Taking the temperature

Monitoring temperature with a data acquisition system is easy. Making accurate, repeatable measurements is a little harder

By Chris Webb

Temperature is a deceptively simple measurement quantity,’ says Hewlett Packard’s product marketing manager Barry Scott. We often think of it as a single number, but it is really a statistical construct whose accuracy and repeatability can be profoundly affected by thermal mass, measurement time, electrical noise and measurement algorithms.’

True enough, and as Scott points out, the difficulty was highlighted in 1990 when the committee reviewing the International Practical Temperature Scale adjusted the definition of one reference temperature by nearly a tenth of a degree Celsius.

In other words, temperature is difficult to measure accurately in the best of circumstances, and real-life test conditions only make it harder. Understanding the advantages and disadvantages of the various approaches to measuring temperature will help you avoid problems and get better results.

A straw poll of those companies involved in temperature sensing and control reveals that precious few are involved in research, but plenty are active in development.

Better electronics associated with temperature sensors, has spawned a new generation of better stability and precision. There are smart transmitters and smart sensors designed to detect and measure their own degradation and failure.

Of the four most common types of temperature transducers used in data acquisition systems resistance temperature detectors (RTDs), thermistors, IC sensors and thermocouples no single device is the best for every situation. When choosing one, factors to take into consideration include performance, accuracy, useful range, cost and convenience.

Thermocouples are popular for their broad temperature range, typically between -250 and 3,000 C, and ruggedness. They don’t require any form of power and their low cost makes them a good choice for large data acquisition systems.

There have been many developments in the area of transducer heads, like Pepperl+Fuchs’ loop powered H-UT-Ex1 with a 4-20mA output and suitable for hazardous areas. There’s also the SC4400 two wire programmable head and field mounted temperature transmitter from Rochester.

RTDs are most stable and accurate, but are slow and expensive; thermistors have a high output and are fast, but serve only a limited temperature range and are fragile.

The choice of platinum in top-quality RTDs offers the most accurate and stable measurement up to about 500 C. One disadvantage, however, is that they can be self-heating. The need to apply a current means heat, distorting results.

Thermistors, resistive detectors typically made of ceramic semiconductors, offer much higher impedance than RTDs, so reduced lead errors make it feasible to use the simpler two-wire technique. Their high output yields high resolution measurements, further reducing any impact of lead resistance.

Low thermal mass

Their low thermal mass presents a disadvantage, however which is the potential for higher self-heating from the current source used in the measurement.

It is perhaps in the area of IC sensors where the most change has taken place of late. While solving the linearity problem, they are still relatively inexpensive and offer good accuracy at room temperatures; but they do still require a power source. Leading the trend towards smart sensors they possess enough on-board intelligence to help with data reduction and analysis in data acquisition systems.

Meanwhile, nowhere is the need to measure temperature more important than in the power generation industry, says Peter Orrell, sales and marketing manager of York Sensors. Temperature measurement is used to spot leaks in pipelines, to measure power line performance and to detect the formation of hot spots.

His company offers fibre optic-based sensing, in which distributed temperature sensing systems measure temperature and signal changes. Linked to a single processing unit, the sensor cables can be configured linearly along pipelines to provide a continuous, rather than point, measurement.

Finally, on the control side, there is much activity in the development of communicating controllers. There is the CAL 3300 miniature instrument from CAL Controls. It uses a standard ModBus protocol via RS232 or 485 serial link. And there’s the Series XT16 from Athena Controls.

Then FGH Controls gives us the Proteus 3000, a dual loop thermal head ratio heat treatment system in a single box, which speeds up the heating of large loads in ovens, kilns and furnaces by up to 50%, compared with setpoint systems.