Project Background

A provincial institute of metrology and technology is responsible for the verification and calibration of current measuring instruments in the province's electric power, transportation, communications and other industries. In order to improve the calibration capacity and efficiency, the institute decided to develop a new generation of portable standard current source for on-site calibration of various current measuring instruments.

The core of the standard current source is the high-precision shunt, whose accuracy directly determines the measurement uncertainty of the entire device. This is one of the application scenarios that require the highest accuracy of the shunt.

Technical specification requirements:

• Current output range: 0.1A-100A
• Current output accuracy: ±0.02%
• Shunt measurement uncertainty: ≤0.01% (k=2)
• Long-term stability:<20ppm>
• Temperature coefficient:<5ppm>

Technical challenges faced

1. Extremely high precision requirements

The measurement uncertainty of 0.01% places extreme demands on all performance indicators of the shunt, including resistance value accuracy, temperature coefficient, long-term stability, and thermal potential.

2. Very low temperature coefficient

Conventional shunts have a temperature coefficient of 20-50 ppm/°C, which is not sufficient for the<5ppm>

3. Long-term stability

As a measurement standard, the shunt resistance value must not drift significantly during its service life, otherwise the reliability of the traceability chain will be affected.

4. Thermoelectric potential effects

In µV-level signal measurements, the thermal potential of the shunt can be a major source of error. The selection of low thermopotential materials and the optimization of the structural design are required.

Technical Solutions

1. Selective grade shunts

Selection of a specially selected version of the metering stage of the FL-2 shunt with key parameters:

• Resistance value: 1mΩ ±0.01% (factory calibrated)
• Rated current: 100A
• Temperature coefficient:<5ppm>
• Long-term stability:<20ppm>
• Thermopotential:<0.5µV/°C

2. Specialty alloy materials

The resistor body is made of Zeranin, a copper-manganese-nickel alloy with a very low temperature coefficient (typical ±5 ppm/°C) and thermal potential ((<1µV/°C), ideal for precision resistors.

3. Low thermopotential structural design

• Fully symmetrical construction of both ends of the resistor body
• The voltage sampling point is located symmetrically in the temperature field of the resistor body
• Specialized low thermo-electric potential welding process
• The system is equipped with a forward and reverse measurement function to eliminate the effect of residual thermoelectric potentials

4. Temperature compensation

The system is equipped with a high-precision platinum resistance temperature sensor (Pt100) to measure the shunt temperature in real time. The software carries out online compensation according to the temperature characteristic curve of the shunt (factory calibration) to further reduce the temperature influence.

Project results

• Measurement uncertainty:The measurement uncertainty of the shunt reaches 0.01% (k=2), which meets the design requirement
• Temperature Characterization:TCR in the range of 20-30°C<5ppm>
• Stability validation:Continuous follow-up test for 2 years, annual drift<15ppm
• Metrological certification:Traceability through the China Academy of Measurement Sciences, obtaining calibration certificates
• Application effects:After the equipment was put into use, the calibration efficiency was increased by 3 times, which was well received by the users.

Summary of technical parameters