pressure transmitter calibration equipment endress hauser

Basics of Calibrating Pressure Transmitters Pressure transmitters need to be calibrated on a regular basis for maximum performance.

When do you do it How do you do it And who does it By Keith Riley, Ehren Kiker and Duane Muir, EndressHauser Pressure transmitters used in the process industries are very durable and reliable instruments.

Even so, they still require periodic maintenance and calibration to ensure optimal performance.

This is an area of confusion for many, with these and other questions typical: Are we calibrating our transmitters too often, resulting in excessive downtime and unnecessary mainte-nance expense Are we calibrating our transmitters too infrequently, resulting in quality issues and possible loss of product Are we calibrating our transmitters correctly As with most things in life, there is no one size fits all answer.

However, there are simple best-practice guidelines, which can be modified to fit specific applications.

This article helps answer the basic questions facing process plant personnel with regard to calibration.

How Often Each process plant has to determine correct calibration intervals based upon historical performance and process-related requirements.

Direct mounted pressure transmitters installed inside in a controlled environment on a process with stable conditions should be calibrated every four to six years.

Direct mounted pressure transmitters installed outside on a process with stable conditions should be calibrated every one to four years, depending upon ambient conditions.

If a remote diaphragm seal is employed on a pressure transmitter, the calibration interval should be reduced by a factor of two; i.e., a four to six year interval is reduced to two to three years.

This is because a remote diaphragm seal will employ more fill fluid than a direct mounted configuration.

Consequently it will experience more mechanical stress from process or ambient temperature fluctuations.

Most remote diaphragms are flush faced where the diaphragmmembrane is susceptible to physical damage (dents or abrasions) that can cause offset or linearity issues.

If the process regularly experiences significant pressure swings or over pressurization events, reducing the calibration interval by a factor of two is a good rule of thumb.

How Accurate How good is good enough In other words, what is the Maximum Permissible Error (MPE) for your calibration Many make the mistake of adopting the manufacturers reference accuracy as their calibration target.

Unfortunately, this means they will have a MPE that is too tight, with a high rate of non-conformance in their calibration process.

In the worst case with a very tight tolerance MPE, it may not be possible for their field or lab test equipment to calibrate some of their transmitters.

A manufacturers reference accuracy is based upon tightly controlled environmental conditions seldom if ever duplicated in a plant environment.

Using that reference accuracy for a calibration target also fails to take into account the long term stability of the instrument.

Over time, all instruments will experience slight accuracy degradation due to aging and simple wear and tear on mechanical components.

In general, unless there are mitigating circumstances, it is better to set a reasonable MPE achievable with standard field and lab test equipment.

The corresponding output is measured with a multimeter for a 4-20mA transmitter, or with a specialized device for smart transmitters with digital outputs such as HART, Foundation Fieldbus, Profibus or EtherNetIP.

The test equipment you intend to use should be traceable to the National Institute of Standards and Technology.

Pressure Transmitter Calibration Equipment Endress Hauser