Sensors used in industrial equipment, like those that measure temperature, humidity, and pressure, are designed to be accurate and precise. However, in applications where these measurements are critical for process control or product safety, calibration is needed to confirm that sensors are working properly.
In highly regulated industries, like pharmaceutical manufacturing, calibration is also required as part of validating and maintaining the quality management system. In addition to initial calibration, periodic re-calibration and adjustment is needed for sensors that can drift over time.
In its most basic terms, calibration is the comparison of a measurement provided by a sensor (sometimes called the “unit under test” (or UUT) with one that is known to be accurate (the “standard”).
A fundamental question about calibration is: how do we confirm the accuracy of the standard unit? In the US, the answer to this problem is provided by the National Institute of Standards and Technology (NIST), a government organization that provides calibrations that are traceable to a known and accepted primary standard. Instruments that are calibrated through NIST can then be used as standard “known good” units for calibration of instruments in the field. We’ll be covering the role of NIST and how this process works in more detail later in this article. In other parts of the world, this role is filled by local national metrology institutes. The International Bureau of Weights and Measures (known by its French abbreviation BIPM) coordinates activities globally in this area.
Calibration is related to verification, but the two procedures are somewhat different and distinct in practice. Verification is the evaluation of calibration results, specifically comparing the accuracy of the unit under test, determined in calibration, to a specification, regulation, or customer requirement. These concepts are sometimes grouped together.
In the context of industrial processes, calibration usually involves confirming the accuracy of a measurement unit used for process control (a temperature sensor, flow controller, or flow meter in a chemical reactor, for example) or environmental monitoring (temperature and humidity sensors in a warehouse, for example). For a temperature sensor, calibration could involve placing the UUT and a unit known to be accurate in a controlled temperature chamber, then comparing the readings from the two units.
The terms used when discussing calibration and related procedures can be confusing to those not familiar with this area. The International Bureau of Weights and Measures has published a dictionary of terms used in metrology (the science of measurements, including calibration).
To summarize some of the key terms:
- Calibration strictly means the comparison of a measurement device to a known standard, which can be a material, object, physical process (like melting or freezing), or a second device which is known to be accurate.
- Verification is the evaluation of the calibration results against a specification. For example, a particular quality policy may specify that a temperature sensor must be accurate to 1 °C. If the calibration shows a discrepancy between the sensor and the true value of <1 °C, the sensor has passed verification.
- If the sensor fails verification, then an adjustment can be made to the sensor to improve its accuracy, followed by re-calibration and re-verification. Calibration data collected before and after adjustment is referred to as “as-found” and “as-left” data, respectively.
Note that verification can have different meanings, even in this area. Verification can also mean a less formal functionality check of measurement equipment performed between normal calibration intervals.
Not following rigorous calibration procedures, or neglecting calibration altogether, can have major consequences. As an example of why calibration would be needed, consider a thermocouple-based temperature sensor. Thermocouples work by measuring a small voltage difference that is generated at the junction of two dissimilar metals. The voltage is temperature-dependent, so it can be translated into a temperature by the measurement unit.
Thermocouples are considered to be rugged compared to other temperature measuring devices, however, any small changes to the metallurgy of the junction (which can be caused by aging or repeated temperature swings), damage to the wiring at the junction or between the junction and the voltage measurement circuit, or minor changes to the measurement circuit can cause drift in the sensor. If the temperature accuracy of the unit drifts, then any control or monitoring that is based on that sensor will also drift. In a pharmaceutical storage warehouse, this could result in degradation of the stored material and recalled product.
Likewise, if a new flow controller installed on a manufacturing unit has an unknown offset between the flow setpoint and the actual delivered flow, process excursions could result, leading to scrapped product. This could be avoided by proper calibration of the controller during installation.
In highly regulated industries, calibration is often considered to be part of an overall quality management system. Because of this, non-compliance with calibration procedures can result in citations from government auditors. From 2015-2020, the FDA issued over 500 citations related to issues with equipment calibration in the context of manufacturing and distribution of pharmaceuticals (21 CFR 211) and medical devices (21 CFR 820).
How to Obtain an NIST-Traceable Calibration
21 CFR 820.72 requires that the standards used for calibration in medical device manufacturing be “traceable to national or international standards”. NIST is a source for these standards. The scope of NIST is actually much more broad, but for this article, this is what we’ll be focusing on.
To perform calibrations traceable to a recognized national or international standard, an organization or company would follow these steps:
- First, obtain a piece of measurement equipment that has been calibrated by NIST using the applicable standard. For temperature, humidity, and pressure measurements, this is handled by the thermodynamic metrology group of NIST. Typically a measurement sensor (like a thermocouple gauge, a pressure sensor, or humidity sensor) will be shipped to NIST for onsite calibration.
