Every measuring instrument needs calibration but not all measuring instruments are required to be calibrated. Below are some of the reasons or criteria to consider before having an instrument calibrated. This may save you some time and money.

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1. It is not critical in your process ( just to display a certain reading for the purpose of functionality check).

2. It functions as an indicator only (for example: high or low and close or open).
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3. As an accessory only to support the main instrument. For example, a coil of wire is used to amplify current. Current is measured, but the amplification is not that critical, used as an accessory only to amplify a measured current.
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4. Its accuracy is established by a higher or reference to a higher or more accurate instrument within a group. (for example a set of pressure gauges that are connected in series in which one of them is a more accurate gauge where they are compared or referenced to).
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5. If the instruments are verified regularly or continuously monitored by a calibrated instrument that is documented in a measurement assurance process. For example, a room thermometer that is verified by a  calibrated thermometer regularly.
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6. If the instrument is a part of a system or integrated into a system where the system is calibrated as a whole. For example, a thermocouple that is permanently connected to the oven (some thermocouples are detached after usage and then transferred to other units).

7.