Improving Long-Term Stability of Batemika UT-ONE Thermometers with Sealed High Precision Resistors

Abstract: Batemika Measurement Company improved the accuracy of UT-ONE series thermometers by using VHP101T sealed foil resistance as an internal reference resistance. The annual resistance drift has dropped from more than 20ppm to less than 5ppm.

Background: Valentin Batagelj is the manager and chief engineer of Batemika. Batemika specializes in thermometry and thermometry. Production of high-precision equipment and measurement software primarily for calibration and R&D laboratories. Its main product line is UT-ONE’s series of thermometers with an accuracy as high as a few thousandths of a degree Celsius.

Product Keywords:

VHP101T Ultra High Precision Sealed Foil Resistor

SMR3DZ Ultra High Precision Z-Foil Molded Chip Resistors

challenges

The precise temperature measurement of platinum resistance thermometers requires extremely accurate resistance measurement equipment. 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 4ppm in resistance. With the advent of modern 24-bit sigma-delta analog-to-digital converters, this accuracy has become simple to achieve, and today the main challenge is to guarantee short- and long-term drift.

solution

Batemika has earlier started the development of the UT-ONE series thermometers with an overall reading accuracy target below 0.01°C. The first generation of UT-ONE devices can achieve the expected effective resolution and short-term drift, but are trapped in the inability to achieve the expected long-term drift parameters.

Improving Long-Term Stability of Batemika UT-ONE Thermometers with Sealed High Precision Resistors

Figure 1: Batemika UT-ONE B03A 3-channel thermometer

The measurement circuit for the UT-ONE thermometer readings is based on a 24-bit sigma-delta analog-to-digital converter in a 4-wire ratiometric configuration.

Figure 2: Simplified measurement circuit of the thermometer

The advantage of this configuration is that the accuracy of the current source and 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 the two is indistinguishable. In earlier designs, the reference resistors were SMR3DZ foil resistors with a load life of 50ppm (70°C at 2000 hours rated power).

Customers expect resistance stability to be around 10 to 20ppm per year. The long-term drift results shown in Figure 3 confirm previous predictions. Long-term drift has a one-year cyclical variation that may be related to seasonal changes in humidity. Drift does not have a specific trend and does not accumulate over the years.

Figure 3: Long-term resistance drift of UT-ONE thermometer with 100Ω SMR3DZ resistance

The VPG foil resistor field design engineer received the above long-term drift results, and suggested that the customer replace the SMR3DZ molded chip foil resistor with the VHP101T sealed foil resistor to improve long-term stability. Finally, the customer took a VHP101T 100Ω resistor sample for evaluation.

The resistor is housed in a small metal box, which provides good mechanical and electrical protection for the resistor, and is combined with a thermometer as a test standard.

The results shown in Figure 4 show that the long-term temperature drift for this sample is less than 1 ppm per year.

Figure 4: Long-Term Resistance Drift at 100Ω VHP101T

Amazed by the performance of the VHP101T, Batemika intends to replace the reference resistor in a component in the UT-ONE with the VHP101T. Exceeding customer expectations was a dramatic improvement in the long-term drift results shown in Figure 5. Coordinates at the same scale in Figures 3 and 5 allow us to clearly see the contrast. Long-term drift has been reduced from greater than 20ppm per year to less than 5ppm per year. The VHP101T resistor also brings better short-term stability and lower initial drift, which brings more confidence to the customer’s quality control process. Batemika now only uses VHP101T resistors as reference resistors in the thermometer, and even refurbished a bunch of existing parts.

Figure 5: Long-term resistance drift of UT-ONE thermometer under 100Ω VHP101T

This is based on a VPG foil resistor field design engineer’s suggestion? ——The internal reference resistor of the product is replaced by Vishay foil resistor VHP101T sealed metal foil resistor, which greatly improves the long-term stability of the entire device and greatly improves the measurement results, once again proving to customers the excellent foil resistance long-term stability.

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