Instrumentation Calibration

We recently surveyed more than 2,000 of our customers regarding control system issues that were important to them in their job performance, and 75 percent of respondents indicated interest in instrument calibration issues. Therefore, our first in a series of monthly posts will address this important, but often overlooked subject. First let’s understand what calibration is. ISA defines it as “a test during which known values of measure are applied to the transducer and corresponding output readings are recorded under specified conditions.” In essence, comparing an instrument with a unified standard to verify its accuracy and to make adjustments to meet the standard. Why do you need to calibrate instrumentation in your plant? In some cases, particularly clean water treatment applications, there are regulatory requirements regarding flow and condition of the plant’s effluent. These can include analytical instruments such as  turbidity and suspended solids, among others. Inaccurate instrument readings can affect your compliance and leave you vulnerable to fines. Regulatory specifics vary from state to state, so you will need to check with your state’s Department of Environmental Management to verify those measurements applicable to your situation. In other situations, particularly industrial applications, processes have stringent performance criteria for such measurements as pressure and temperature. Inaccurate readings can affect product quality, with potentially huge financial impacts. Some processes require a gas analyzer to measure combustibility, and errors in such instruments can result in safety hazards. All new instruments are calibrated to assure proper performance, but over time error, or drift, can occur for any of a variety of reasons. These can include environment changes, addition of components to the output loop, variations in the electrical supply, or changes to the process. We recommend that every plant using control instrumentation establish a periodic asset management schedule, including instrument calibration. Most process industries perform instrument calibration annually, though some industries, such as pharmaceutical, are more likely to perform calibration twice a year. It is also possible, using calibration trending history software, to identify those instruments in a plant that tend to be very stable, requiring less frequent calibration, and those that exhibit greater drift, thus requiring more frequent attention. Our ISA CCST-certified field service technicians can perform calibration on an extensive array of instrumentation.