BENCH (Workshop, Laboratory) Calibration.

When Calibrations are being performed in a laboratory or workshop after removing the process instrument from its original field location, it comes under the banner of Bench Calibration. To minimize process downtime, spare pieces of equipment can be installed in place to that Instrument which is disconnected for bench calibration. In bench calibration, power gets supplied to an instrument from an external source, if required.

Benefits of Bench Calibration ​

Can be performed in a workshop prior to commissioning which can provide assurance regarding the instrument’s working condition.

Saves a lot of time in comparison to the field calibration which needs to be performed once the instrument is installed to its intended location within the process.

Since bench calibration is performed under controlled environmental conditions with more accurate calibration equipment, less total uncertainty can be achieved.

Consistency in bench calibration can be achieved in a more decent way because of the well written operating procedures.

Safety-related issues can be addresses easily within the workshop

Primary standards can be used in the workshop to calibrate portable working standards in the house.

Access to various calibration equipment is easy within the workshop.

Dedicated Workshop or Laboratory for calibration can be beneficial in applying for accreditation to ISO/IEC 17025 as total uncertainty can be lessened within controlled conditions.

Individual Instrument calibration Vs. Loop calibration

When calibration gets performed by disconnecting the Input and Output, it comes under the banner of Individual Instrument calibration.

Throughout the instrument’s calibration range, the output can be measured on various data points by applying a known value of the input.

By doing so, the correct instrument can be adjusted.

Since calibration is getting performed on the individual instrument so compatibility with multifunction calibrators can be achieved.

It takes way too much time to check each instrument independently in comparison to Loop testing in which all loop instruments can be checked altogether.

When a calibration of the set of instruments gets performed together within a single loop, it comes under the banner of Loop Calibration.

Pass / Fail Calibration status can be determined by observing the relationship between Input and Output values of a complete loop and not individual instruments within it.

The entire loop (Sensor and Loop components) can be verified against tolerance.

Since calibration is getting performed in a loop so compatibility with multifunction calibrators can be compromised.

It doesn’t take too much time as we don’t need to check each instrument separately.

Figure: 1 Source Maximo Calibration.

Note: RTD or Thermocouple as temperature sensing element senses the temperature and sends the signal in Ohms and Millivolts respectively to temperature transmitter which in turn transmits milliamperes signals to local display with which it is connected and finally to DCS. The signal is displayed on display units which is used to control the process.

End to End Loop Testing Vs. Portion of Loop