Calibration is one way of verifying that a lab instrument is working properly. Calibration is a check of certain instrument parameters—velocity, weight, distance, etc. at a specific point in time. Keep in mind that calibration is just a check. If the check finds everything to be within tolerance, the instrument is considered “calibrated.” If the check turns up a problem, further action is needed. An instrument is considered “out of calibration” or “uncalibrated” if some parameter is not within the specified tolerance, or if the check has not been performed within the specified interval.
A calibrated instrument doesn’t guarantee that tests performed by the instrument will yield accurate results. It doesn’t even guarantee that the calibrated parameters will be the same the next time a test is performed. Maintaining a good calibration procedure and schedule does greatly improve the probability that the instrument will produce accurate results with each use.
Some calibrations can be performed by laboratory staff without sophisticated tools or training. In fact, some instruments need to be calibrated so frequently (daily, or before each use) that it would be impractical to calibrate them any other way.
Certain calibrations are performed as part of the associated test method. For example, the first step in the procedure for AATCC Test Method (TM) 22, Water Repellency: Spray Test is to “Calibrate the apparatus.” This simply requires pouring 250 mL of water into the apparatus funnel and measuring the time for it to pass through.
A lab may do other calibrations on a routine basis. AATCC Monograph (M) 6, Standardization of Home Laundry Test Conditions, provides “simple procedures…to calibrate the top-loading washing machines.” It is recommended that these be performed “at a minimum once a year.”
Some calibrations are not explicitly described in a test method. They may be prescribed by the manufacturer or they may be developed by the lab. Placing check weights on a scale is one example of this type of calibration. This can be done on a regular basis —weekly, monthly, or annually or before each use.
For calibrations that are too complicated, or just too time consuming, to be performed by lab staff, a specialist may do the job. These calibrations may require sophisticated tools or traceable standards.
Some manufacturers will calibrate their own instruments, either by sending a representative to the lab or by having the instrument shipped to the manufacturer’s facility. There are also companies that calibrate a range of instruments.
For instruments that fail a calibration check, adjustment or repair may solve the problem. Only in extreme cases will they need to be completely replaced. There are some tools, however, that should be replaced regularly rather than calibrated. AATCC Gray Scales fit into this category.
Each scales come with a certificate of conformity based on measurement of the individual grey chips. The scale is difficult to measure accurately once it is assembled. Instead of in-house calibration with a spectrophotometer, AATCC committee RA36 recommends replacing scales at least annually.
Other items, such as multifiber strips, are suitable only for single use. For items that will not be calibrated regularly, it is important to store them as recommended and verify the shelf life.
Calibration is necessarily an ongoing process. How often it needs to be done depends on a number of factors, including how often an instrument is used, the required accuracy, and the instrument’s tendency to drift. Test methods and manufacturer’s instructions often include recommended calibration schedules, but individual labs may have valid reasons for modifying these schedules.
So, how does one know if and when a lab calibrates its apparatus? If one visits the lab, he will probably see calibration stickers on some instruments. Make sure the due date has not expired. For stickers with space to list multiple calibration dates, the dates should be at regular intervals.
Not all calibrations are documented with a sticker. There may be a log, certificate, or other records. If these aren’t posted near the instrument, don’t hesitate to ask. They may be filed elsewhere for safekeeping. Some instruments may also store calibration data electronically.
Generally, a current calibration sticker is a good sign that the lab maintains its instruments in good working order. If one wants to be particularly diligent, check the frequency of calibration. One can also ask what the calibration entails. Was every bulb/filter combination in the lightbox checked, or just the one most frequently used? Was the tensile tester calibrated for load AND speed? In what range, and to what accuracy, was the scale calibrated?
There is a lot that goes into a calibration program that cannot be covered here, but knowing what to look for and what to ask is a good first step.