Electrical Metrology (DCLF)

An electrical metrology is usually the first to be set up in a country. It is relatively simple to establish, and in general the demand from industry is relatively high. In addition, several other metrology disciplines require the backing of the electrical standards to derive their traceability, especially those organisations that will be sending their standards for calibration to the national laboratory·

This section has been limited to what is commonly known as DCLF, which stands for dc to low frequency (≤ = 1MHz) electrical metrology and excludes the more specialist and complicated sections that use higher frequencies·

Like several other disciplines in metrology, the pinnacle of the dc traceability chain is based on a physical phenomenon known as the Josephson Effect, which in turn is traceable to frequency. There are only a few Josephson junction systems operational in the world, and they are very costly to purchase and even more costly to run. The Josephson junction apparatus is typically only activated, by the few national research institutes that have them, a few times a year to transfer 10 Vdc (the characteristic voltage for most systems), to very stable, zener-based, reference sources.

In turn, the users then maintain a group of zener references and, by periodic inter-comparison of them, maintain their laboratory standard for dc voltage. Some laboratories prefer to use the absolute value of one of the zeners, while most use the mean value of the group of zeners. If a laboratory wants to improve its dc capability, it simply increases the number of zener references in the group·

To put this in perspective, the Josephson junction apparatus can deliver approximately 0.1 ppm, while a group of about 8 zener references will be able to maintain 0.3 ppm, and a group of only 4 about 0.8 ppm per year. In contrast, it is relatively easy to maintain the groups of zener references, while establishing the transfer value of the Josephson junction is very difficult and requires a great deal of equipment and experience·

In practice, very few national laboratories actually need to have a Josephson junction system, and a group of zener references is usually sufficient. From this group, one zener reference is sent once a year to a laboratory that has a Josephson system to import traceability to the group·

However the zener references only have an output voltage fixed at a nominal 10 Vdc, and they need to be treated well and not loaded with low impedance measuring devices. This means that, in most cases, the zener references are used to transfer the 10 Vdc standard into a more robust electrical multifunction calibrator which will then source any voltage (ac or dc) from zero up to approximately 1100 V. These calibrators have a one-year specification of 3.5 ppm on their basic Vdc function·

The traceability of Vac is done by comparing a Vac signal to the equivalent Vdc by using thermal transfer standards. Sometimes these are inside the multifunction calibrators and sometimes they are external devices. They are relatively simple to use but require lots of practice and patience to get consistent results.

Resistance capability, at this level, is maintained by having a series of standard resistors. In some very high-accuracy cases, special standard resistors are placed in a stirred oil bath, but the more modern "air resistors" are quite stable enough at <3 ppm per year. One or two of the set (1, 10, 100, 1k, 10k, 100k, 1M & 10MΩ ) are then sent to a high-level laboratory once a year to import traceability, and the imported value is compared against the rest of the set in the laboratory, by inter-comparison. Once again, the standard resistors are of a fixed value and their value is also transferred into a multifunction calibrator to allow general high-level calibration over a wide range·

The calibration of a multifunction calibrator is done using a procedure known as artifact calibration. Using only one calibrated 10 Vdc zener reference, a 1Ωand a 10kΩ standard resistor as it's source traceability, the calibrator is able to deliver a very wide range of Volts (ac and dc), resistance, and current (ac and dc), all of which are traceable to international standards. Multifunction calibrators, together with a null detector or a good long scale digital multimeter (DMM), can then be used to calibrate most 8½ digit multimeters and other industrial level calibrators·

Following down the calibration chain, there is another step before the general purpose test equipment level is reached. For example, a simple 3½ digit multimeter used by the thousands in industry is difficult (almost impossible) to calibrate using only a multifunction calibrator because the multifunction calibrator is not designed to do this level of work. Even though the multifunction calibrator is very accurate, it lacks the range and additional functions required by the typical multimeter, such as 20 amps, thermocouple simulation, capacitance, frequency etc. To calibrate all of these functions on a DMM, a multi-product calibrator is used·

The multi-product calibrator has a Vdc specification of about 12 ppm per year and is thus suitable for calibrating most DMMs up to 6½ digits, as well as many of the other lower level calibrators, DMMs, panel meters etc. typically seen in industry·

There are many countries that simply use a multi-product calibrator with a few accessories as their national standard and, invariably, even the highest-level laboratory requires the lower level equipment to be able to perform their tasks adequately. So the decision about what to install is usually based from the bottom up, starting with a multi-product calibrator and then, if the market requires the laboratory to calibrate other laboratory's multi-product calibrators, installing a multifunction calibrator with a few accessories. In limited cases, the national laboratory will need to go to fixed values such as zener references and standard resistors and, very rarely, a Josephson junction system· The tables below show how traceability is derived and the typical equipment and workload associated with each level of laboratory. Do not be misled by the terminology used that a national laboratory has to be a primary level laboratory. The most important issue is that the national laboratory is capable of producing traceable calibration at the level required by its market, with competence and efficiency·

The equipment that would normally be in a typical high-level laboratory (mixture of laboratory levels) which could cater for most industrial and laboratory work loads would include the following:-

Lab standards

2 x Reference standards (8 zener reference cells)

1 x External battery and charger (required for inter-comparison journeys)

1 x Transit case (to hold one zener and one ext battery for inter-comparison)

1 x Set standard resistors (1, 10, 100, 1k, 10k, 100k, 1M & 10MΩ )

1 x Transit case (to hold two/three standard resistors for inter-comparison)

High-level station

1 x Reference DMM (8½) with Ratio function

1 x Multifunction calibrator

General work station

1 x Multi-product calibrator with scope cal opt. (600MHz)

1 x 6½ Digit DMM

1 x 50 Turn coil

2 x Set low thermal leads

1 x Set thermocouple test leads

1 x Hand held DMM

Other useful items

1 x Dual bench power supply (0-30V >2 amps)

1 x Decade resistance box (1Ω to 10MΩ )

1 x Decade inductance box (100µH to 11H)

1 x High voltage probe (40kV)

1 x LCR Meter

1 x 25Ω Resistance standard (for temperature calibration work)

1 x Hand held thermometer with selection of probes

1 x PC, Printer & met/cal software etc.

1 x Cal Book (Philosophy of Calibration in Practice)

If required to calibrate insulation and installation testers

1 x Multifunction electrical tester calibrator or 1 x Set high voltage resistors

1 x Set low Ω , high current resistors