When do instruments need to be calibrated

Manufacturing plants need to be absolutely confident that their instrumentation products – temperature sensors, pressure transducers, flow meters and the like – are performing and measuring to specified tolerances.

If sensors drift out of their specification range, the consequences can be disastrous for a plant, resulting in costly production downtime, safety issues or possibly leading to batches of inferior quality goods being produced, which then have to be scrapped.

Most process manufacturing plants will have some sort of maintenance plan or schedule in place, which ensures that all instruments used across the site are calibrated at the appropriate times.

However, with increasing demands and cost issues being placed on manufacturers these days, the time and resources required to carry out these calibration checks are often scarce.

This can sometimes lead to instruments being prioritised for calibration, with those deemed critical enough receiving the required regular checks, but for other sensors that are deemed less critical to production, being calibrated less frequently or not at all.

But plants can improve their efficiencies and reduce costs by using calibration history trend analysis. With this function, the plant can analyse whether it should increase or decrease the calibration frequency for all its instruments.

Cost savings can be achieved in several ways. First, by calibrating less frequently where instruments appear to be highly stable according to their calibration history.

Second, by calibrating instruments more often when they are located in critical areas of the plant, ensuring that instruments are checked and corrected before they drift out of tolerance.

This type of practise is common in companies that employ an effective preventive maintenance regime.

Current practices

The analyses of historical trends and how a pressure sensor, for example, drifts in and out of tolerance over a given time period, is only possible with calibration software that provides this type of functionality.

But in reality, how often do process plants actually calibrate their instruments and how does a maintenance manager or engineer know how often to calibrate a particular sensor?

Sensors that are highly stable need not be re-calibrated as often as sensors that tend to drift.A few years back, Beamex conducted a survey that asked process manufacturing companies how many instruments in their plant required calibrating and the frequency with which these instruments had to be calibrated.

The survey covered all industry sectors, including pharmaceuticals, chemicals, food and beverage, oil and gas, paper and pulp.

Interestingly, the survey showed that from all industry sectors, 50 percent of the respondents said they calibrated their instruments no more than once a year.

However, in the pharmaceuticals sector, 42 percent said they calibrated once a year and 42 percent said they calibrated twice a year.

Perhaps unsurprisingly, due to it being a highly regulated industry, the study proved also that the pharmaceuticals sector typically possesses a significantly higher number of instruments per plant that require calibrating.

In addition, these plants also calibrate their instruments more frequently than other industry sectors.

Calibration history

But regardless of the industry sector, by analysing an instrument's drift over time (i.e. the historical trend) companies can reduce costs and improve their efficiencies. Pertti Mäki is area sales manager at Beamex in Finland.

He specialises in selling the Beamex CMX to different customers across all industry sectors.

He comments: "The largest savings from using the History Trend Option are in the pharmaceuticals sector, without doubt, but all industry sectors can benefit from using the software tool, which helps companies identify the optimal calibration intervals for instruments."

The trick, says Mäki, is determining which sensors should be re-calibrated after a few days, weeks, or even years of operation and which ones can be left for longer periods, without of course sacrificing the quality of the product or process or the safety of the plant and its employees.

Doing this enables maintenance staff to concentrate their efforts only where they are needed, therefore eliminating unnecessary calibration effort and time. But there are other, perhaps less obvious benefits of looking at the historical drift over time of a particular sensor or set of measuring instruments.

As Mäki explains: "When an engineer buys a particular sensor, the supplier provides a technical specification that includes details on what the maximum drift of that sensor should be over a given time period.

"With CMX's history trend option, the engineer can now verify that the sensor he or she has purchased, actually performed within the specified tolerance over a certain time period. If it hasn't, the engineer now has data to present to the supplier to support his findings."

But that's not all. The history trend function also means that a plant can now compare the quality or performance of different sensors from multiple manufacturers in a given location or set of process conditions.

This makes it an invaluable tool for maintenance or quality personnel who, in setting up a new process line for example, can use the functionality to compare different sensor types to see which one best suits the new process.

Calibration software such as CMX can also help with the planning of calibration operations.

Calibration schedules take into account the accuracy required for a particular sensor and the length of time during which it has previously been able to maintain that degree of accuracy.

Sensors that are found to be highly stable need not be re-calibrated as often as sensors that tend to drift.

Analysing trends

The history trend option is basically a utility for viewing calibration history data. It is easy-to-use and is available both for positions and devices.

The data is displayed graphically and is also available in numeric format in a table.

The function enables users to plan the optimal calibration intervals for their instruments. Once implemented, maintenance personnel, for example, can analyse an instrument's drift over a certain time period.

History trend displays numerically and graphically the instrument's drift over a given period. Based on this information, it is then possible to make decisions and conclusions regarding the optimal calibration interval and the quality of the instruments with respect to measurement performance.

Users already familiar with CMX may confuse this function with the standard calibration results window, but the history trend window enables users to view key figures of several calibration events simultaneously, allowing users to evaluate the calibrations of a position or a device for a longer time period compared to the normal calibration result view.

For example, the user can get an overview of how a particular device drifts between calibrations and also whether the drift increases with time. Also, the engineer can analyse how different devices are suited for use in a particular area of the plant or process.

Reporting is straightforward and the user can even tailor the reports to suit his or her individual needs, using the report design tool.

[This article was written and supplied by Beamex.]

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