As found values represent the equipment’s state upon arrival. Even if the equipment is obviously incorrect, technicians will take all as found measurements before making adjustments. This procedure documents a baseline of the equipment’s performance as it has been running in the facility. If the equipment is found to be out-of-tolerance or measuring incorrectly, these numbers can be used to form an impact analysis and determine how large of an effect the faulty measurements had on your operation.
The Five Key Components of a Calibration Certificate
In the metrology industry, Calibration Certificates (certs) are the official reporting document for all calibration services performed. You may feel the sudden urge to doze off after hearing the word “reporting.” Unfortunately, many people quickly skim certs to make sure all equipment passes, then it’s stuffed in a filing cabinet labeled, “In case of Audit.” This usually happens because people on our end have done a poor job explaining what everything means and why it matters. With the proper knowledge, certs can provide valuable insight into your equipment’s health and efficiency.
Although calibration certificates can vary between companies and equipment types, there are five key components included on most certificates. In this article, we will breakdown each component and explain what it means for your equipment and operation.
1. Measurement Uncertainty (MU)
If you’ve ever worked on a carpentry project, you probably understand why the age-old adage measure twice cut once exist. Whether it’s a user error or a slight miscalculation, variations in measurement are common. Even the best measuring equipment under perfect conditions has a margin of error created by small uncontrollable and often undetectable factors. In the metrology world, we seek to quantify this doubt with a number known as measurement uncertainty (MU). Even though many of the factors are unpredictable and hard to control by themselves, our lab combines existing data, established mathematic formulas, and a series of tests to quantify and validate the possible effects of these factors. This process is called creating an uncertainty budget. MU is often denoted as a +/- value that will be taken into account throughout the entire calibration process. In the case of accredited certificates, MU will either be listed alongside the results or referenced in a footnote. For unaccredited certificates, MU is often unlisted and not taken into account.
Since measurement uncertainty can have a direct impact on overall process quality, it’s important to minimize MU without sacrificing quality. When selecting a calibration lab, making a comparison of each lab’s MU can be a determination of the quality and consideration put into their calibration procedure. An accredited lab can minimize uncertainty by considering/calculating more variables or controlling the calibration environment more thoroughly. There is one important caveat, small MU is only good if it is built on good science. Since unaccredited certificates do not have to follow a standard for measurement uncertainty, they can produce an unsubstantiated and inaccurate number by ignoring important factors. Since different accreditation bodies often have different standards that are more or less flexible with certain variables, it’s also important to note the accrediting boy when comparing two certificates MUs.
For a deeper dive, check out our article on Measurement Uncertainty.
2. Accreditation/Testing Statement
Most metrology companies will include an accreditation statement as part of their certificate. These statements typically define the accrediting body and a reference to which process the technicians followed.
Although the names and wording vary, most companies have a handful of similar certificate types available to their customers. By default, most technicians will calibrate to either customer standards or perform a simple verification. Please know, these defaults are not considered accredited calibrations. Although non-accredited calibrations are not necessarily bad, it’s important to have the appropriate calibration type for your procedure. For many non-critical processes, anything more than a verification may be considered overkill. For internal calibration standards and critical components, accredited calibrations can provide the highest possible quality. When in doubt consult your quality department. Your quality procedures should outline the types of tests required for your process.
A final note on accreditation, it is very important to pay attention to the wording of the accreditation statements. Many certificates may state that the particular lab or standard used is accredited, but this does not necessarily mean the calibration itself is accredited. Unless a certificate has reference to their scope and specifically states “this calibration is accredited,” it is probably not an accredited certificate.
Results are the reported values measured during calibration. These values are typically displayed under the columns as found and as left. To establish the equipment’s ability to make a full range of measurements, most calibrations utilize a minimum of three test points taken in ascending then descending order.
