FEATURE CALIBRATION TOP TIPS FOR CALCULATING CALIBRATION FREQUENCY
In order to optimise process performance we need to ensure measurement and analytics equipment is working accurately. David Bowers, product specialist for pressure and process flow for ABB UK Measurement & Analytics, explains the importance of calibration in promoting good practice and profitability
A
t a time when companies across all industries are under financial
pressure, everyone is looking to squeeze the maximum value that they can from their assets. We all know that you can’t manage what you can’t measure, so maintaining an accurate monitoring and control regime is essential. That’s just one reason why it’s the right time to think about how well your calibration regime is supporting your business. Properly scheduled and effective
calibration ensures that the readings emerging from instrumentation are accurate. Without it, measurements are essentially meaningless. 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 aging from any number of factors, such as mechanical stresses or temperature fluctuations.
THE BOTTOM LINE Ensuring instrumentation is calibrated has an impact on the bottom line. However, the impact varies depending on the business. While flow meters carrying out fiscal measurements on petroleum products may need to be accurate to ±0.025%, a meter controlling the level in a tank used for washing down machinery may only need to achieve ±10%. It’s about optimising the process to
generate the most income from the available assets, whatever your 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. Alongside process optimisation there is
growing pressure for more measurement accuracy emerging from the legislative arena too. Environmental Permitting Regulations (EPR) require companies in the process and water industries to maintain effective monitoring of their emissions to air and water.
8 APRIL 2020 | PROCESS & CONTROL Increasingly, EPR compliance requires
plant operators to use monitoring equipment that has been officially approved under the MCERTS scheme and ensure regular calibration.
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. As a guide, instruments should be calibrated when they are first installed, as per manufacturer guidelines, when a physical shock or vibration could cause damage, following severe weather or sudden changes in weather, or when the output seems doubtful. The calibration frequency will also
depend on how critical it is to the process. If high performance and accuracy is crucial to production, then the instrument should be checked regularly. In safety critical applications,
companies may need to have their pressure transmitters checked every 12 months in a SIL1 application or every three months in a SIL3 application, this frequency is determined by the target reliability required. When calculating the calibration
frequency that works for your business, this five-stage process designed for pressure transmitters is a useful guide: 1. Determine the performance
required for the application. Some applications have a direct bearing on safety or plant efficiency and therefore accurate readings in the order of 0.5% of span or less. This will normally increase the required calibration frequency. If all that is needed is to indicate a
Properly scheduled and effective calibration ensures that the readings emerging from instrumentation are accurate. Without it, measurements are essentially meaningless
performance figure of 10% of span, the device need never be calibrated at all. 2. Determine the operating
conditions. Operating conditions such as static pressure, the ambient temperature and vibrations are another vital aspect. These conditions will each have an associated error figure. 3. Calculate the TPE (Total Probable
Error). This is determined by a formula which incorporates terms for the quoted base accuracy of the device and the likely effects of static pressure and temperature errors on performance accuracy. 4. Determine the stability for a
month. This data should be provided by the vendor. Normally the stability will be expressed for a given time period e.g. 36 months. 5. Calculate the calibration
frequency. 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.
Ensuring instrumentation is calibrated has an impact on the bottom line, and is key to optimising the process to generate the most income from the available assets
WHO CAN HELP? It’s advisable to stick to using accredited calibration services. In Britain calibration services are accredited by The United Kingdom Accreditation Service (UKAS) and is the sole accreditation body recognised by government to assess, against internationally agreed standards. ABB offers a range of pressure
measurement equipment, including a portfolio of sensors and transmitters, backed up by expert service and support. For more advice on how, when and why
to calibrate your pressure transmitter equipment, email
enquiries.mp.uk@
gb.abb.comor call 0870 600 6122 ref. ‘pressure transmitter calibration’.
ABB
www.new.abb.com
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