- NIST will then generate a traceable calibration using one of a variety of possible methods, as specified by the customer. One method is to use the equipment under test to measure a physical property or process that has been well-characterized or defined. For example, temperature sensors can be calibrated by measuring the melting points of various materials. Since the melting points of the materials are known, they represent an absolute, internationally recognized source of temperature calibration points. A full list of the standard materials used for temperature calibrations is given by the International Temperature Scale of 1990 or ITS-90. Rather than this direct approach, NIST can also provide traceable calibrations using documented comparisons of sensors calibrated directly using the ITS-90 scale. These are called “comparison calibrations”.
- When the newly NIST-calibrated equipment is returned to the customer, additional standards can be generated at the customer site by calibration against it. The details of how to perform these calibrations are described in the ISO/IEC 17025 standard. This is a quality standard, similar to ISO 9001, that applies specifically to labs that perform testing and calibration.
- The equipment with calibrations that can be traced back to NIST can then be used in turn to calibrate equipment in the field.
The key to traceability is documentation, and the string of calibrations leading back to NIST must be fully documented and unbroken for traceability to be maintained. According to 21 CFR 820.72, documentation should include “equipment identification, calibration dates, the individual performing each calibration, and the next calibration date”. In addition to this, auditors should also check documentation to confirm that the lab performing the calibration is competent (shown, for example, through ISO certification), and that calibrations are performed at appropriate intervals.
Another element to traceability is that the contribution of measurement uncertainty in each step of the calibration process needs to be accounted for, and calculated to ensure it meets the requirements of the customer or regulatory body. NIST publishes a helpful guide of measurement uncertainty for various types of temperature sensors in different temperature ranges. A more in-depth guide to measurement error, and how it propagates through a series of measurements, has also been published by the BIPM.
NIST has explicitly stated that the responsibility for supporting claims of traceability is the responsibility of the organization asserting the claims, not of NIST. The role of NIST in the context of traceable calibrations is to ensure traceability to an accepted reference standard, and to provide calibration and reference materials. However, NIST provides a checklist for calibrating instruments at a third party calibration lab, as well as guidelines for calibration and other lab practices.
For temperature calibration specifically, NIST also administers the Measurement Assurance Program (MAP), in which measurement of the ITS-90 scale is monitored at a customer site through NIST-calibrated measurement sensors. This program allows other organizations to verify measurements against the ITS-90 standard, and to claim NIST traceability of their calibrations, provided the MAP procedure is done on regular intervals.
The Different Levels of NIST Calibration
An important consideration in a calibration is the number of data points that are used. For example, in a temperature calibration, the number of data points refers to the number of temperatures at which the performance of the device under test is compared to the standard. In a 1-point calibration, the device would be checked against the reference at a single temperature. In a 3-point calibration, three different temperatures would be used.
1- and 3-point calibrations are the most common. For applications where accuracy within only a narrow range is needed, a 1-point calibration within that range is often sufficient. When accuracy over a wider range is important, a 3-point calibration can be used to cover the range. Custom point calibrations are sometimes used for special applications where the temperature range of interest is not covered by the available pre-set calibration points.
Different types of adjustments can be made if the accuracy of a device is found to be insufficient during calibration. The most common are zero and span adjustments. A zero adjustment is used to correct a constant offset, while a span adjustment is used to correct the slope of the response curve of the device. The reliability of these corrections is directly related to the number of points used in the calibration, and the range of these points.
How to Make Sure You Are Meeting NIST Standards
The traceability and accuracy of the primary calibration performed at NIST is documented in test reports and calibration certificates (note that the test numbers associated with calibrations are not generally accepted as proof of traceability). After the equipment leaves NIST, traceability is maintained by following the guidelines established in the ISO/IEC 17025 standard, as well as procedures published by NIST, including a checklist that identifies the responsible party at each step of the calibration process.
The easiest way to ensure that a third-party vendor laboratory is conducting calibrations properly is to check that they are ISO/IEC 17025 certified, which is commonly performed by a recognized third party accrediting organization. One of these well known organizations is the American Association for Laboratory Accreditation (A2LA). The A2LA maintains a searchable directory of certified organizations.
As we discussed earlier in the article, it is the responsibility of the company claiming traceability, not NIST, to support these claims. However, when working with a third party, auditing, documentation of calibration procedures, and calibration certificates can be used to establish traceability.
Calibration of measurement equipment is a straightforward process that ensures the dependability of critical measurement systems. Calibration with measurements traceable to a NIST standard (or standards provided by other national metrology institutes outside the US) is widely accepted as necessary for performing calibrations.
Calibration can be made even easier by working through an accredited third party experienced in the documentation and procedures of NIST traceable calibrations.
About the author: Before coming to Dickson, Director of Services Antoine Nguyen spent more than 18 years in quality and validation roles in the pharmaceutical and medical device industries.