What Is Calibration?
Depending on whom you ask, there are many different answers to the definition/meaning of calibration, but the basic principle remains the same throughout.
To ensure that measurements being made or output provided by equipment are accurate, they need to be compared against a reference that is known to be accurate. This is exactly what the process of calibration achieves – a comparison between measurements of known and unknown accuracy or precision.
When calibration is being done, the equipment with unknown performance is called the ‘unit under test’ or ‘test instrument’; and the other is called the ‘calibration standard’ or simply ‘standard’.
Why Is Calibration So Important?
Calibration is used to define the quality of measurement parameters, like accuracy, range or precision, which are recorded by a piece of equipment. It’s a necessary part of the process like manufacturing, testing, and quality assurance, which form the backbone of a wide range of industries and sectors.
Here’s an overview of why calibration is important for practically every industry (especially those that are heavily regulated by authorities like the FDA) at some point or another:
- Over time, the quality of measurements of just about every tool will deteriorate to some extent. Companies need to ensure that these shifts in accuracy are tracked, and measures are taken to prevent them from affecting final product quality.
- Equipment that operates on certain technologies, or measures shifting parameters like humidity, temperature and pressure, is more likely to be affected by a ‘drift’ in accuracy.
- In situations where the quality of the measurements is imperative for maintaining quality, you need to ensure that the instrument is operating within an acceptable range of error, and calibration is essential for this.
- In order to ensure that you can enjoy complete confidence in the measurements and the output of any piece of equipment, calibration of instruments needs to be performed on a periodic basis.
What is Equipment/Instrument Calibration & What Does It Do?
Instrument calibration is amongst the primary (and often the most crucial) methods of checking and maintaining the quality of measurements made by instruments. The measurements are checked and compared, and the instrument is configured to provide results which are within an acceptable accuracy, precision, and repeatability range.
Minimizing, or altogether eliminating, factors that could cause inconsistencies and errors are a fundamental part of instrument design philosophy. There are various service providers you could turn to for your instrument calibration requirements. These often offer specialized services like laboratory calibration services or pipette calibration services, which are customized to the needs and requirements of various industries.
Though the exact method used for calibration of equipment depends on the instrument in question, the procedure typically involves most or all of the following steps:
- One or more test samples or standards with known values, often called ‘calibrators’, are measured using the test instrument.
- The results obtained are compared with the actual values, thus establishing a relationship between known values and the measurement technique.
- Using this process, the instrument is, in essence, ‘taught’ to produce more accurate results than it would otherwise.
- Post calibration, the instrument can measure unknown samples with higher reliability of precision and accuracy.
Using known standards of different values, multiple calibrations are performed to establish a better correlation at different stages within the entire operating range of the instrument. While you might want to perform calibrations at many different points to plot an accuracy curve, this may not always be the best choice, since:
- Costs associated with labor and time can rack up quickly, so you need to limit the number of calibration points accordingly.
- The relation between number of calibration points and the resulting performance might not be linear.
- Practically speaking, you’ll need to make tradeoffs between the effort and cost of calibrations, and the desired performance levels.
Instruments tend to perform best when they’ve been calibrated according to the recommendations of the manufacturer. The performance specifications include intermediate points, which are used for calibrations. The process specified is designed to ‘zero-out’, i.e. basically eliminate, the inherent errors in the instrument at those points.
What Factors Affect Calibration?
Once you’ve understood the benefits of performing calibration and how critical it can be for maintaining quality, it’s quite apparent that it should be dealt with carefully. While designing the calibration procedures and during the actual calibration, some steps must be taken to eliminate potential error sources that can degrade the quality of the results.
For extremely sensitive instruments, you may need to take them to a calibration laboratory or other calibration service provider, where they can be calibrated under controlled conditions. There are a number of factors that can affect calibration results otherwise, both during the calibration procedure and afterwards.
- Using the Wrong Values
The instructions for calibration need to be followed very closely. The calibrator mentioned in the instructions is the one that the instrument will ‘learn’ from. Disregarding the documentation and choosing a different one or the wrong values changes the way the test instrument behaves. This can produce significant errors within some parts or the entire operating range of the instrument.Some of the newer instruments have a built-in software diagnostic system that can alert operators when the order in which calibrators are tested is wrong (Calibrator-B used before Calibrator-A). However, they may not be able to distinguish between calibrators that use the wrong values.
