Metrology and Calibration
What is metrology and calibration? How does it affect the average consumer or business person? If you have ever had anything weighed, metered or otherwise measured, you likely have run across calibrated equipment. Such equipment can range from calibrated pressure gauges in hospitals and industrial settings, to high accuracy weights and scales used in many businesses. Torque wrenches and other tools are commonly calibrated to ensure your safety when airplanes and other transportation equipment gets rebuilt or as part of routine maintenance. When you buy gas, flow meters determine how much gas is flowing so you can be properly charged -- these meters require frequent calibration. To the average consumer, these events are virtually transparent. Equipment is calibrated for many reasons, including company policy and Government regulation, but the consumer rarely realizes it's happening.
For the business person in any number of areas, calibrated equipment is a necessary part of doing business. This equipment gets calibrated on a certain schedule, perhaps yearly or even several times a year, depending on the equipment. The calibration industry has grown in recent years with the explosion of equipment that requires it. Flow meters, pressure gauges, micrometers, specialized airplane maintenance tools, thermometers, scales, electronic measuring equipment and others all require that they be certified as being accurate and repeatable. Most calibration labs boast that their metrology equipment traces back to NIST standards. The National Institute of Standards and Technology is a federal technology agency that develops and promotes measurement, standards, and technology. By being able to trace back to the definitive standards of NIST, the customer is, in theory, getting the most accurate calibration possible. While there are many fine labs out there doing top notch work there are also labs out there that are not. This article discusses what the business owner should be asking their chosen lab to ensure that they are getting their money's worth and most importantly, that their equipment is correctly calibrated.
As indicated, most labs claim that their equipment traces back to original NIST standards for maximum accuracy. For example, if you buy a class 1 weight set and you want to verify that the weights are accurate you can send it to one of these labs. The first question you need to ask your lab is whether or not their standard that they will compare your weights against has the accuracy to properly determine the mass of your weights. For example, the lab could have a set of NIST traceable Class 3 weights. So, yes, they are traceable to NIST. However, since class 1 weights are two levels more accurate than class 3 weights, the lab could not properly use them to determine the accuracy of your weights. So you need to ask your lab exactly what standard they are going to use to verify your equipment. Saying it is NIST traceable is not enough! They need to show you that their standards exceed the accuracy of your equipment by at least a margin of 2-to-1. Many labs will use a 4-to-1 criteria to ensure you get the best calibration possible. Additionally, you need to ask to see the calibration certificates for the equipment they use to calibrate your equipment. Ensure that their equipment has been calibrated within at least the last year.
We've seen many a calibration report where a torque wrench was listed as reading 200.56092491 foot pounds. All of the numbers after the decimal point imply a certain level of accuracy. Many labs will allow their equipment to report numbers out to many digits even though their equipment can't possibly measure this level of detail. For example, if their calibrated torque transducer is accurate to .01 pounds, then any number after the 2nd digit is meaningless. Since the equipment can't read the third decimal place that number is bogus and only provides and illusion of great accuracy that does not exist! However, many people see these numbers on their reports and go away thinking that their torque wrench is capable of discerning between 200.00000001 and 200.00000002 foot pounds. Which it could never be able to do. The same thing frequently occurs with the calibration of weights. Their scale may have been calibrated with weights that were good to .0001 grams. If your report shows a 50 gram weight as reading 50.00192 grams it means nothing, since the test equipment could not possibly resolve to this fine number. Once again, you are left with the illusion of accuracy that doesn't truly exist.
Whenever equipment is calibrated, certain procedures have to be followed in order to make sure it is done correctly. Both government, industry and equipment manufacturers have developed procedures for just about every type of equipment imaginable. These procedures are developed by engineers who know the equipment and specialize in testing and design. When bringing equipment to a lab for calibration you need to ask what procedures they are following. Many labs make up their own procedures. In some cases they just copy the equivalent government or manufacturers procedure and make it their own. This is a perfectly valid thing for them to do assuming they have not made fundamental changes to the procedure. However, some labs -- due to limited equipment -- will create procedures from scratch to enable them to use equipment they have on hand instead of the specialty equipment called out by the official procedure. While this can sometimes be valid if enough engineering goes into the process, there is no guarantee to you, the customer, that it is. You have no real way of knowing if the procedure they created actually tested your equipment properly and to the required specifications. If your lab cannot point to a government, military or industry published procedure that is the basis for their testing -- be wary!
Many labs claim either accreditation or certification from various sources. Being accredited or "ISO 9001 certified" is a plus when choosing a lab. However, it is not in itself any kind of guarantee. Labs get these certifications in order to be able to do business with customers that require it. While accreditation or certification requires some up front work and inspection it means very little on a day to day basis. These certifications are designed to show that the lab follows a pre determined flow in their work process in order to ensure that processes are repeatable and, in theory, that quality is maintained. For example, it could be the quality policy of a company, as defined under their certification, to always use an approved procedure when doing a test. That sounds great. But the definition of what a approved procedure consists of is up to the lab. They could write up a procedure with their own methods and specification, approve it internally and then use it and not violate their quality plan.
In the end all of this depends on the character of the business. A non-certified business that bends over backwards to always follow best practices and procedures might be a better choice than one with all kinds of certifications and accreditations but does their work shoddily and without concern for the final product or safety of those using the product. Asking the questions discussed above will help you determine if your calibration shop is genuine or just out to make a quick buck.