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What is calibration, and why do it?

There is no such thing as a perfectly accurate measurement instrument, no matter how expensive or high end. Every measurement device will have some amount of error. The allowable measurement error is normally specified by the manufacturer as its “Accuracy”. For example, a temperature data logger might be offered with an accuracy of ±0.5°C. This means that whatever temperature the logger records, it is guaranteed to be within 0.5 degrees Celsius above or below the real temperature that it’s trying to measure.

LITE5032L-EXT MicroLite Temperature Data Logger with External Probe

Photo courtesy of Fourtec

This is perfectly OK for the purposes of most instruments. We don’t need to have “perfect” measurements in order for them to be useful. For example if a data logger is recording the temperature inside a vaccine fridge, we might need to be sure that the vaccines were stored between 2 and 8 degrees Celsius at all times. So, using our example logger with accuracy ±0.5°C, as long as the logger recorded temperatures between 2.5 and 7.5 degrees C, we can be very confident that our vaccines are safe. 

However, as time goes by, instruments can “drift”, meaning that their measurements change compared to what they measured when new. Continuing the above example, a data logger that measured 0.2°C below the real temperature when new, might gradually drift over a period of 2 years until it’s eventually measuring 0.7°C below the real temperature, which is outside its accuracy specification. The end result of this could be that we think our vaccine fridge is safely running at 7.5°C (as reported by our logger) but in reality it’s running at 8.2°C, and our vaccines are no longer safe. In the case of vaccine storage, this could have serious health and economic implications. We can see from this example how important it is to get instruments checked regularly, to make sure they remain within their accuracy specification.

Calibration is the procedure that checks your measurement equipment, to make sure it is measuring correctly within its rated accuracy specification. The procedure may also include adjusting the instrument to bring it within the specification. This is normally done by a specialised laboratory using equipment that has already been calibrated to national standards. It is only by arranging regular calibration checks, that we can stay confident in the measurements made by our instruments. 

How often should I calibrate my equipment?

It is best to arrange equipment to be calibrated on a regular schedule, to make sure it stays within manufacturer specifications. The best time period between calibration checks depends on many factors including:

  • The nature of the sensor and expected drift: Modern temperature instruments & sensors tend to be relatively stable, but other sensor types such as humidity / CO2 / water quality /etc tend to drift more over time.
  • The importance of the measurements’ accuracy to the user: Some users require very accurate measurements to guarantee their processes are valid; others only want a rough indication and don’t need high accuracy.
  • The recommendations of the manufacturers: Manufacturers often give a guideline as to how often calibration should be checked.
  • The environment in which the sensors are used: Harsh environments with dust / condensation / chemicals / temperature extremes (etc) will cause sensors to drift faster than benign environments.
  • Regulatory requirements: Sometimes government regulations require that instruments must be calibrated at particular intervals, eg. US FDA guidelines.

The most common interval for calibration checks is 1 year, as it is short enough to capture the drift of most common sensor types, but long enough to minimise costs and down time. However it is ultimately up to you to decide what calibration interval works best for you. If you’re not sure, give us a call to discuss.

It is also a good idea to review the calibration interval from time to time: whenever you send a device to get calibrated, you will receive a report showing how far it has drifted from the true measurement. Hence you can see how the measurements are changing over time and decide whether to shorten or lengthen the calibration interval.

How is calibration done?

Calibration can be done by sending your equipment to a calibration lab, or by arranging an on-site calibration by a specialist company. You may also be able to perform basic calibration checks yourself if you have the required reference equipment, and know the correct procedures. However in most cases it’s best to leave calibration to the experts, as there are many pitfalls and mistakes that could result in invalid measurements.

Most people send their equipment to a lab for calibration. This means taking the equipment out of service for a week or more. If the equipment is critical, it is common for a second device to be purchased so that there’s always one available even when the other device has been sent for calibration.

DENT TCA-5 Electrical Current Transducer

Photo courtesy of DENT Instruments

Within the lab, trained technicians do the calibration procedures, which depend very much on what the equipment has to measure, plus other factors. For example, temperature can be calibrated by placing the temperature sensors in a water bath or temperature chamber, allowing them to stabilise, and measuring the same bath / chamber with a calibrated reference instrument at the same time. Humidity calibrations may be done in a humidity chamber, or in a small chamber with special salts that maintain a particular humidity. Electrical calibrations are done with specialised electrical equipment, and so on. 

