Endress+Hauser iTHERM TM401, Technical Information

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iTHERM TM412
Hauser 13
Calibration Calibration of thermometers
Calibration involves comparing the measured values of a device under test (DUT) with those of a
more precise calibration standard using a defined and reproducible measurement method. The aim is
to determine the deviation of the DUT's measured values from the true value of the measured
variable. Two different methods are used for thermometers:
• Calibration at fixed-point temperatures, e.g. at the freezing point of water at 0 °C,
• Calibration compared against a precise reference thermometer.
The thermometer to be calibrated must display the fixed point temperature or the temperature of
the reference thermometer as accurately as possible. Temperature-controlled calibration baths with
very homogeneous thermal values, or special calibration furnaces into which the DUT and the
reference thermometer, where necessary, can project to a sufficient degree, are typically used for
thermometer calibrations.
The measurement uncertainty can increase due to heat dissipation errors and short immersion
lengths. The existing measurement uncertainty is listed on the individual calibration certificate.
For accredited calibrations according to ISO17025, the measurement uncertainty shouldn’t be twice
as high as the accredited measurement uncertainty. If this is exceeded, only a factory calibration can
be performed.
Evaluation of thermometers
If a calibration with an acceptable uncertainty of measurement and transferable measurement
results is not possible, Hauser offers customers a thermometer evaluation measurement
service, if technically feasible. This is the case when:
• The process connections/flanges are too big or the immersion length (IL) is too short to allow the
DUT to be immersed sufficiently in the calibration bath or furnace (see the following table), or
• Due to heat conduction along the thermometer tube, the resulting sensor temperature generally
deviates significantly from the actual bath/furnace temperature.
The measured value of the DUT is determined using the maximum possible immersion depth and the
specific measuring conditions and measurement results are documented on an evaluation certificate.
Sensor transmitter matching
The resistance/temperature curve of platinum resistance thermometers is standardized but in
practice it is rarely possible to keep to the values precisely over the entire operating temperature
range. For this reason, platinum resistance sensors are divided into tolerance classes, such as Class
A, AA or B as per IEC 60751. These tolerance classes describe the maximum permissible deviation of
the specific sensor characteristic curve from the standard curve, i.e. the maximum temperature-
dependent characteristic error that is permitted. The conversion of measured sensor resistance
values to temperatures in temperature transmitters or other meter electronics is often susceptible to
considerable errors as the conversion is generally based on the standard characteristic curve.
When using temperature transmitters from Hauser, this conversion error can be reduced
significantly by sensor-transmitter matching:
• Calibration at three temperatures at least and determination of the actual temperature sensor
characteristic curve,
• Adjustment of the sensor-specific polynomial function using Calendar-van Dusen (CvD)
coefficients,
• Configuration of the temperature transmitter with the sensor-specific CvD coefficients for
resistance/temperature conversion, and
• another calibration of the reconfigured temperature transmitter with connected resistance
thermometer.
Hauser offers this kind of sensor-transmitter matching as a separate service. Furthermore,
the sensor-specific polynomial coefficients of platinum resistance thermometers are always provided
on every Hauser calibration certificate where possible, e.g. at least three calibration points,
so that users themselves can also appropriately configure suitable temperature transmitters.
For the device, Hauser offers standard calibrations at a reference temperature of
–20 to +500 °C (–4 to +932 °F) based on the ITS90 (International Temperature Scale). Calibrations in
other temperature ranges are available from your Hauser sales center on request.
Calibrations are traceable to national and international standards. The calibration certificate is
referenced to the serial number of the device. Only the insert is calibrated.
Minimum insertion length (IL) for inserts required to perform a correct calibration
The insert length IL is automatically calculated for every thermometer configuration in the
Hauser Configurator +Temperature software application. The system also automatically
checks whether the selected insert length suffices to perform a factory calibration. For further
information, see the 'Accessories' section →  38