What is calibration?
Calibration is the process of comparing the reading of one instrument or equipment with a standard instrument. The reference instrument is already calibrated and referenced to a known set of parameters. The reference instrument should itself be directly traceable to equipment that is calibrated. Calibration is done to minimise any type of uncertainty in measurement by ensuring the accuracy of test equipment. For most standard organisations accuracy ratio will be 3:1. The main objective of calibration is checking the accuracy of the given instrument and determining the traceability of measurement.
The above diagram shows a micrometer. The main parameter for calibration is the accuracy of the scale. A calibrated slip gauge is used for calibrating the scale. For checking flatness and parallelism a calibrated optical flat is used.
What are calibration standards?
In simple words, calibration is the process of matching the standard value of the instrument to the master value (true value) of the instrument. IIM & TE item is taken as standard reference for calibration.
The below table shows the different valve type and common test standard:
| VALVE TYPE | COMMON TEST STANDARD |
| Cast Iron gates | API 598, MSS SP-70* |
| Control valves | ISA-S75, FCI 70-2 |
| Pressure relief valves | ASME PTC 25, API 527 |
| Cryogenic valves | BS 6364, API 598 |
| Steel ball valves | API 598 |
| Steel butterfly valves | API 598 |
| Cast iron checks | API 598, MSS SP-71* |
| Cast iron plugs | API 598, MSS SP-78* |
| Bronze gates, globes and checks | MSS SP-80* |
| Pressure seal valves | ASME B16.34* |
| Steel valves larger than NPS 24” | ASME B16.34* |
| Pipeline valves | API 6D* |
| Cast iron globes | API 598, MSS SP-85* |
What is calibration point?
Calibration point is a single source for calibration of wide range of instruments in electronics traceable to NABL (NATIONAL ACCREDITATION BOARD FOR TESTING AND CALIBRATION LABORATORIES).
What is calibration error?
Most calibration errors are some combination of hysteresis, zero, span, linearity errors. The zero error always accompany with other type of errors. The efficient identification and correction of instrument calibration errors are important. A hysteresis calibration error occurs when the response of the instrument become different to the increasing input compared to the decreasing input. Hysteresis errors are always caused by some mechanical friction on moving element such as bellows, levers, bourdon tube etc. The output of an instrument with hysteresis problem always lags behind the changing input, because friction always acts in a direction opposite to relative motion.Technicians, perform a single-point calibration test as a qualitative indication of its calibration. If the instrument performs within the specification, then it is good. If not, the instrument definitely need to be re-calibrated.
Why calibration is important?
Due to normal wear and tear, the accuracy of almost all measurement devices decreases. The accuracy may degrade over time to time. Also a hazardous manufacturing surrounding may cause damage to the accuracy of the instrument, sometimes it may be degraded due to long use. In short,these all factors results in improper functioning of the manufacturing unit. The quality of the products also reduced if there is no accuracy in measurement devices. For proper functioning, calibration is an important process to be done regularly.
How frequently we need to calibrate instruments?
A measuring device/instrument should be calibrated according to the recommendation of the manufacturer. It should be calibrated periodically, that is annually,quarterly or monthly. Mechanical or electrical shock may happen to measuring device, calibration should be done after any such kind of incident occurs to the measuring device. As compared with the cost of calibration, hidden costs and risks associated with the uncalibrated device is high. So it is better to calibrate the instrument at regular intervals as recommended by the manufacturer to reduce the error and to increase the accuracy of measurements.
What is the difference between QC and Calibration?
Qc (Quality Control) make sure that the instrument is working properly. While calibrators give a reference point to adjust the instrument. If the working is improper; if QC is out of the specified range, calibrators are used to adjust the instrument to make it into the specified range.
What are the advantages of calibration?
- Calibration is proof that the instrument is working properly.
- Increases the confidence of instrument user.
- Calibration fulfils the requirement of traceability.
- Increases power saving and cost saving.
- Reduced rejection and failure rate, hence gives higher productivity.
- Interchangeability.
- The improved product quality and service quality leading to satisfied customers.
- Increases safety.
These are the benefits or advantages of calibration.
