The Truth About NIST Certification

NIST stands for National Institute of Standards and Technology. The institute is a government agency, which started out as the National Bureau of Standards (NBS) and changed its name in 1988. (Many specifications and data sheets still refer to NBS). NIST’s mission is to maintain the ultimate accuracy standard for physical properties such as temperature, humidity, pressure, and any other measurable entity imaginable.

NIST traceability has long been used in government specifications and has found its way into many commercial HVAC specifications. Some environments require very high accuracy, such as a clean room that may destroy millions of dollars’ worth of pharmaceutical products if the environment is off by 2°F or 2%RH. In critical environments such as this, it makes sense to specify a sensor with accuracy traceable to NIST standards.

In order to understand traceability, we must come to grips with the language. The American National Standards Institute (ANSI) definition of traceability is “the property of a result of a measurement whereby it can be related to appropriate standards, generally national or international standards, through an unbroken chain of comparisons”. A certificate of traceability must include documentation of all the primary and secondary standard equipment used for calibration.

A standard is a reference value or piece of equipment to which we can compare and measure the accuracy of a test device (such as an RH transmitter). The identification and date of the calibration or comparison are the basis of traceability. A primary standard is equipment that has been sent to NIST for calibration and certification. Most companies have them re-calibrated at least once a year at considerable cost. The next level is termed a secondary transfer standard or working standard. This equipment is calibrated and certified annually from the primary standard. Secondary transfer standards are used in calibrating production units such as thermistors, pressure transmitters, RH transmitters, etc…, and are the ones from which NIST traceable certificates are issued for products.

It is important to note that NIST does not check individual products. NIST only verifies its own standard and the primary standards it calibrates for outside labs and industry. NIST does not involve itself with claims of accuracy for secondary transfer standards or the subsequent products calibrated against them. There is no policing body or watchdog over claims of accuracy for secondary transfer standards. This is why the industry relies on traceability back to our national reference standard at the NIST.

Manufacturers offer many different versions of “NIST Traceable” certificates. The lowest level indicates the NIST traceability of the test equipment used to calibrate the product (usually the secondary transfer standard). A higher level certificate is to test the product at a single point (such as 50% RH or 77°F), and the accuracy at that point is identified on an individual serialized certificate to verify the exact difference (uncertainty or error) from the secondary transfer standard. An even higher level would be if the product were tested at multiple points (such as 30%, 50% and 80% RH) with an individual certificate issued.

Each level of certification involves a calibration cost, which is indicative of the effort needed to issue a traceability certificate. At the lowest level, suppliers often give these certificates away for free, which gives a good idea of their value. Reputable manufacturers already use traceable calibration equipment to verify that their products will perform within the accuracy figures stated in their normal specifications, so producing a certificate is a simple copy-machine operation.

When a single-point or multiple-point certificate is specified, each individual product in question must be compared to the standard, and the certificate must indicate the exact variance at the specified point(s). These certificates are therefore costly. Unless these costs are already buried within the product price, a fee can be expected. Of course, this type of certificate must also be specified when ordering since the manufacturer needs to have the product in hand to do the testing.

Kele offers a number of products with NIST certificates as standard. Many manufacturers offer optional NIST certificates for their products.

To summarize, a traceability certificate does not make a sensor more accurate. It only verifies that its accuracy has been compared to a standard. For critical applications, when extra proof is needed on the front end to provide peace of mind, contact your Kele Sales Associate for availability.