Sealed high-precision resistors can be used to improve the long-term stability of the pyrometer.
Accurate temperature measurement with platinum resistance thermometers requires extremely precise resistance measuring devices. In order to achieve an accuracy of 0.001°C, an industry standard Pt-100 probe at 0°C needs to have an accuracy of 4 ppm on the resistance value. With the advent of modern 24-bit sigma-delta analog-to-digital converters, this accuracy has become easier to achieve, and today the main challenge is to ensure short- and long-term drift.
Batemika has earlier started the development of the UT-ONE series of thermometers with a total reading accuracy target of less than 0.01°C. The first generation of UT-ONE devices is now available. The first generation of UT-ONE devices were able to achieve the expected effective resolution and short-term drift, but were stuck with the inability to achieve the expected long-term drift parameters.The measurement circuitry for the UT-ONE thermometer readings is based on a 24-bit sigma-delta analog-to-digital converter in a 4-wire ratiometric configuration.
The advantage of this configuration is that the accuracy of the current source and the low-frequency noise do not affect the measurement results. The long-term drift is determined only by the reference resistor and the programmable amplifier, but at the same time the drift caused by these two cannot be distinguished. In an early design, the reference resistor was an SMR3DZ foil resistor with a load life of 50 ppm (70°C, 2000 hours at rated power).
The customer expected the resistor to be stable at roughly 10 to 20 ppm per year. The long-term drift results confirmed the previous prediction. The long term drift has a yearly cyclic variation, probably related to humidity changes due to the seasons. The drift does not have a specific trend and does not accumulate over the years.
The VPG foil resistor field design engineer received the above long term drift results and suggested the customer to replace the SMR3DZ molded chip foil resistor with a VHP101T sealed foil resistor to improve the long term stability. Finally, the customer took a sample of the VHP101T 100Ω resistor for evaluation. The resistor was mounted in a small metal box that provided good mechanical and electrical protection for the resistor, combining it with a pyrometer as a test standard. The long-term temperature drift of this sample was less than 1 ppm per year.
Amazed by the performance of the VHP101T, Batemika intends to replace the reference resistor in one of the components within the UT-ONE with the VHP101T. Exceeding the customer's expectations, the long term drift results have improved dramatically. The coordinates at the same scale give us a clear picture of the comparison. The long term drift was reduced from >20ppm per year to less than 5ppm per year. the VHP101T resistor also brought better short term stability and lower initial drift, which gave the customer more confidence in their quality control process. batemika now only uses the VHP101T resistor as the reference resistor in the pyrometer and even refurbished a bunch of existing parts.
This based on the advice of the VPG foil resistor field design engineer? --The internal reference resistor of the product was replaced with a Vishay Foil Resistor VHP101T hermetically sealed metal foil resistor, which greatly improved the long-term stability of the whole device and made a big difference to the measurement results, proving to the customer once again the excellent long-term stability of the foil resistor.
