Calibrating instruments: the basics explained

'To measuring is to know' is an important principle in many industries. In our data-driven society, a lot is being measured. Therefore, the number of sensors and measurement equipment used is increasing every year. To guarantee quality and safety, it is important that the measured data is accurate. To ensure a high degree of data reliability, (regular) calibration of measurement equipment and sensors is a must.

Calibration is a process that requires careful attention and is based on a number of important principles. In this blog you will read all about the basics of calibration through a number of terms and concepts that are an integral part of the calibration process.

What is calibration?

To calibrate is to compare the output of the instrument to be calibrated with the values of a proven accurate reference instrument. This way it can be determined whether the measuring instrument does not deviate too much and is suitable to perform measurements. 

For example, suppose you want to measure the pressure in a system with a pressure sensor, in order to monitor the quality of the process. You have selected a sensor with an (in)accuracy of 0.1%. During the calibration you can compare the measurements of this sensor with a reference sensor or pressure calibrator that, for example, has an accuracy of 0.01%. By comparing the pressure sensor with the reference at several measuring points, for example 0, 2, 4 6, 8 and 10 bar, you establish the deviation. This is a calibration. Based on the determined deviation (the calibration result) you can then determine whether the sensor is still accurate enough for its purpose. The same principle applies when calibrating other types of instruments, such as calipers, torque wrenches, etc.

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Calibrating, gauging and adjusting

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Gauging and adjusting are two terms that are related to calibration. Sometimes terms are used interchangeably as if they were synonyms, but that is not entirely correct. In the case of gauging, as with calibration, you determine whether and how far an instrument deviates from a certain standard. From a technical point of view, gauging often involves the same work as a calibration.

The difference is that a gauging is an inspection in which a certifying body determines whether a meter meets the legal requirements. Is the instrument rejected? Then it may no longer be used for the application described in the law. For example, a speedometer used by the police to determine violations must be calibrated by law, therefore this calibration can be called a gauging. 

Calibrations in general are not prescribed by law. However, there may be indirect legal consequences related to a calibration. Consider the following example: A company signed a contract to deliver a product of a certain quality. If a dispute arises, one can be asked to prove that the product meets the quality criteria mentioned in the contract. A recent calibration report can be used to prove that the instruments used for these measurements are functioning well and can be used for this purpose.

Adjusting is the act of altering or correcting a measuring instrument after you have determined that the device is outside the desired specifications. Adjusting is therefore often part of the calibration process and takes place after the initial (or as found) calibration. After an instrument has been adjusted/corrected, you can recalibrate it to determine the deviation after adjustment.

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The calibration plan

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Good preparation is also essential for calibration. A well thought out calibration plan forms the basis for a good calibration process. Such a plan describes, among other things, the chosen calibration method, which reference instrument you are going to use, which measurement points are to be used and how often you have to calibrate a certain instrument. And what requirements there are for the calibration technician. It is also good to record the reason for calibration. Is this an internal requirement from your own quality system or are there external factors, such as legislation and regulations or agreements with a customer or other (third) party. By recording this, it is easier to determine what should be done when, for example, an instrument deviates too much or when the calibration has expired.

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The calibration interval

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Another important term is the calibration interval. How often will you calibrate an instrument to ensure the reliability of measurements? The answer to that question depends on several factors. When determining the calibration interval, consider the following 3 points: 

• Technical factors: for example, if a measuring instrument naturally exhibits a certain drift, you can determine the calibration interval based on this. The degree to which the instrument is used also plays a role in this.
• Impact if the instrument is out of spec: if an instrument is out of spec, and you only find out during calibration, what would be the impact? Do products have to be recalled, are previous measurement results no longer valid or has your process been less efficient?
• Practical matters: you don't just take an installation out of operation and a mechanic also needs his instruments to be able to do his job. The chosen calibration interval must also be practically feasible.
  

Technical factors, such as drift, are often mentioned in the data sheet or manual of the instrument. This does not include the influence of use. This can only be estimated by gaining experience. With new measuring instruments it is wise to follow at least the calibration interval indicated in the manual of the device. After a number of calibrations you can, depending on the determined deviation and stability of the measuring instrument, extend or shorten the interval step by step. 

Determining the right calibration frequency is all about carefully weighing costs and risks. 'Better safe than sorry' is usually the best motto. If an instrument is calibrated regularly, a deviation will be detected earlier and the deviation will also usually be smaller. The risk of having to recall products or redo measurements will therefore be lower if calibration is done more frequently. The cost of this type of measure quickly exceeds the cost of performing regular calibrations. The accuracy of a sensor can also affect operational costs. For example, if a temperature sensor indicates a value that is too low, more energy will be used for heating, etc.

If a chosen interval is not practical, in practice,  it will lead to non-compliance. In some cases, it is good to extend or shorten an interval so that instruments can be calibrated at a time when they are not in use. If a machine is down due to a breakdown or planned maintenance, this can be a good time to calibrate the instruments without incurring additional costs due to downtime. By defining the leeway on the calibration period, you can schedule calibrations at a convenient time.

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Automation & data collection

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Performing calibrations and keeping track of all documentation can be time consuming and labor-intensive. When calibrations are done manually it is also prone to errors.

Working with calibration software helps to handle calibrations efficiently and accurately. You can automate almost the entire process and have one central information platform where all your calibration data and activities can be found. Manual calculations and manual data entry are replaced by automated workflows that allow you to analyze, retrieve and check calibration data without having to go through a complicated search path. 

Creating a historical overview of your calibration data is much easier with calibration software.. You know which calibrations have been performed and which are still on the program, just by taking a glance into the dashboard . An overview of the data from each individual calibration is immediately available. This also makes it easy to analyze the trends of different measurement instruments and provides you with the necessary insights to optimize your calibration process. The software not only allows you to perform calibrations, but also to record your calibration plan and process.
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AutoCal+ 

Do you want to automate your calibration process? AutoCal+ is the solution for you.

• AutoCal+ supports more than 30 quantities and units, allowing you to use the software to calibrate almost any instrument. Drivers are available for some popular calibration instruments to fully automate the calibration.
• With AutoCal+ you have a complete overview of all your instruments in one system. Accuracies and calibration intervals are stored in the system.
• AutoCal+ subscriptions are flexible. Does your usage change over time? Simply add or delete users or instruments or change your subscription. 
• AutoCal+ is available starting at € 39.50 per month. This makes it an affordable and cost-effective solution. The benefits of calibration software ensure a quick return on investment.

Would you like to know more about AutoCal+? Feel free to call us at +31 79 203 31 33, fill in the contact form or start a free trial.