Text updated april 2021
Sensor tolerance classes as defined in IEC 60751
The international standard IEC 60751 specifies the requirements and temperature/resistance relationship for industrial platinum temperature sensors. The standard was revised by the IEC in 2008 – a revision that can still cause confusion about which tolerance classes are currently valid.
This technical information sheet takes you through the main points of IEC 60751:2008 and the updated tolerance classes.
Resistance temperature sensors
Resistance temperature sensors, or Resistance Temperature Detectors commonly known as RTDs, measure temperature using electronic resistance. Industrial resistance temperature sensors most often use a platinum measuring resistor. Platinum is a highly stable metal that can withstand a broad temperature scale. The platinum resistors are made from platinum wire or film and are embedded in an insulator, most made of glass or ceramic. It is a highly accurate technology with very short reaction time to increase or decrease in temperatures.
Resistance temperature sensors are normally used for temperature measurement between measurement between -50°C and 600°C (91°F to 1092°F) depending on sheath material and choice of element. Standards define the measuring values.
Senmatic develops and delivers resistance temperature sensors for high and low temperature ranges using Pt100, Pt500, and Pt1000 for a wide range of industrial applications.
Platinum is shortened Pt, which is why such sensors are named Pt. 100,
500, and 1000 define the number of ohms measured at 0°C (32°F).
What is new in the 2008-version of IEC 60751?
The 2008 technical revision of the standard cancels and replaces the former edition originally published in 1983. The most significant changes include a change in the scheme that tolerance classes follow and tests for tolerance acceptance, hysteresis, and individual tests.
A significant change is that the accuracy classifications now must cover the complete thermometer.
Before 2008: In the previous version, only the element (the resistor) should fulfill classification accuracy.
After 2008: The complete thermometer – not only the element – must now fulfill classification accuracy at connection end.
However, now outdated requirements are sometimes still cited in quote requests today.
“Old requirements and specifications are still cited in quote requests, although they were updated in 2008. We experience that buyers or EPC’s copy specifications from old quotes and transfer them directly to current procurements.”Allan Jørgensen Key Account Manager at Senmatic
Resistor tolerance classes
For resistors, the standard defines four tolerance classes for wire-wound resistors and four for film resistors. Doing so provides a common technical understanding throughout installation and collaborations. Simply put, the classification is synonym with a common understanding of the accuracy in the installation.
The tolerances classes in the standard are defined as:
Thermometer tolerance classes – “Class A” and “Class B” most common
Tolerance classes are defined by a single nominal resistance/temperature characteristics curve. The standard defines four classes – AA, A, B, and C:
Due to high accuracy, classes A and B are most common. However, the class used should match the actual need for accuracy. If you, for example, use temperature figures in custody transfers for the purpose of calculating volume to settle accounts, accurate measurement is highly important. If you, on the other hand, use temperature figures to detect a leak in an oil tank, accuracy is less essential – in that case the most essential feature might instead be response time to detect leakages in proper time.
Senmatic delivers sensors of higher accuracy than the classes included in the standards. In fact, we can be as accurate as a tenth of B. For information about accuracy exceeded the standard’s classes, please contact us.
As seen in the table above, you must be aware of tolerance classes, if working with cryogenic temperatures. For cryogenics, you must use classes B or C, as AA and A are not valid for the lower cryogenic temperatures.
2-, 3-, or 4-wire connection?
Whether the sensor is to be connected by 2, 3 or 4 wires depends on the needed accuracy and signal type.
2-wire connections are used when accuracy is not the central factor e.g., when used for temperature control of exhaust gasses on ships. 2-wire connections are the less accurate of the three options.
3-wire connections are mostly used for long distances, typically for 10 meters or above. If operating in tolerance classes A or AA, the standard requires that 3- or 4-wire connections must be used. 3-wire connections are most often used in resistance temperature sensors.
4-wires give the highest accuracy, as it eliminates tolerance in the wires. For tolerance classes better than B, you must use 3 or 4 wires. But accuracy also depends on the element used.
Example – error percentage depends on both element and wires:
Pt100 Ω + 1 Ω = 1 % error
Pt1000 Ω + 1 Ω = 1 ‰ error
Example – Impossible to verify accuracy for “3-wire, Class A” scopes: For thermometers with long wires internally as well as externally, the standard’s 3 % resistance tolerance of the wires makes it impossible to verify sensor accuracy on 3 wires. Still, many scopes are described as 3-wire, Class A. This will always create a deviation. To meet IEC 60751:2008’s accuracy Class A or better, sensors must be ordered as 4-wire, which has no length-dependent uncertainty.
Senmatic follows the universal wire colour code for single spot sensors.
Resistance thermometer or thermocouple?
Whether a resistance or thermocouple sensor is the most optimal to use depends on several factors. General factors to consider and compare include:
Temperature range: Resistance sensors are normally used for temperature measurement between -50°C and 600°C (91°F to 1092°F), whereas thermocouples can be used for much higher temperatures – up to as high as 2500°C (4532°F).
Accuracy: Resistance temperature sensors are generally more accurate than thermocouples.
Stability: Resistance temperature sensors generally have higher stability than thermocouples. 4-wire connections are the most stable. To create the utmost stability, we expose sensors to so-called “temperature shocks” in our laboratory.
Resistance temperature sensors from Senmatic
At Senmatic, we deliver both resistance temperature sensors and thermocouples. When contacting us regarding sensors, you do not need to predefine the tolerance class that matches your exact need. In collaboration with our technical team, we will advise and recommend a suitable sensor type and class according to the type of installation and temperature range the sensor must perform in. We can also guide beyond the sensor type, for example regarding cable type, as cables must also match the environment.
Simple apparatus sensors
Senmatic sensors are considered simple apparatus when the sensor is a passive and used in intrinsically safe circuits, as they do not transmit electricity to the remaining installation. That is also why sensors are not covered by ATEX, as in the standard EN 60079:
“Simple apparatus is defined in 3.5.5 and includes:
- passive components, e.g., switches, junction boxes, resistors and simple semi-conductor devices;
- sources of stored energy consisting of single components in simple circuits with well-defined parameters, e.g., capacitors or inductors, whose values are consided when determining the overall safety of the system;
- sources of generated energy e.g., thermocouples and photocells, which do not generate more than 1,5V, 100 mA and 25 mW.”
Please contact us, if you have questions regarding the above or if you are interested in purchasing sensor solutions for your business.
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