Temperature sensors and calibration

Introduction

The temperature sensor from Disruptive Technologies offers excellent accuracy, very good long-term stability, and continuous liveliness checks without any need for re-calibration in most use-cases.

Key takeaways

  • Excellent accuracy of +/- 0.5 °C measuring in the range from 0 °C to +65 °C based on factory calibration on each individual sensor.
  • Long-Term stability of better than 0.04 °C per year results in an accuracy better than +/-1 °C, in the range 0 °C to +65 °C, after 15 years
  • The sensor reports to the cloud several times per hour which makes it easy to spot and replace broken or misplaced sensors

Accuracy over time

The temperature sensor from Disruptive Technologies is calibrated at manufacturing. Re-calibration is in many cases not required due to the excellent long-term stability of the Disruptive Technologies sensor as shown in the graph below.

 

Absolute temperature accuracy versus sensor age

The temperature measurement unit in the sensor is based on CMOS technology. It is an industry-leading, fully integrated, solution with excellent long-term stability. This is achieved by integrating the sensing element, readout electronics, and calibration on a single integrated circuit with a digital interface.

Calibration

Transfer standards (for further detail see here) are used in the calibration of individual sensors during manufacturing. The transfer standards are subject to scheduled calibration procedures. The calibration of the reference itself used for the calibration of the transfer standards is performed by an ISO/IEC 17025 accredited laboratory which is a full member of the International Laboratory Accreditation Cooperation (www.ilac.org). The qualification procedures used to establish the accuracy and long-term stability is performed based on the JEDEC JESD47 qualification test method.

The requirement for re-calibration does, however, not only stem from the long-term stability of the sensing solution itself. In some industries, legal requirements demand checking or calibration at regular intervals. Such checks are then part of the total quality assurance system. A regular check would identify far more errors than drift in the sensor element itself. It could discover damage to the complete sensing setup and equipment. Broken mechanical assembles or measurement probes could be found and one could even verify that the temperature sensor has not been removed or relocated.

The cloud-connected regular measurements from the Disruptive Technologies’ sensors make it easy to automatically detect if a sensor stops reporting. This will, together with the excellent long-term stability of the sensors, render most regular calibration unnecessary.

It is possible to perform regular inspections in cases where even better reliability is required to verify the accuracy of the sensors. This could be done by comparing measurements over a period with a 3rd party temperature sensor. It could also be done by temporarily placing a secondary, newly factory calibrated, Disruptive Technologies temperature sensor next to the first one for a period of time and inspect the difference in temperature. If this difference is outside the acceptable range, then the first sensor should be replaced.

A regular re-calibration or inspection could by the means described above be run as follows:

  • Check the setup visually and ensure correct sensor placement
  • Check the difference between redundant sensors and verify that it still is within limits (if no automatic monitoring is implemented)
  • Check that all sensors are reporting as expected

In the cases where legal requirements demand strict regular calibrations at a certain interval, an alternative approach would be to regularly replace the sensors with new sensors. New sensors are already factory calibrated, and will, therefore, be calibrated at the time of installation. Sensors should not be attached with tape in these cases, but with brackets that allow the sensor to be removed without being damaged.