As left values are recorded after necessary adjustments are made.* These numbers are important to use as a marker of the equipment’s current performance. When possible and a customer request adjustments to be made, these values should be within an acceptable tolerance or as close as it was possible at the time. If the equipment can not be adjusted into an acceptable tolerance, it may be time to look at purchasing new equipment.
*If no adjustments are required as found, the as left column may be left blank or contain duplicates of the as found values.
4. Statement of Conformity
The statement of conformity is a clear declaration of the conformance or non-conformance to particular calibration standards. In simpler terms, a statement of conformance clearly states if the equipment is reporting acceptable results or not. Certificates commonly denote conformity using terms such as pass/fail, in-tolerance/out-of-tolerance, or within specifications/out-of-specifications.
For the most part, this section is pretty straightforward. Either the equipment is working correctly or not. However, there is one aspect of conformity that tends to raise a few eyebrows. Depending on the type of certificate, there may also be a not possible (NP) option listed with pass/fail. Not possible statements of conformity occur when the measurement uncertainty is greater than the difference between the measured result and tolerance. For example, if we have a scale with a tolerance of +/-1lb at 100lbs. When calibrating at 100lbs you may assume the scale can pass calibration by reporting a value anywhere between 99lbs and 101lbs, however that’s not quite true. For the sake of this example, let’s assume the measurement uncertainty is +/-0.5lbs. During calibration, the scale reports a certified 100lbs as 100.7lbs. Even though the result is within the acceptable tolerance, determining a pass/fail is considered “not possible.” This is because the 100.7lbs reported could possibly be anywhere between 101.2lbs and 100.2lbs when the uncertainty is taken into account. Since the measurement itself could have been up to 101.2lbs, it’s not possible to determine whether the equipment is in tolerance or not.
How to Respond to Each Type of Conformity
You may think a pass warrants no further action, but there are some important steps that can be taken to ensure process quality. If equipment constantly passes calibration with a wide margin, it may be possible to tighten your process tolerances and increase production efficiency. Conversely, if the equipment is starting to drift and just barely passing every time, it may be time to invest in new equipment before it starts to fail.
Although no one ever likes to see the word fail, the truth is, it happens sometimes. When it does, it’s important to determine the impact the out-of-tolerance equipment has had on your process. Was the out-of-tolerance test point in the equipment’s normal operating range? Could the mismeasurement cause and quality or safety issues? When was the last calibration? Could the equipment be adjusted back into tolerance? These and other questions can only be answered by your team to determine the next course of action. If there was no huge impact on your process and the failure didn’t have any effect, it may be worth expanding your process tolerances. If there is a possibility of small variations a random quality check of possible affected parts may be in order. For more extreme quality and safety issues, a recall may be in order.
If failure becomes a regular occurrence for equipment it is time to assess the possible reasons for failures. Common issues include improper operation, too wide of calibration and adjustment intervals, or bad equipment.
Since they live in a sort of grey, not possible results can be a little more difficult to determine the next course of action. Depending on the process a random quality check of affected parts could be in order just to be on the safe side. If your equipment is consistently marked not possible, it may be time to look at expanding your process tolerances or purchasing more precise equipment.
Traceability is established through reference numbers that connect calibration results back to the standard(s) used for reference. These numbers allow each measurement made to be traced back to the international standard through an unbroken chain of calibrations. As calibrations move up the traceability chain towards the international standard unit (SI), the tolerances become tighter and tighter and the amount of measurement uncertainty diminishes. This chain of traceability and the subsequent calibrations are the reason a screw produced in Louisville, Kentucky can fit a machine halfway across the world in Japan without being specifically designed for that application.
It’s worth noting, just because a traceability reference is listed does not mean the calibration is accredited or that the standards themselves are adequately traceable. Many tool manufacturers issue what’s called a conformance certificate that only provides traceability to their standards. These conformance certificates are not true accredited calibration certificates.
Need More Help Deciphering your Certificates?
Our calibration team is here to help you breakdown your certificates and answer any questions you may have.