω at 0°C.
Platinum is used because it has a stable temperature coefficient and being a noble metal is not very susceptible to contamination. Pt100 (PRT) is the most commonly used and has a temperature coefficient of ? = 0.00385 (European standard), which corresponds to an average resistance change, over the temperature range 0 to 100°C, of 0.385Ω per °C.
Both the absolute resistance value and the change in resistance per °C are both relatively small and give rise to measurement problems, especially when the resistance of the connection leads are taken into consideration. There are other standards also in use – for instance, the US standard for pt100 has an alpha of 0.00392.
Types of measurement
When measuring the resistance of a Pt100 a test current is forced through the component and the test metre measures the voltage at its terminals. The meter then calculates and displays the resulting resistance and is known as a two-wire measurement. It should be noted that the meter measures the voltage at its terminals and not across the component.
As a result of this, the voltage drop across the connection leads is also included in the resistance calculation. Good quality test leads will have a resistance of approximately 0.02Ω per meter. In addition to the resistance of the leads, the resistance of the lead connection will also be included in the measurement and this can be as high as, or even higher in value than, the leads themselves. The two-wire measurement is not recommended.
(See figure one)
A three-wire connection is quite common in industrial applications and will eliminate most of the effect of the lead resistance on the measured value. Care must be taken to ensure that all three wires are of equal resistance but this is almost impossible to achieve in practice. The three-wire method will not deliver the same degree of accuracy as a true four-wire system but is better than two wires.
(See figure two)
Four wire measurements are the most accurate configuration. Two-wires are used to pass a constant current through the Pt100 and the volt drop across the unit is then measured. The impedance of the voltage measurement circuit is high and as a consequence only a very small current flows in the potential circuit, which for practical purposes can be ignored. The result is that the measurement lead resistance can also be ignored.
(See figure three)
When measuring PRTs the measurement current used by most temperature indicators is either DC or low frequency AC. If AC is used, then care in selecting a non inductive sensor is essential as the measurement will be the impedance of the sensor rather than its true DC resistance.
There may also be some differences in the temperature measurement between sensors from different manufacturers, as their construction technique may differ, resulting in slightly different impedance values. This AC measurement does, however, eliminate any thermal emf errors that may arise.
Using DC current measurement, the true resistance value is measured and used to calculate the corresponding temperature. In this instance impedance errors are not a problem, but errors due to thermal emf must be considered. The best method of countering any thermal emf is to measure the sensor resistances with current flowing in one direction, then reverse the current and take a second measurement.
The average of these two measurements is the true resistance without any thermal emf. This is often called the switched DC method and is selectable on the Cropico thermometers. To obtain the best measurement results, the resistance of the Pt100 sensor must be measured with a high degree of accuracy. A temperature change of 1°C will correspond to a resistance change of 0.385Ω so to obtain a measuring accuracy of 0.01°C (10mK) the resistance must be measured to ±0.0385Ω.
For example: for a temperature of 100°C the resistance value will be 138.5Ω. To measure this with an accuracy of ±0.01°C, this resistance value must be measured to ±0.0385Ω, which is equal to ±0.028%.
If a current of 1mA is used as the measuring current to measure 138.5Ω (100°C), then a voltage of 138.5mV will need to be measured to ±138.5μV, and to measure the temperature change of 0.01°C, a change of 3.85μV must be measured. So it’s clear that a small error in the voltage sensing measurement can create widescale temperature measurement errors.
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