Improvements in High Resistance Measurements at CENAM Benjamin Rodriguez Medina, Alepth Pacheco Estrada, and Felipe Hernandez Marquez

Centro Nacional de Metrología (CENAM) brodrigu@cenam.mx; apacheco@cenam.mx; fhernand@cenam.mx

Abstract — This paper describes 3 important improvements

in high resistance measurements: the fabrication of a container to provide a moisture free environment to the resistive element; the development of high value standard resistors that can be connected to a split guard potential to reduce leakage currents, and the automation of an Active-Arm Bridge which improved the measurement results and reduced the calibration time.

Index Terms — High Resistance, Active-Arm Bridge, resistor container, leakage resistance.

I. INTRODUCTION

The measurement of resistance values above 10 MΩ requires dealing with physical effects that can influence the resistance value in a significant way. These effects can be: leakage resistance, thermal EMFs, moisture, mechanical shock, vibrations on the resistive element, electrostatic noise, temperature coefficients, and capacitive effects in the measurement system. These effects occur regardless of the measurement system being used. In order to minimize the undesirable effects already mentioned, it is necessary an adequate use of a “guard” connection and the construction of an appropriate container for the resistive element to be measured to protect it, mainly, from moisture [1]. At CENAM it was developed an Active-Arm Bridge for the measurement of high resistance values [2], it was implemented by substituting two of the resistive arms of a conventional Wheatstone bridge circuit with a low output impedance and high stability voltage sources [3, 4]. Recently the automation of the system was completed in order to reduce uncertainties and time in the measurement process. II. STANDARD RESISTORS A cylindrical container was made using a drilled stainless steel rod of 15 cm long and 5 cm OD (see figure 1) in which a high-value resistor can be installed (it can be a resistor from 10 MΩ up to 10 TΩ). At both sides of the cylindrical container an insulator-metal interface was used which consists of three basic parts: a high-resistance electrical insulator made of a machined polytetrafluoroethylene (PTFE) washer, a central machined brass hub which reaches one of the terminals of the resistor, and a circular stainless steel fastener, which in turn also allows the hermetic sealing of the container in combination with a 1/32” o’ring. The container has a pair of thin copper tubes for filling it with dry nitrogen to provide a moisture free environment to the resistive element. To ensure the seal of the container, we carried out a moisture test that consisted, firstly, in filling the container with dry white paper and then a total immersion of the sealed container in water

dyed black for at least 24 hr, no evidence of humidity was found in the container after this test. The fully assembled and tested container was finally placed inside an aluminum case using a couple of supports made of PTFE; these supports were fitted to the container to electrically isolate the container from the case. The supports and the aluminum case protect the container against mechanical shocks and vibration.

Fig. 1. Cylindrical container parts

III. MEASUREMENT SYSTEM DESCRIPTION AND AUTOMATION

ALGORITHM

At CENAM we developed an Active-Arm Bridge for the measurement of high resistance values [2]. This measurement system consists of two programmable multifunction calibrators as the voltage sources, an electrometer as detector and a conventional PC with an Intel Core I5 and 3 Gb of RAM. Fig. 2 shows the Active-arm Bridge circuit, with the instruments already mentioned, a standard resistor RS and the unknown resistor RX. To control the instrument inputs and outputs and to get a system capable of performing multiple measurements automatically an algorithm was developed with an appropriate graphical user interface. The program starts asking the operator to enter the parameters related to the measurement and begins the measurement sequence which aim is to find the VX voltage value which reaches the balance ratio in the Active-arm Bridge: . (1)

A brief description of the measurement sequence consists of: a) the voltage sources are set to have the same nominal ratio of Rx and Rs and the current (IA) is measured; b) a variation in VX is calculated from IA to get zero current in the detector (VX0); c) a voltage proportional to a difference of 100 μΩ/Ω of the relation between Rx and Rs is set and the current (I100) is recorded; d) step “c” is repeated to obtain differences of (80,

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60, 40, 20, 0, -20, -40, -60, -80 and -100) μΩ/Ω and the corresponding currents are measured; e) with a least-squares interpolation of the I100 to I -100 currents points the voltage VX+ is calculated; f) to reduce thermal EMFs, the sequence is repeated from “a” to “e” in reverse polarity of the voltage sources and a VX- is obtained; g) finally with the average of the two voltages (VX+ and VX- ) the balance voltage VX is estimated. Each of the currents measurements are the average of 100 readings of the electrometer and between all the changes in the voltage sources is placed an appropriate delay to avoid the capacitive effects in the system.

