Perhaps you?ve already noticed that in some instances a Pt100 is specified with an accuracy class B or A. At other times, it has the class F 0.3 or F 0.15. This website post looks at the specifications for Pt100 in the international IEC 60751 standard and explains the difference in accuracy classes.
Characteristics of a Pt100
The corrosion-resistant, precious metal ?platinum? has a high long-term stability. In addition, a Pt100 features a high reproducibility and a minimal non-linearity. Other very useful properties are a very good thermal shock resistance and high measurement accuracy. Finally, the wide temperature range which can be realised with a Pt100 helps it be the most frequently used measuring element in industrial temperature measurement.
Meaning of the international IEC 60751 standard
For industry, standards have become important. They make sure that products possess a constant, traceable quality. In case a product is manufactured in accordance with international standards, all market participants could be confident that the characteristics described within it are met. Thus, a Pt100 relative to IEC 60751 always has the same base resistance values in addition to a defined tolerance curve. This allows the user, for example, to displace a defective thermometer with a new one, and never have to readjust the control loop. Likewise, one controller can simply be exchanged for another, provided the latter includes a Pt100 input.
Differentiation between measuring resistor and thermometer
Fig. left: Pt100 in thin-film design
Fig. centre: Pt100, wire-wound, glass measuring resistor
Fig. right: Pt100, wire-wound, ceramic measuring resistor
With the revision of IEC 60751 in 2008, new accuracy classes and measuring ranges for Pt100s were introduced. Thus the typical differentiated between measuring resistors and resistance thermometers for the very first time. A measuring resistor consists of a platinum wire (wire-wound measuring resistor) or a platinum film (film measuring resistor) and is made for installation in resistance thermometers. A resistance thermometer (c), on the other hand, by definition consists of:
exactly the same measuring resistor (a or b), installed in protective components
internal connecting wires and external terminals for link with electrical measuring instruments
Mounting elements, connecting cables (d) or connection heads, based on thermometer version.
Comparison: Measuring resistor (a = wire-wound, b = film resistor) and resistance thermometer (c)
Development history for the IEC 60751 standard
IEC 60751 originally only recognised classes A and B for Pt100s. It did not differentiate between measuring resistors and thermometers. There is also Bootleg in the measuring resistances between wire-wound and film measuring resistances. Triggered by complaints from their customers, the thermometer manufacturers (independently of every other) measured the accuracy of their own and third-party instruments. The result: Thermometers with film measuring resistors show a different behaviour at higher temperatures than described in the standard. The standards committee took this into account in the revision of IEC 60751. The accuracy classes A and B for resistance thermometers were retained. Classes AA and C were added. The addition taken care of immediately customer demand for more accurate thermometers (class AA) and considers the greater inaccuracy of film measuring resistors at temperatures above 500 �C (class C).
Bases and outcomes of the revision of the IEC 60751 standard
For the measuring resistors themselves, the standards committee has consequently introduced new classes. Tests have shown that a measuring resistor behaves differently under laboratory conditions when compared to a measuring resistor installed in a thermometer. This behaviour affects the range of validity and the tolerance value. Thus it could happen that a measuring resistor originally has class A ? the thermometer in which it really is installed, however, includes a different validity range. Also the tolerance value can thus be altered. To carry out justice to this fact, a separate table for measuring resistances was created. The differences in the temperature ranges between a wire-wound Pt100 and a Pt100 in thin-film design (film measuring resistor) are considered. Wire-wound Pt100?s are available in classes W 0.1 / W 0.15 / W 0.3 / W 0.6 (W for ?wire wound?). Film measuring resistors match the classes F 0.1 ? F 0.6 (F for ?thin film?).
Measuring resistors
Wire-wound measuring resistors
Film measuring resistors
Tolerance value
[�C]
Class
Range of validity
[�C]
Class
Range of validity
[�C]
W 0.1
-100 ? + 350
F 0.1
0 ? +150
+/- (0.1 + 0,0017 * t)
W 0.15
-100 ? +450
F 0.15
-30 ? +300
+/- (0.15 + 0,002 * t)
W 0.3
-196 ? +660
F 0.3
-50 ? +500
+/- (0.3 + 0,005 * t)
W 0.6
-196 ? +660
F 0.6
-50 ? +600
+/- (0.6 + 0,01 * t)
Table 1: Accuracy classes and temperature ranges for Pt100 ? Measuring resistors relative to IEC 60751
Thermometers
Wire-wound measuring resistors
Film measuring resistors
Tolerance value
[�C]
Class
Range of validity
[�C]
Class
Range of validity
[�C]
AA
-50 ? +250
AA
0 ? +150
+/- (0.1 + 0,0017 * t)
A
-100 ? +450
A
-30 ? +300
+/- (0.15 + 0,002 * t)
B
-196 ? +600
B
-50 ? +500
+/- (0.3 + 0,005 * t)
C
-196 ? +600
C
-50 ? +600
+/- (0.6 + 0,01 * t)
Table 2: Accuracy classes and temperature ranges for Pt100 thermometers in accordance with IEC 60751
Differences between wire-wound and film measuring resistors
Other than the temperature ranges, there are further differences in both Pt100 versions. The most important is the design. A wire-wound measuring resistor is considerably bigger than a Pt100 in thin-film design. Very short insertion lengths, which are generally necessary in machine building, can only be achieved practically with a film measuring resistor. The low mass of the film measuring resistor results in a shorter response time of the thermometer. Also, the vibration resistance is therefore much better than with a thermometer with a wire-wound Pt100.
Meaning of the measuring resistor type
The thermometer marking in accordance with IEC 60751 does not specify the kind of measuring resistor. Normally, this is of no concern to an individual as long as the specifications required for the application are met. However, due to the different advantages of the two types, it can be helpful in individual cases to learn the sort installed. Thus, for instance, a thermometer with a film measuring resistor could be fitted even where it really is only immersed several millimetres into the medium. Whereas, with a wire-wound resistor ? simply because of its length ? a measuring error may appear because the measuring element may not be able to be fully immersed in the medium.
EExchange of experience
Film measuring resistors will be the standard design in WIKA thermometers, unless the temperature range or an explicit customer request exclude them. What exactly are your experiences with resistance thermometers? Which tolerance specification can you prefer and why? You are welcome to utilize the comment function or write to me.
Note
You can get further information on resistance thermometers on the WIKA website or in the video: How does a resistance thermometer work? Inside our Technical Information ?Operating limits and tolerances of platinum resistance thermometers per EN 60751? you will see more info on the differences between wire-wound and film measuring resistors.
Also read our articles
Pt100 in 2-, 3- or 4-wire connection?
Pt100, Pt1000 or NTC ? which Threatened may be the right one?