Emerson Exchange 365

To Calibrate or Not to Calibrate Pressure Transmitters? The Cost Saving Answer.

Manufacturers of pressure transmitters generally do not explicitly recommend calibration intervals in product literature. Estimates can be extracted from published specifications with application specific data in hand. These estimates can be especially powerful when compared with historic field calibration results.

How can estimated calibration intervals be extracted from published specifications?

The translation of published specifications to an estimated calibration interval is a 5-step process:

1. Determine the installed performance required for the application
2. Define the operating conditions
3. Calculate the total probable error, TPE, based on published specifications
4. Identify the appropriate stability specification
5. Calculate the estimated calibration interval

Where to start?

First, identify the required performance for the measurement point. Typically pressure transmitters are required to deliver anywhere from ± 0.5% to 2% of calibrated span installed performance. Installed performance considers the impact of ambient temperature and elevated static pressure on a differential pressure transmitter which can add significant uncertainty relative to the reference accuracy of the device.

Let’s walk through an example using the 5-step approach…

1. The application requires ± 0.5% of calibrated span installed performance. The device will be reading 250 inH2O differential pressure under normal operating conditions, so this converts to a required installed performance of ± 1.25 inH2O.
2. The device will be exposed to 1400 psig static pressure with an expected ambient temperature variation of ± 50°F.
3. Start with a Rosemount 3051S Ultra performance class transmitter. A range 3 sensor with a 1000 inH2O upper range limit, URL, will give flexibility to handle variations in measured differential pressure. A search of the Rosemount 3051S Product Data Sheet provides the necessary performance specifications.

                                    Reference Accuracy = ± 0.025% of span

                                    Ambient Temperature Effect [0.009*(1000/250)+0.025] % of span

                                    Calculated Ambient Temperature Effect of ± 0.061% of span

                                    Static Pressure Effect (± 0.025% URL per 1000 psi) = ± 0.035% of span

            A root sum of squares approach combining these uncertainties yields

            ± 0.075% of span which converts to a TPE = ± 0.187 inH20.

      4.  The stability specification for the Rosemount 3051S Ultra performance class is ± 0.15% of URL for 15 years. With a 1000 inH2O URL, the specification can be translated to ± 1.5 inH2O for 15 years, or ± 0.1 inH2O per year.

      5.  The estimated calibration interval is calculated by dividing the difference between required performance and TPE by the stability of the transmitter.

                                                     (1.25 - 0.187) inH2O / 0.1 inH2O per year = 10.6 years

Using all the same application details with only a change to the performance class of the device shows how transmitter selection impacts the expected calibration interval.

Higher performance class transmitters can reduce measurement uncertainties, extend calibration intervals, and deliver calibration maintenance cost savings that offset the initial purchase price increase. These estimated calibration intervals can also be compared with field calibration data and serve as justification for extending calibration intervals to more closely match the stability and installed performance of today’s pressure measurement technology.