Getting the best performance from pressure transmitter ­equipment is a more involved process than many would have you believe. Trevor Dunger, product specialist for pressure and level for ABB’s UK Measurement Products business, explains the importance of calibration in promoting good practice and profitability

While companies across all industries are under financial pressure, everyone is looking to squeeze maximum value out of their assets. Maintaining an accurate monitoring and control regime is one reason to think about how well your calibration regime supports your business.

Properly scheduled and effective calibration ensures the readings from instrumentation are accurate. Without it, measurements can be meaningless. Maintaining an effective monitoring regime is not a ‘fit it and forget it’ job. Periodic calibration is essential to accuracy of instruments.

This may be self-evident in some types of instrument, such as pH meters that rely on the sort of wet chemistry that can easily be disturbed. But even seemingly robust, solid state sensor elements such as thermocouples rely on electronics to relay their readings to plant control systems and electronic components are subject to ageing from any number of factors, such as mechanical stresses or temperature fluctuations. The same applies to the electronics of input cards, which provide the signals to controllers, so these should also be calibrated regularly.

The bottom line

Some process measurements are more critical than others. So while flow meters carrying out fiscal measurements on petroleum products may need to be accurate to ±0.025 per cent, a meter controlling the level in a water tank used for washing down machinery may only need to achieve five per cent. Other critical examples include monitoring the alcohol levels in beverages to determine the excise duty, or blending tiny amounts of expensive ingredients such as perfume essences with cheaper ones such as ethanol. Essentially it’s about optimising the process to generate the most income from the available assets, whatever the business. The ultimate goal might be increased production yields, reduced product give-away and over-charges, more consistent product quality and/or reduced product liability.

Legislative pressure

There is also growing pressure for measurement accuracy from the legislative arena. Environmental monitoring under the Environmental Permitting Regulations (EPR) is an example, with companies in the process and water industries now required to establish and maintain effective monitoring of their emissions to air and water. A slip up and companies could face prosecution, fines or lose a licence to operate.

Technological progress

Improving technology is also playing a role in driving calibration standards, because more accurate calibration is called for as the measurement accuracy of the instruments being tested improves. The existing IEC 61298 standard states the measuring equipment for process measurement and control devices should display a measurement uncertainty that is four times better than that of the device being calibrated. This is increasingly difficult to achieve, as the test equipment being calibrated might be just as accurate. The result is that calibration can be a difficult and expensive challenge. The cost for supporting some plant-based instrumentation can be 10 per cent of the original purchase price on a yearly basis. Nevertheless, calibration is essential, so the key is to get the right regime in place to ensure that the investment in testing is not wasted.

When should you calibrate?

The right time to calibrate a specific piece of equipment will depend on the type of instrument and the nature of the job it’s doing. There are some common situations such as; with a new instrument; at time intervals specified by the manufacturer or regulator; after a specified number of operating hours or cycles; when an instrument has had a physical shock or vibration; following sudden weather changes; whenever the output appears doubtful.

Look beyond the hype

Depending on the application, the accuracy needed, the ambient conditions and a range of other factors, the calibration frequency of a device may differ from its manufacturer’s recommendation. The user can calculate the calibration frequency himself by taking his particular conditions into account. The calibration frequency depends on three things: the application of the device, the performance the user needs from it and the conditions it will operate in.

The calibration frequency is given by desired performance minus the total probable error, divided by the stability per month. This determines the frequency with which the calibration needs to be checked in order to maintain the desired accuracy.

Who can help?

Calibration is all about standards, so it’s advisable to stick to using accredited calibration services. Suppliers and laboratories offering calibration services in Britain are accredited by The United Kingdom Accreditation Service (UKAS).