- Calibrator Formulation Tolerance
Just like your equipment, the quality of the calibrator you use can affect the results of the calibration. Using calibrators manufactured by reputable and trustworthy manufacturers or calibration labs, which are built to precise specifications and tolerances, is essential for obtaining repeatable performance and dependable results. There is another tolerance that is associated with the design and formulation of a control or calibrator. This is due to normal inaccuracies and variations in quality control processes and the instrument itself.For example, if you’re using calibrators whose nominal values are 50 and 800 mOsm/kg (H2O), and if they’re both manufactured to perform at the lower end of the required range, the net effect of calibration may be to lower the accuracy or precision. This would result in additional errors in the range of several mOsm/kg, over the entire range the calibration was performed on.
- The calibration process will ‘teach’ the instrument to read 800 incorrectly as 796, so the actual results curve will be higher than if the instrument was calibrated as per the correct value of 796, or if the calibrator was at the actual required formulation of 800 mOsm/kg.
- If it is assumed that one calibrator is at its nominal value (800 mOsm/kg), but the true value is off by just a tiny bit, say at about 796 mOsm/kg, the resulting curve is well off the assumed result.
- Ideally, the resultant calibration curve should be linear, but even small errors can have a drastic effect.
- If the instrument is calibrated in this situation, any measurements made with it will be inaccurate, until it has been recalibrated with the correct values.
- Sample Preparation TechniqueAs with normal testing, you should always use good techniques for sample preparation. This is essential for optimizing the resultant performance through calibration. A similar situation to the one discussed above can result if the sample itself has not been prepared properly for the calibration.
Good sample preparation techniques can help eliminate a number of sources of possible inaccuracies and contamination in the sample; some of which include:
- Pipetting different volumes of the sample.
- The presence of air bubbles in the sample.
- Inconsistencies resulting from evaporation (which is caused by preparing samples too early).
All of these situations can cause more variations in the results obtained from the equipment calibration process. The increase in number and scale of the variations can cause the mean values obtained through calibration to vary significantly. The result would be that the calibration curve would erroneously shift and the errors in all the results would increase.
- Ambient Temperature Effects
Even when you perform calibration of instruments using the correct values, reliable calibrators with the correct tolerance and the correct sample preparation technique, errors can still result from other factors. Environmental factors, like the temperature of the surroundings, can have a huge impact on the results of the calibration:
- Instruments should be calibrated in an environment where factors that can affect the performance, like temperature, pressure and humidity, are closest to those of the surroundings it is operated in.
- Variations and differences in operating temperature can affect the performance of electrical and other components.
- Instruments calibrated at one particular temperature, or in fluctuating temperatures may be prone to temperature-induced errors if it is operated in a significantly different environment. This can degrade the accuracy of the calibration results.
How Frequently Should An Instrument Be Calibrated?
There’s rarely a set ‘perfect’ calibration frequency for any instrument, since there are a number of factors that need to be taken into account while designing a calibration regimen. This means that the correct frequency can often only be described as – “as and when it’s needed”.
You can create a history for different instruments by tracking the changes in measurements of a known value and by comparing the “as-found” and “as-left” results of each calibration.
- Of all the things you should consider, the most weightage will probably be given to just how much of an effect the instrument in question has on the overall quality.
- A close second would be the manufacturer’s recommendations and the instrument’s tendency to ‘drift’ out of calibration.
- Recalibration may be warranted after any events that could throw the precision or accuracy. Such as an electrical fault, a fall, or other impacts.
- Another time when you may need to perform an unscheduled calibration is just before a particularly important measurement is made.
- Calibrating an instrument each time you plan to use it, just to check its performance isn’t always practical and it can get very expensive, very quickly.
- Control solutions with known values can be tested every day or periodically, which can provide an indication of the performance and establish a history.
- If the results from the control data do not indicate any issues or inaccuracies in the instrument’s performance, then you can continue using it till the next scheduled calibration.
Slight variations between measurements are to be expected, and will occur despite your best precautions. As long as they fall within the limits of the acceptable range for errors, there’s no reason to perform an unscheduled re-calibration.
However, if it looks like the measurements are close to the limits of, or beyond the acceptable performance criteria, it might be a good idea to calibrate it. This is also true of significant short-term shifts (like while operating the instrument in different environmental conditions).
When you’re designing a calibration routine, it’s extremely important to take into account any regulations governing your field of operations. Check the requirements of quality compliance organizations, as well as specific standard operating procedures for laboratories and government regulatory authorities.
These may require instruments to be re-calibrated even if there’s no evidence that it is needed. The requirements should be followed nonetheless and should always be given precedence over all else. Any issued guidelines can be used at times, especially if you’re unsure about whether an instrument needs to be calibrated to improve accuracy.