The calibration checks may be done at one, two or three different measurement points, to confirm the instrument measures correctly over a particular range. For example you might calibrate a temperature logger at 0, 25 and 50 degrees Celsius – this would give you confidence that the logger is measuring correctly for all temperatures between 0 and 50 degrees C.

When the technician performs the calibration checks, it is important to know the technical limitations and pitfalls of the procedure, such as how long it will take for a sensor to stabilise to the surrounding temperature, before a measurement can be taken.  Each kind of measurement has its own potential issues, and rigorous procedures must be followed to make sure the calibration is done correctly.

All reference equipment that’s used to calibrate devices, will also need to have been calibrated against a higher standard. Ultimately it’s up to the national standards reference body to maintain the official measurement standards. In Australia this is the National Measurement Institute (NMI). Hence, there must be a traceable chain of calibrations of all equipment, back to the NMI, to make sure that every calibration is valid. If any instrument along this chain has gone outside its calibration period, then all measurements taken by it are not valid, nor are any calibrations done from it.

NATA Accredited vs. Traceable Calibrations

One of the most misunderstood things about calibration is the concept of “NATA Accredited” or “NATA Certificates”.

The job of calibrating equipment is highly specialised and requires scientific training and strictly followed procedures. If a technician does not follow the correct procedure (eg. not leaving a temperature probe long enough to stabilise) then the results of calibration are completely useless. For this reason, NATA (National Association of Testing Authorities) runs an accreditation program, which gives us confidence in each calibration lab’s procedures and equipment. For a calibration lab to become NATA Accredited, it must undergo regular assessment by NATA to make sure that all technical staff are properly trained, procedures are documented and strictly followed, and all equipment is kept in calibration.

NATA Offices in Sydney
In addition, NATA accreditation is always done for specific types of measurements. For example a lab may be NATA accredited for performing temperature calibration, but not for humidity. This is an important thing to look out for when choosing a NATA accredited lab: are they accredited for the actual measurements you need calibrated?

There are also a number of calibration labs that are not NATA accredited, but still perform the calibration services in a professional and competent way. These labs normally provide calibration certificates that are “Traceable to National Standards”, meaning that the equipment they used to calibrate with, has been itself calibrated in a way that can be traced back to the NMI. However there is no third party accreditation to prove that their technicians performed the calibration work correctly. This kind of lab will generally charge less for calibration services, as they don’t have the overhead costs of undergoing regular NATA assessment and audits.

Hence when you need your equipment calibrated, you will need to choose between getting a NATA Accredited lab to do it, or just opt for a “Traceable” calibration. The decision will depend on several factors, including:

  • What budget is available for calibration;
  • How critical the measurements are;
  • Any mandated requirements your organisation has, for calibrations to be from NATA accredited labs;
  • Your level of confidence in the procedures and skills of the technicians in the non-accredited labs.

7 Point Calibration Checklist

When you purchase and use measurement equipment, it is important to decide on your calibration approach right from the outset. The points you need to consider are as follows:

  1. What measurements are you using that need to be checked for calibration regularly? (eg. temperature, humidity)
  2. What accuracy is required, and over what range of measurement? (eg. ±0.5°C accuracy needed between 0 and 10 degrees C)
  3. What are the real world consequences of the equipment drifting out of calibration? (eg. a whole fridge full of medicines & vaccines may need to be discarded, and people’s health could be harmed)
  4. What is the best calibration interval for your equipment? If you’re not sure, start with a 12 month interval. If you know the sensors are exposed to a harsh environment such as condensation or harsh chemicals, opt for 6 months or even 3 months for a very harsh environment. After a couple of calibration checks, observe how much the measurements have drifted over time, and lengthen or shorten your calibration period accordingly.
  5. Do you require a NATA accredited lab to do the calibration? Or is it adequate to just use a lower cost lab to do a “traceable” calibration check? If you need NATA accreditation, make sure the lab you use is actually accredited for the measurement you’re getting calibrated.
  6. What are the key measurement points that you MUST be certain the measurements are accurate? eg. if you need to keep goods between 2 and 8 degrees, the 2 and 8 degree points are critical to be correct. When requesting calibration, specify that these points must be calibrated.
  7. Do you need to purchase additional sensors or instruments, to cover the period when your main instrument is away for calibration?

If you’re not sure about any of these points, the first thing to do is have a chat with your equipment supplier to find out more. ESIS’ engineers are always happy to talk through the calibration requirements with you when you purchase equipment through us. ESIS can also arrange for calibration of your equipment when needed.

ESIS is the leading supplier of industrial measurement and data logging equipment in Australia since 1971. Contact us to discuss your measurement and calibration needs today.

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