Batemika Measurement Solutions was able to improve the accuracy of its UTONE family of thermometer readouts by implementing a VHP101T HermeticallySealed Bulk Metal^{®} Foil resistor as the internal reference resistor. Longterm drifts were improved from over 20 ppm per year to less than 5 ppm per year.
Industry/Application Area: Precision Instruments
Products used:
The Challenge
Precise measurement of temperature with platinum resistance thermometers requires resistancemeasuring instrument with extremely tight accuracy requirements. To achieve 0.001°C accuracy in temperature requires 4 ppm accuracy in resistance for the industrystandard Pt100 probe at 0°C. With the advent of modern 24bit sigmadelta analogtodigital converters, achieving linearity and effective resolution on the level of a few ppm has become relatively straightforward, so the main challenge now remains assuring low shortterm and longterm drifts.
The Solution
At Batemika we started the development of UTONE family of thermometer readouts with the total accuracy target under 0.01°C. Our first generation of UTONE devices was able to achieve all our targets in effective resolution, linearity, temperature coefficients and shortterm drift, but was struggling to achieve the expected longterm drift specifications. Based on the suggestion of the VPG Foil Resistors field design engineer, we replaced our internal reference resistor with the Vishay Foil Resistors VHP101T hermeticallysealed Bulk Metal^{®} Foil resistor. This single step has dramatically improved the longterm stability of the entire instrument and gives a new level of confidence in its measurement results. The typical accuracy specification that we were able to achieve is 0.006°C for Pt100 probe at 0°C, and as the longterm drift data is slowly accumulating, we are now considering further improving the accuracy specification.
Figure1: Batemika UTONE B03A 3channel thermometer readout

The User Explains
The measurement circuit of the UTONE thermometer readouts is based on the 24bit sigmadelta analogtodigital converter in 4wire ratiometric configuration. The simplified measurement circuit consists of the current source with 1 mA measurement current, which creates a voltage drop on the unknown resistance RRTD and reference resistor RREF. The voltage drop over unknown resistance is amplified with the programmablegain amplifier and fed into the ADC input. The voltage drop over the reference resistor is used as the reference voltage of the ADC.
Figure2: Simplified measurement circuit of UTONE thermometer readouts 
The advantage of this configuration is that the accuracy and lowfrequency noise of the current source has no effect on measurement results. The longterm drift is determined only by the reference resistor and the programmablegain amplifier, and the contributions of these two sources can’t be distinguished from each other. In our initial design, the reference resistor was a Bulk Metal^{®} SMR3DZ molded Vishay Foil Resistor, which has a load life stability of 50 ppm at 70°C and rated power. As our instrument is specified only up to 36°C with almost negligible power, we assumed the stability of the resistor would be between 10 and 20 ppm per year. We also assumed that the stability of the programmablegain amplifier would be in the same range, so further improvement of the reference resistor would be pointless. The longterm drift results presented in figure 3 appeared to confirm our assumptions. The longterm drift seems to have a yearly period and might be related to seasonal variation in relative humidity. The drift has no particular trend and does not accumulate over several years.
Figure3: Longterm drift of the UTONE thermometer readout at 100 Ω with SMR3DZ resistors

The longterm drift results were shown to the VPG Foil Resistors field design engineer, who suggested the replacement of the Vishay Foil Resistors SMR3DZ molded resistor with the VHP101T hermeticallysealed resistor in order to improve further the longterm stability. We were initially very skeptical about the change, as the assumption was that the improvement in the reference resistor would be overshadowed by the drift of the programmablegain amplifier. Nevertheless, we acquired one sample of the VHP101T 100 Ω unit for evaluation purposes. This resistor had been installed in a small metallic case, which provides good mechanical and electrical protection. We now produce it as a check standard in combination with our thermometer readouts.
This resistor was regularly recalibrated over a period of more than two years with the resistance bridge with uncertainty better than 1 ppm. The results in figure 4 show that the longterm drift for this particular unit is less than 1 ppm per year.
Figure4: Longterm drift of the Vishay Foil Resistors VHP101T 100 Ω hermetically sealed resistor 
Impressed by the longterm performance of the VHP101T resistor, we decided to replace the reference resistor in one of our UTONE units with the VHP101T. This would at least allow us to examine the drift of the programmablegain amplifier, as in this case the influence of the reference resistor would be negligible. Contrary to our assumptions and expectations, the results for the longterm drift, as presented in figure 5, show a huge improvement. The scale in figures 3 and 5 is the same to simplify the comparison. The longterm drift was reduced from over 20 ppm per year to less than 5 ppm per year. This improvement is consistent with all units tested. The VHP101T resistor resulted also in a better shortterm stability and lower initial drift after assembly, which simplifies our qualitycontrol procedures and gives a new level of confidence in our products. We now use only VHP101T resistors as the reference in our thermometer readouts and we even retrofitted most of our existing units.
Figure5: Longterm drift of the UTONE thermometer readout at 100 Ω with VHP101T resistors 
Acknowledgement:
Batemika specializes in measurement solutions in thermometry and temperature metrology. We produce highprecision instrumentation and measurement software solutions for calibration and R&D laboratories. Our lead product is the UTONE family of thermometer readouts, which is capable of measuring temperature with all commonlyused temperature probes with accuracy down to a few thousandths of a degree Celsius. Our background in both electrical engineering and temperature metrology allows us to provide solutions that are specifically tailored to your everyday measurement needs. www.batemika.com