The AC-DC transfer differences of thermal converters due to thermoelectric effects in the heater circuit are directly measured with a fast reversed DC to an uncertainty of a few parts in 10/sup 7/. This new and independent method allows former theoretical methods for evaluating thermoelectric effects in thermal converters to be checked. >

Measuring thermoelectric effects in thermal converters with a fast reversed DC

The thin-film multijunction thermal converter (PMJTC) developed in cooperation between Physikalisch-Technische Bundesanstalt (PTB) and Institut fur Physikalische Hochtechnologie e.V. (IPHT) is today's most sensitive and accurate standard for the precise measurement of electrical AC quantities in the frequency range of 10 Hz-1 MHz. Thin-film technology and micromechanics in silicon were essential for this success. The thin-film heater and bismuth/antimony thermocouples with high Seebeck effect deposited on a thin membrane of low heat conductance result in the attractively high sensitivity of the PMJTC which allows voltage measurements down to 100 mV to be performed. The statistics of the mass production of the PMJTCs show that PMJTCs built into a housing with an N-connector at the input can be reproducebly manufactured with an AC-DC voltage transfer difference smaller than 0.1 /spl mu/V/V at 1 kHz, 8 /spl mu/V/V up to 100 kHz, and below 40 /spl mu/V/V up to 1 MHz for a heater resistance of 90 /spl Omega/. A compensation circuit has been added on the chip which results in low-frequency PMJTCs (LF-PMJTCs) with AC-DC transfer differences below 0.3 /spl mu/V/V at 10 Hz.

Development of thin-film multijunction thermal converters at PTB/IPHT

This paper describes a new ac-dc current-shunt calibration system with a current range of 1 mA-100 A and a frequency range from 10 Hz to 100 kHz. The new system consists of an ac-dc comparator controlled via a universal serial bus and a set of reference coaxial shunts with dedicated thermal voltage converters. The design of the custom-built shunts has been optimized through a theoretical modeling. The results of the calibrations of the reference coaxial shunts are presented.

AC-DC current shunts and system for extended current and frequency ranges

A micro-machined thermal flow sensor to measure a minute amount of gas flow rate is developed, and tested to quantify the flow measuring characteristics over the variation of flow rate. A bifilar-Evanohm-R alloy heater and chromel-constantan thermopiles were formed on a sandwich membrane, Si3N4/SiO2/Si3N4, which consist a core part of the sensor. In performance test of the sensor, range of gas flow rate was regulated from 0.2 to 1.0 slm and the heater power was variated from 0.72 to 5.63 mW. In the present experiment, the Reynolds number based on the diameter of a flow passage was ranged from 81 to 408. It is confirmed from the experiment that the sensitivity (S = ∂ΔV/∂Q; ΔV, voltage difference between thermopiles; Q, flow rate) of the sensor is enhanced with increasing the heater power and decreasing the measuring flow rate range. Also, the sensor response time required to a stable state is reduced with increasing the flow rate range.

Sensitivity enhancement of a silicon micro-machined thermal flow sensor

We have measured the thermoelectric transfer difference of two thermal voltage converters using a Josephson source and compared the results to similar measurements made with a conventional semiconductor source. Both sources use the fast reversed DC method. The Josephson source is an array of 16 384 superconductor-normal-superconductor Josephson junctions that is rapidly switched between voltage states of +0.5 V, 0 V, and -0.5 V. A marginally significant difference is detected between measurements with the two different sources.

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Thermoelectric transfer difference of thermal converters measured with a Josephson source

It is shown that by using a fast-reversed DC instead of a steady-state DC of both polarities for AC-DC transfer, the speed and the accuracy of the transfer are improved, because the second-order Thomson and Peltier effects are eliminated. An accurate high-speed buffer has been developed for fast reversing with analog switches. The buffer has high input impedance, high-speed switching capability from plus to minus DC. 10-V and 10-mA output capability, and high amplitude accuracy. Measurement results are reported. >

Improving speed and accuracy of AC-DC transfer using a fast reversed DC

An AC-DC voltage transfer module has been designed around the planar multijunction thermal converter (PMJTC) to measure AC voltage by an automatic AC-DC transfer It operates in the frequency range from 10 Hz up to 1 MHz with an uncertainty below 20 V/V for frequencies up to 100 kHz and below 50 V/V up to 1 MHz The module uses the standard PTB-PMJTC with one heater in an isothermal way In the first step of the AC-DC transfer procedure, it applies the internal DC reference voltage to the heater to calibrate the system, and in the second step, it applies the unknown AC voltage from the input side to the same terminal of the heater and a DC feedback voltage from the other terminal The feedback loop adjusts the output voltage of the PMJTC to the same value as with the reference DC From the feedback DC value and the reference DC value, an accurate value for the AC voltage is calculated With this isothermal design, the time for an accurate AC-DC transfer is reduced considerably to several seconds

AC-DC voltage transfer module with thin-film multijunction thermal converter

An ac module for a dc multimeter has been designed around the Planar Multijunction Thermal Converter (PMJTC) to measure ac voltage by automatic ac-dc transfer for frequencies from 10 Hz up to 1 MHz. The electronic uses the standard PMJTC in an isothermal way. It applies the sum of the ac signal and the dc feedback at the same time directly to one heater. The dc therefore is an accurate measure for the ac quantity. With this design the time for an accurate ac-dc transfer is reduced considerably.

http://ieeexplore.ieee.org/iel5/8039/22207/01034747.pdf

B. Stojanovic

Papers

Measuring thermoelectric effects in thermal converters with a fast reversed DC

Improving speed and accuracy of AC-DC transfer using a fast reversed DC

AC-DC voltage transfer module with thin-film multijunction thermal converter

AC voltage converter module with thin-film Multijunction Thermal Converter