Frequently Asked Questions about platinum RTD elements

Platinum RTD SpecificationsThin film vs. Wirewound RTD elementChoosing a Thin Film ElementTemperature CoefficientResistance vs. Temperature CharacteristicRTD Element Physical PropertiesAssembly Considerations

Platinum RTD Specifications

What is the difference between the IEC 60751 specification and the DIN EN 60751 specification?
The IEC 60751 and DIN EN 60751 specifications are identical. The DIN specification is basically the IEC spec with a cover page added.

What is the difference between the IEC 60751 specification and the ASTM E1137 specification?
Both specifications apply to the standard 3850ppm temperature coefficient platinum curve, and are based upon the ITS-90 temperature scale. One primary difference between the two specifications is the definition of tolerance classes, as follows:

 'F' indicates thin film element. If the tolerance of a wirewound element is being defined, substitute 'W'.

A customer asked for a temperature sensor assembly with a platinum RTD element that meets the requirements of DIN 43760. Is this a valid specification for a platinum RTD sensor?
No. DIN 43760 Sept 68 applied to both 100 ohm nickel and platinum RTD elements. The next version of the specification, DIN 43760 Sept 87, applied only to nickel elements, and no longer applied to platinum elements. DIN EN 60751 is the applicable DIN specification for platinum RTD elements.

A customer has asked me to build a temperature probe with a “JIS curve Pt100” RTD element. What is this curve, and can Heraeus supply an element that meets this curve?
The customer may be referring to a Pt100 element with a temperature coefficient of 3916 ppm, which was defined by the JIS C1604-1987 (and earlier) specification. Heraeus Sensor Technology supplies TC 3916ppm ceramic wirewound elements, in resistance values up to Pt500.

The more recent JIS C1604-1997 version specifies a temperature coefficient of 3850ppm, matching the DIN/IEC standard. Please confirm your customer’s temperature coefficient requirement before ordering.

I’ve seen references to a F0.3 tolerance. What does this mean?

“F0.3” tolerance is equivalent to a class B tolerance (F=thin film, W=wirewound, 0.3 indicates ±0.3 Deg C @ 0 Deg C). The tolerance nomenclature for platinum temperature sensing elements was revised in the IEC 60751 2008-07 specification (also in the DIN EN 60751 2009-05 specification). The following table relates the old tolerance designation to the new.

Thin film vs. Wirewound RTD element

Thin film or wirewound platinum RTD element—which one should I choose?
The requirement of the specific application dictates the type of element used, but typically the default choice is a thin-film element. Intrinsically vibration-resistant and lower in cost than a wirewound element, the thin film element meets the needs of most temperature sensing applications. The following table summarizes the advantages of each type:

Choosing a Thin Film Element

What are the key characteristics of each thin film type? Why would I choose one type over another?

The thin film element types supplied by Heraeus Sensor Technology differ primarily by operating temperature range. The table below summarizes the various types. The min/max operating temperature of your application must fall within the operating range specified for the part. Using a part outside the rated operating range may produce unpredictable results, and is not recommended. Please see individual data sheets for the specific properties of each type.

I notice that some of the thin film element types are available in a variety of sizes. Why would I choose one size over another?

For a new application, we typically recommend the M222 type (2.3mm L x 2.1mm W). The M222 has a relatively low unit price, and will fit in a variety of probe sizes. For existing applications, a larger size element, such as the M1020 (9.5mm L x 1.9mm W) may be required to match an existing size footprint. The following table summarizes some size-dependent properties.

Temperature Coefficient

How is the temperature coefficient of a platinum RTD element defined?
The temperature coefficient, also referred to as the “alpha value”, is the average change in resistance between 0 and 100 °C, and calculated using the formula

Where R₁₀₀ is the resistance at 100 °C and R₀ is the resistance at 0 °C

What temperature coefficients are available for platinum RTD elements?
Heraeus Sensor Technology supplies platinum RTD elements with the following temperature coefficients:

Resistance vs. Temperature Characteristic

How is the resistance vs. temperature characteristic of a platinum RTD element defined?
The Callendar–Van Dusen equation describes the relationship of resistance to temperature in platinum RTD elements.
For temperatures t equal to and above 0 Deg C, the equation is R(t) = R0*(1+A*t+B*t²)
For temperatures t below 0 Deg C, the equation is R(t) = R0*(1+A*t+B*t²+C*(t-100°C)*t³)
Where A, B, & C are constants for specific RTD curves.

The constants for the IEC 60751 TC 3850ppm curve are

A = 3.9083*10^-3 °C^-1
B = -5.775*10^-7 °C^-2
C = -4.183*10^-12 °C^-4

RTD Element Physical Properties

What is the self-heating constant?
The self-heating constant defines the temperature rise in degrees Kelvin per mW of applied power. The constant of each RTD element is measured in a standard condition of ice water at 0 Deg C. Since the constant is measured under conditions that don’t necessarily reflect a typical application environment, the self-heating constant is primarily used to compare the self-heating properties of one element to another. In addition, the actual use conditions greatly influence the self-heating constant. For example, potting the element in a thermally conductive material increases the surface area and thermal mass, effectively lowering the self-heating constant. If an element is used in a full or partial vacuum, however, the opposite can occur—the self-heating constant can increase due to the reduced thermal conductivity of the surrounding medium. In temperature-sensing applications, self-heating, if excessive, can introduce significant measurement error. The dependency of self-heating on the thermal conductivity of the surrounding medium can also be exploited to measure fluid level, flow, thermal conductivity, fluid density, etc.

Heraeus Sensor Technology platinum RTD data sheets date response time data for each part. A response time is stated for “T_0.5” and “T_0.9”. What does this mean?
“T_0.5” means the time elapsed to respond to 50% of a step change in temperature. Similarly, “T_0.9” means the time elapsed to respond to 90% of a step change in temperature. Let’s use actual response time data from the M222 thin film data sheet (below) as an example.

The data sheet states a t_0.5 response time of 0.05 seconds in water. This means if an element is exposed to a step change in temperature from 50 to 100°C, after 0.05 seconds, the element body temperature will be 75°C (50% of step change between 50 & 100°C), and after a total of 0.1 seconds, the element body temperature will be 87.5°C (50% of the step change from 75 to 100°C. The following table provides illustration of the concept.

Assembly Considerations

Are there any special precautions need be taken when handling platinum RTD elements, or incorporating them into assemblies?
Please refer to the document Handling & Installation of Platinum RTD Sensors for detailed information on handling and assembly.