Instrument Accuracy Checks and Calibration
Does your facility have an instrument calibration or accuracy check program with procedures to verify that the devices are precise and accurate? If not, it’s time to implement one.
Consider all the instruments and devices that measure something in your facility. These measurements may include temperature, weight, pressure, humidity, pH, salinity, and the list continues. Each device has a purpose, and the resulting measurements are needed for monitoring and managing the equipment or product. But what happens when these devices become inaccurate and fail to provide precise results? Does your facility have an instrument calibration or accuracy check program with procedures to verify that the devices are precise and accurate? Management should be confident that their measuring devices are reading correctly. The following steps will help you establish an instrument calibration procedure, or tune up your existing one.
Step 1: Identify the Measuring Devices/Instruments
Develop a comprehensive list of every device in your facility that is used to measure something. This includes scales, thermometers, gauges, pH meters, and so forth. Then determine if all the listed devices are still of benefit. Those devices that are no longer of use should either be removed or marked as not being routinely checked for accuracy. Before removing any device, check with the managers, supervisors, and employees in the area to make sure it is not used. One suggestion is to place a sign on the instrument stating that it will be removed unless management is notified that it is used.
If the instruments do not have a manufacturer identification number, clearly mark each device with a unique number by attaching an asset number tag, etching the device, or using another labeling method. Identification by location (i.e., table scale) may be appropriate if there is only one of the devices; however, a numbering system is recommended, especially if any of the following circumstances apply:
- The device is the standard used to calibrate other devices (such as a NIST-certified thermometer).
- The device is used to monitor a critical control point (CCP).
- Your facility has more than one of the devices. For example, table scales can be moved easily and the same one may not necessarily be returned to the same location.
As appropriate, identify the location of the instrument, such as: Receiving Office, Mixer 1, Laboratory, Packing Line 3, End of the Pasteurizer Hold Tube, and so forth.
Identify the range of use by specifying the actual temperatures, weights, etc. that will be measured by the device. For example, if a floor scale is used to weigh 500 pounds, test the accuracy at 500 pounds rather than at 20 pounds. Also identify the required or accepted range from the certified device (such as +/- 2 pounds, +/-0.50C, etc.)
Assign a procedure number for the method of certification, calibration, and accuracy checks used for each device. Documented procedures should include:
- How to conduct the check.
- Acceptable deviations/deviation range.
- Corrective action if the results are not within the accepted deviation range.
- Documentation requirements.
The instrument check/calibration procedure should include frequency of the checks for each device, such as daily, weekly, monthly, annually.
Also, specify the department or position responsible for checking the devices or for arrangements if an outside agency will be used. Finally, make sure the location of the affiliated results and corrective actions is identified in the procedure.
Step 2: Determine Certification, Calibration, and Accuracy Check Requirements
Certification of Accuracy. The use of a recognized standard, such as calibration against the National Institute for Standards and Technology (NIST), is a regulatory requirement for some segments of the food industry. The use of a NIST-certified device or certified-reference standards is typical for most measuring devices, based on the critical nature of the measuring device accuracy, Good Laboratory Practices, and other standards of the industry.
At a minimum, the traceable reference standard should be recertified annually. Obtain and maintain the documentation provided by the certification company. Note: for digital thermometers with interchangeable probes, both the digital device and affiliated probe are to be recertified.
If an outside company is used to calibrate the devices, the resulting documentation should include the NIST-certified standard’s identifying number.
Standardization. Historically, NIST-certified thermometers were glass and mercury, thus, not routinely removed from the lab or brought into production areas. In this situation, a second thermometer was often calibrated against the NIST-certified thermometer. The resulting standardized thermometer was then used for accuracy checks. With today’s digital thermometers, most locations are using NIST-certified digital thermometers as their standard for all calibration and accuracy checks.
Some segments of the industry (such as low-acid canned foods1) are still required by regulation to use certified, mercury-in-glass thermometers.2
Weights used for in-house checks are commonly overlooked. They can fluctuate in weight based on their condition. Chipped or broken weights may weigh less, while rusting weights can weigh more. If an outside contractor is used to calibrate scales, include your weights in the contract for an accuracy check.
