Transmission trouble?

Pressure transmitters are now ubiquitous throughout industry, measuring everything from pressure to flow rate and levels of liquids in tanks. In applications such as petrochemical plants, there may be thousands of pressure transmitters installed on just one site.

Of course, users of pressure transmitters want them to be accurate, particularly in safety critical applications. For a variety of reasons, no device will remain accurate indefinitely and eventually will need to be re-calibrated to make it give a true reading of actual conditions.

Yet, calibration of pressure transmitters is a skilled job requiring specialist equipment and is something that companies don't want to do too often, particularly if it means closing revenue earning plant. This is why so many pressure transmitter users are increasingly being attracted to devices which their vendors claim to have zero drift or only to require recalibration every five years.

On the face of it, such devices offer a convenient fit with the five-year plant maintenance cycles operated by many companies. How often you check your pressure transmitter calibration depends on how critical it is to the process.

If high performance and accuracy is crucial to production or health and safety, then the transmitter should be checked regularly. Some applications will have a financial implication, for instance measuring the flow rates of materials in the petrochemical industry for fiscal purposes.

In safety critical applications, companies may need to have their pressure transmitters checked typically every 12 months in a SIL1 application or every three months in a SIL3 application This frequency is determined by the target reliability required.

Can manufacturers' figures for calibration frequency really be believed? The answer has to be, 'possibly'. The figures quoted are based on a specific set of conditions for temperature and pressure, but these may have little to do with the conditions in a real plant.

Depending on the application, the accuracy needed, the ambient conditions and a range of other factors, the actual calibration frequency of a device may differ markedly from that quoted by its maker. It could be shorter. It could be a lot longer.

The user will only know if he takes his particular conditions into account and calculates the calibration frequency himself.

The calibration frequency of any pressure transmitter depends on three things: the application of the device, the performance the user needs from it and the conditions it will operate in.

Operators should follow a five-stage process to calculate calibration frequency:

<b>Determine the performance required</b>

Application is an important factor because it affects the accuracy needed. Some applications have a direct bearing on safety or plant efficiency and therefore accurate readings are extremely important and will lead to the user setting a high performance figure, in the order of 0.5% of span or less. This will normally increase the required calibration frequency. Other applications may not demand a very high performance, for example measuring the level in a water tank. If all that is needed is to indicate that the water level is approximately in the center of the tank a good enough performance figure could be something around 10% of span. An accuracy of this order could lead to calibration frequencies of hundreds of years, suggesting that the device need never be calibrated at all.

<b>Determine the operating conditions</b>

Operating conditions such as static pressure and the ambient temperature are another vital aspect. For each pressure transmitter, these conditions will each have an associated error figure.

<b>Calculate the TPE (total probable error)</b>

This is determined by a formula which incorporates terms for the quoted base accuracy of the device and the likely effects of static pressure and temperature errors on performance accuracy.

<b>Determine the stability for a month</b>

This data should be provided by the vendor for the particular model to be used. Normally the stability will be expressed for a given time period e.g. 36 months.

<b>Calculate the calibration frequency</b>

The calibration frequency is given by desired performance minus the Total Probable Error, divided by the stability per month. This determines the frequency with which the calibration needs to be checked in order to maintain the desired accuracy.

To determine the calibration frequency, the user must ensure he gets certain information from the pressure transmitter's manufacturer. This includes temperature error, stability (drift), static pressure error and base accuracy.

Above all, a customer must not rely solely on the assurances of a manufacturer, as different models in a manufacturer's range will have differing performances. Making a purchasing decision based on 'headline' statements in literature can result in over specified instrumentation or a poorly performing pressure transmitter which can adversely affect process control, resulting in poor product quality and increased waste.

Checking pressure transmitter calibration at the right intervals will avoid these problems, while keeping the cost of ownership to a minimum.

Trevor Dunger is a product specialist with ABB.