NIST STANDARD

The NIST (National Institute of Standards and Technology) Standard can be as accurate as .001% with roughly the same TCR as Precision Wirewound Resistors and are very stable. These are used as a standard in verifying the accuracy of resistive measuring devices. They are normally the Primary Standards of a company's test lab. They are returned to the NIST for measurement and their accuracy is tracked through out the standards life to determine the Standard's stability. Most companies will have two sets of standards so that they can continue to measure while one set of standards are being measured by the NIST . They will alternate returning these NIST Standards to the NIST , one set one year and the other set the next year. For extremely accurate measurements, the Standard with the longest history and the best stability will be used. If erratic readings are received from the NIST over a period of years, the Standard is retired. Also, if the reading has significantly changed since the last NIST reading, the standard is suspect and all measurements made using that standard must be checked. Normally, a standard will take about 3 years to stabilize and becomes more stable with time unless it has had excessive power applied or has been dropped. These standards are generally stored in an oil bath at +25oC. During measurement, a thermometer is placed in a cavity in the top of the Standard, called the oil well, and the temperature is recorded for each measurement so that the exact value can be determined. That is the value at +25oC plus or minus the change in value caused by the temperature coefficient. Each standard will have a temperature correction chart for exact values. Being stored in the oil bath prevents the Standard from being stressed by changes in room temperature. These are highly precision devices and are expensive to buy and expensive to maintain, but they are the primary resistor reference for any test lab. These resistors are furnished in a totally enclosed metal case and for values above 1 ohm, this enclosure is filled with mineral oil (other type of oil may contain additives that can cause corrosion in later life). The values below 1 ohm may be built in an enclosure that is perforated and these must be submersed in oil. If power is applied without it being submersed, the Standard will be ruined. All NIST Type Standards are equipped with provisions for two, three, or four terminal measurements. The applied power is calculated and the temperature of the Standard is monitored during test. The lowest power level consistent with sufficient resolution to get the desired measurement is used (in the area of 0.01 watts) and any appreciable rise in temperature will dictate that the measurement should be suspended and the test set-up reviewed for ways to reduce the power level. These Standards are rated for operation at room temperature only but their other characteristics are the same as Precision Wirewound Resistors.