Calibration. Calibration involves testing the device with two different measurements or standards, typically just above and below the range of actual use. Examples include:
- Two different buffer solutions would be used to calibrate a pH meter (such as 4.0 and 7.0 if the products being tested are at a range of 4.2 to 5.0).
- A thermometer would be checked against the NIST-certified thermometer at two temperatures, such as ice chip (for cold) or oil tank (for hot) AND at room temperature.
Ensure the instrument is capable of handling the calibration standards. An extreme example would be testing a sensitive table scale with a 100-pound weight. Similarly, a bi-metal or dial-type thermometer can be damaged if exposed to rapid temperature extremes (such as plunging the probe into ice water directly after being in a hot oil bath).
Accuracy Checks. Accuracy checks determine if the instrument is reading a true or correct value at a single point. For example, routine accuracy checks of a thermometer could involve immersing the probe into an ice slurry to determine if the thermometer measures a temperature of 32ºF.
A second type of accuracy check would be to compare a wall thermometer in the refrigerated warehouse against a standardized thermometer (the standardized thermometer being the standard). For seafood HACCP and other regulated segments of the industry, the refrigerator thermometer/RTD probe is to be calibrated at least annually; a single point accuracy check may not be sufficient.
Frequency Determination. The frequency of conducting each type of accuracy check depends on multiple factors, including:
- Regulatory requirements.
- Manufacturer’s recommendations.
- Industry standards/Good Laboratory Practices.
- Level of risk to product safety if the device is inaccurate.
- Inherent reliability. (For example, the least reliable types of thermometers are dial and bi-metal probe thermometers, whereas most digital thermometers are fairly reliable.)
- Environment of use. Is the measuring device moved (e.g., table-top scale moved during daily sanitation)?
- Potential abuse, such as receiving thermometers that may be put into pockets, sat on, etc.
- Historic information about accuracy/past performance.
Step 3: Methodology
Each method of determining the accuracy of a measuring device is to have clearly outlined procedures that an individual can follow with training. A key resource for accuracy checks or calibration procedures is the equipment manufacturer. These procedures do not necessarily need to be elaborate, such as the Thermometer Calibration Guide and affiliated poster provided online by Kansas State University.3
Step 4: Corrective Action
Clearly defined corrective action is needed when a device is identified as being out of the accepted range. The obvious first step is to immediately correct the device. If that is not possible, determine if it should be used. If you decide to continue using it, make sure to include a documented explanation of any resulting monitoring record as to the deviation.
A second and often overlooked corrective action is to determine if product safety (such as measurement of a critical control point) or quality has been compromised. This may require holding all products back to the last acceptable, documented check/calibration of the device while conducting the investigation. The investigation itself would then involve a review of all affiliated records to determine if the product is safe (such as the documented operating temperatures were well within the outlined CCP) or if the product quality is acceptable for distribution.
Step 5: Verification
Verification encompasses at least four activities, including:
- A scheduled, periodic review of the associated records should be conducted by identified management.
- Ensure the individual(s) doing the check/calibration are following the procedures and are questioned about what they would do if the device was inaccurate.
- Ensure the procedures and acceptable deviation range remains valid.
- At a minimum, ensure the list of devices to be calibrated remains accurate. This list should be reviewed annually and updated whenever a new measuring device is added or removed or the use is changed for a particular purpose.
Step 6: Documentation and Record Keeping
Remember the adage, “If it is not documented, it was not done.” A number of documents have been identified, including the procedures for conducting the accuracy check/calibration, the National Institute for Standards and Technology certification, the accuracy check/calibration findings and potential corrective action, and record verification. The record retention schedule for these documents should match any federal, state, local, or company requirements for the documents that can recreate the history of the product.
A Calibration Program is necessary to ensure that the instruments and devices used to measure key processing or product parameters are accurate. These suggested steps will help any food plant set up a program so they can rest easy knowing that their measuring devices are reading correctly.
(1) Title 21 CFR Part 113 – Thermally Processed Low-Acid Foods Packaged in Hermetically Sealed Containers, Subpart C—Equipment (Part 113.40 Equipment and Procedures)