Instrument Classification

Instrument classification categorizes instruments according to their function. Their classification acts as a reference when such instruments are selected, purchased, tested, and used because each may have different criteria. Such a classification typically covers the identification of critical (as they relate to safety, health, and the environment) and non-critical equipment. The classification may also consider the results due to loss of performance and loss of the instrument's required accuracy. Sometimes such a classification resides in a plant's quality assurance manual. It all depends on the way a plant does business and runs its operation. In this chapter, and as an example, instruments are divided into four classes. Plant management and maintenance personnel may decide that a different classification of instruments is required.

Class 1 Instruments

Class 1 instruments are the plant calibration standards. These types of instruments are used to calibrate Class 2 instruments and are generally traceable to an outside, nationally recognized standards or calibration organization. These instruments are kept in the maintenance shop in an environmentally controlled area that meets the manufacturer's specifications. Class 1 instruments are calibrated annually by an independent calibration lab. After each calibration, they are returned to the plant with a certificate approved by the calibration lab. They are typically sent and received back within 30 days of the anniversary due date. The anniversary date of these instruments is staggered to ensure the presence of a working and calibrated set in the maintenance shop at all times.
In the event that a Class 1 instrument is found out of tolerance, the calibration lab should immediately advise the plant. The maintenance supervisor at the plant will then assess the effect this out of tolerance may have had on Types 2, 3, and 4 instruments.
When the calibration equipment is received back, it is checked for obvious shipping damage. If there is damage, the equipment must not be used and must be immediately returned to the calibration lab for repair and recalibration.

Class 2 Instruments

Class 2 instruments are the plant instrument calibration standards. These instruments are used by maintenance personnel to calibrate Class 3 and 4 instruments. Class 2 instruments are calibrated semi-annually by plant maintenance using Class 1 instruments. In addition to the scheduled semi-annual check, calibration may be performed whenever the accuracy of a Class 2 instrument is questionable.  Calibration forms must be completed for each Class 2 instrument every time a calibration is performed. These forms are then signed by and filed with the supervisor of plant maintenance.

Class 3 Instruments

Class 3 instruments are critical process instruments that prevent situations that are either threatening to safety, health, or the environment or that have been defined as critical to plant operation or to product quality. The calibration frequency of Class 3 instruments is based on their required reliability—for critical trips, it is defined by the calculated Trip Testing frequency (T) (see chapter 10). Class 3 instruments may also require calibration when the instrument is replaced or when its accuracy is questioned (e.g., when its reading is compared with other indicators).
Calibration sheets are completed for each instrument every time a calibration is performed.
These sheets are then signed by and filed with the supervisor of plant maintenance.

Class 4 Instruments

Class 4 instruments are used for production and represent the majority of the instrumentation and control equipment in a plant.  Calibration of Class 4 instruments is done when required, when an instrument is replaced, or when the instrument's accuracy is questioned (e.g., when its reading is compared with other indicators). After a certain time in service, the records are checked to determine if this approach is adequate. Calibration sheets are completed for each instrument every time a calibration is performed, and these sheets are then signed by and filed with the supervisor of plant maintenance.

Source : The Condensed Handbook of Measurement and Control 3rd Edition