Fig. 2. Active Arm Bridge circuit.

IV. HIGH RESISTANCE MEASUREMENT SYSTEM GUARDING

CIRCUIT

Fig. 2 shows the shielding of the Active-Arm Bridge, using shielded single-wire cables with PTFE-insulation and the aluminum case that allows the resistor to be operated in 4 modes: floating mode, grounded mode, driven to a split guard potential and driven to a complete guard potential. This is possible because the shielding of the BPO connectors is electrically isolated from the case with PTFE circular plates and at the same time the resistor container is fitted to the aluminum case with two PTFE supports [1]. In order to operate the resistor at a split guard potential, it is necessary to connect the guard to the positive voltage source output. For the complete guard potential mode is necessary to do the same as the split guard potential mode and connect to the A and B guard points an auxiliary resistor ( and ) with the same nominal value as the standard resistor, this ensures that the entire guard shield will be at the same potential reducing the leakage currents in the system [4]. By performing a theoretical calculation of the leakage currents, considering that the PTFE insulation resistance of the cable is 5x1014 Ω, a value of 20 µΩ/Ω of the relation between Rx and Rs using the guard at grounded mode and resistors above 10 GΩ, is obtained. Therefore, it is important to have a system that can be connected in a guard potential mode to reduce the leakage currents.

V. RESULTS

The container sealing test showed that no humidity entered in it. This test exceeds the condition of the actual amount of

moisture that can enter the container in real working conditions, in this way; the tightness level achieved is considered enough. A 100 MΩ resistor was measured using 3 measurement systems to determine its value: An automatic Active-Arm Bridge as the described above, with a test current of 1µA and a 10:1 ratio; a High Resistance Cryogenic Current Comparator (HRCCC) [5], with a test current of 0.1 µA and ratios of 10:1 and 100:1; and an Automatic Ratio Bridge based on a Binary Voltage Divider [6], with a test current of 1µA and a 10:1 ratio. The maximum difference between measurements with these three systems was 0.7 µΩ⁄Ω. Another comparison test was conducted between the Active-Arm Bridge with a test current of 100 nA and a 10:1 ratio and the HRCCC system with a test current of 10 nA and a 100:1 ratio, using a 1 GΩ resistor obtaining a 2 µΩ⁄Ω difference.

VI. FUTURE PLANS

To improve the high resistance measurement system, we are testing a small Faraday shielded chamber with temperature control of (23 ± 0.1)°C, where the resistors of the Active-Arm Bridge will be placed. Also the actual electrometer will be replaced by a new one with better capabilities.

VII. CONCLUSION

With the improvements described in this paper we are able to provide better measurement capabilities for high resistance values. The agreement between the measurement results obtained by the HRCCC, the Automatic Ratio Bridge, and the Active-Arm Bridge confirm that the latter is suitable for use in high precision metrology of high value resistors.

REFERENCES

[1] Ronald F. Dziuba et al., “Fabrication of High-Value Standard Resistors”, IEEE Transactions on Instrumentation and Measurements, Vol. 48 No. 2, 1999

[2] Felipe Hernández Marquez et al. ”Puente de Wheatstone Modificado Utilizado en una Comparación Internacional en el valor de 1 GΩ”. Simposio de Metrología 2008–M2333-1178

[3] Lesley C A Henderson, “A new technique for the automatic measurement of high value resistors”, J. Phys. E: Sci. Instrum. 20, 1996

[4] Dean G. Jarrett, “Automated Guard Bridge for Calibration of Multimegaohm Standard Resistors from 10 MΩ to 1 TΩ”, IEEE Transactions on Instrumentation and Measurements, Vol. 46 No. 2, 1997

[5] R. E. Elmquist et al. “High resistance scaling from 10 kΩ and QHR standards using a cryogenic current comparator” in CPEM Dig., Broomfield, CO, Jun. 2008, pp. 268-269.

[6] A. F. Dunn, “Measurement of Resistance Ratios in the range to 100 Megohms”, IEEE Transactions on Instrumentation and Measurements, Vol. 40, No. 2, 1991

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