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Practical Calibration Standards

Within the context of a calibration shop environment, where accurate calibrations are important yet intrinsic standards are not readily accessible, we must do what we can to maintain a workable degree of accuracy in the calibration equipment used to calibrate field instruments.

It is important that the degree of uncertainty in the accuracy of a test instrument is significantly less than the degree of uncertainty we hope to achieve in the instruments we calibrate. Otherwise, calibration becomes an exercise in futility. This ratio of uncertainties is called the Test Uncertainty Ratio, or TUR. A good rule-of-thumb is to maintain a TUR of at least 4:1 (ideally 10:1 or better), the test equipment being many times more accurate (less uncertain) than the field instruments we calibrate with them.

I have personally witnessed the confusion and wasted time that results from trying to calibrate a field instrument to a tighter tolerance than what the calibrating equipment is capable of. In one case, an instrument technician attempted to calibrate a pneumatic pressure transmitter to a tolerance of +/- 0.5% of span using a test gauge that was only good for +/- 1% of the same span. This poor technician kept going back and forth, adjusting zero and span over and over again, trying to stay within the stated specification of 0.5%. After giving up, he tested the test gauges by comparing three of them, one against the other. When it was realized no two test gauges would agree with each other to within the tolerance he was trying to achieve in calibrating the transmitter, it became clear what the problem was.

The lesson to be learned here is to always ensure the equipment used to calibrate industrial instruments is reliably accurate (enough). No piece of test equipment will ever be perfectly accurate, but perfection is not what we need. Our goal is to be accurate enough that the final calibration will be reliable within specified boundaries.

The next few subsections describe various standards used in instrument shops to calibrate industrial instruments. Click on the links to read more for each topic. 

Electrical standards - Electrical calibration equipment – used to calibrate instruments measuring voltage, current, and resistance – must be periodically calibrated against higher-tier standards maintained by outside laboratories. In years past, instrument shops would often maintain their own standard cell batteries (often called Weston cells) as a primary voltage reference. These special-purpose batteries produced 1.0183 volts DC at room temperature with low uncertainty and drift, but were sensitive to vibration and non-trivial to actually use. Now, electronic voltage references have all but displaced standard cells in calibration shops and laboratories, but these references must be checked and adjusted for drift in order to maintain their NIST traceability. Click here to read the entire subsection...

Temperature standards - The most common technologies for industrial temperature measurement are electronic in nature: RTDs and thermocouples. As such, the standards used to calibrate such devices are the same standards used to calibrate electrical instruments such as digital multimeters (DMMs). For RTDs, this means a precision resistance standard such as a decade box used to precisely set known quantities of electrical resistance. For thermocouples, this means a precision potentiometer used to generate precise quantities of low DC voltage (in the millivolt range, with microvolt resolution). Modern, electronic calibrators are also available now for RTD and thermocouple instrument calibration, able to generate accurate quantities of electrical resistance and DC millivoltage for the simulation of RTD and thermocouple elements, respectively. Click here to read the entire subsection...

Pressure standards - In order to accurately calibrate a pressure instrument in a shop environment, we must create fluid pressures of known magnitude against which we compare the instrument being calibrated. As with other types of physical calibrations, our choices of instruments falls into two broad categories: devices that inherently produce known pressures versus devices that accurately measure pressures created by some (other) adjustable source. Click here to read the entire subsection...

Flow standards - Most forms of continuous flow measurement are inferential; that is, we measure flow indirectly by measuring some other variable (such as pressure, voltage, or frequency) directly. With this in mind, we may usually achieve reasonable calibration accuracy simply by calibrating the primary sensor and replacing the flow element (if inspection proves necessary). In the case of an orifice plate used to measure fluid flow rate, this would mean calibrating the differential pressure transmitter to measure pressure accurately and replacing the orifice plate if it shows signs of wear. Click here to read the entire subsection...

Analytical standards - An analyzer measures intrinsic properties of a substance sample such as its density, chemical content, or purity. Whereas the other types of instruments discussed in this chapter measure quantities incidental to the composition of a substance (pressure, level, temperature, and flow rate), an analyzer measures something related to the nature of substance being processed. Click here to read the entire subsection...

 

References

Calibration: Philosophy In Practice, Second Edition, Fluke Corporation, Everett, WA, 1994.

Lipt´ak, B´ela G., Instrument Engineers’ Handbook – Process Measurement and Analysis Volume I, Fourth Edition, CRC Press, New York, NY, 2003.

 

Click here to go back to the previous page, NIST Traceability and Instrument Turndown

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