Temperature compensation: The key to precision

STS Sensors: When selecting the right pressure transducer, knowledge of the temperatures that may arise is of the utmost importance. If the measurement technology used is not adequately temperature-compensated, serious inaccuracies and other risks will be the net outcome.

This is why end users need to know in advance which temperatures are to be expected within their own specific application. There are two values to consider here: The media temperature and the ambient temperature. Both of these values are important. The media temperature is the value at which the pressure port makes its contact. The ambient temperature, however, is the value arising in the environment surrounding the application and ultimately affects the electrical connections. Both values can be very different from one another, yet each also has differing consequences.

Why is temperature an important factor?

The materials used in piezoresistive pressure transducers display a certain temperature dependency (read more about the thermal characteristics of piezoresistive pressure transmitters here). The measurement behavior of the pressure transducer thus also shifts with temperature. As a result, temperature-related zero offsets and span errors will now arise. Expressed in simple terms, if a pressure of 10 bar is approached at 25 °C and then for a second time at 100 °C, different measured values will be obtained. For users viewing a data sheet, this means that excellent accuracy values are really of little use when temperature compensation itself remains insufficient.

Apart from avoiding serious measurement errors, the mechanical functionality of the measuring instrument also depends upon the existing temperature. This mainly affects components such as electrical connections and the cables used for the transmission of measured values. Very few of the standard materials can withstand temperatures around, yet alone above, 100 °C. The cable sockets and cables themselves can melt or even catch fire here. Besides measuring accuracy, temperature also has an influence on operational safety.

Fortunately, users need not to live with these risks, since pressure transducers can be optimized for different temperature conditions - on the one hand through temperature compensation, and on the other using additional cooling elements and particularly heat-resistant materials.

Temperature errors can be avoided

The manufacturers of pressure sensors employ temperature compensation. Products from STS, for example, are optimized as standard for operating temperatures from -0 °C to 70 °C. The further the temperature deviates from these values, the greater the measurement inaccuracy becomes. A measuring instrument optimized for a range from 0 °C to 70 °C but used at temperatures around 100 °C will no longer achieve its specified accuracy values. In this case, a sensor must be deployed, which is actually compensated for temperatures of around 100 ° C.

There are two forms of temperature compensation:

• Passive compensation: Temperature-dependent resistors are activated in the Wheatstone bridge
• Active compensation (polynomial compensation): Various pressures are approached at rising temperatures within a heating cabinet. These are then compared with the values from a calibration standard. The temperature coefficients determined from this are next entered into the electronics of the pressure transmitter so that the temperature errors in actual practice can now be compensated for "actively".

Active temperature compensation remains the preferred method because it leads to the most accurate of results.

Temperature compensation itself, on the other hand, does have its limitations. As previously mentioned, temperature affects not only the precision of a pressure transmitter. The mechanical components of the measuring cell also suffer at temperatures above 150 °C. At these temperatures, contacts and bondings can become loose and the sensor itself suffers damage. If exceptionally high media temperatures are to be expected, then additional cooling elements will be required to ensure the functionality of the sensor.

Cooling elements at very high media temperatures

To protect the pressure transmitter from very high temperatures, there are four variants which can be employed depending upon the application and the temperature involved.

Variant A: Media temperatures to around 150 °C

In this variant, a cooling fin element is integrated between the measuring cell and the amplifier. It is a matter here of separating the electronics from the actual application, so that these remain undamaged by the elevated temperatures.

Variant B: Temperatures above 150 °C

If the medium is very hot, a cooling element is screwed in front of the pressure port (cooling fins, for example, that can be screwed from both sides). The pressure port thus comes into contact now solely with the cooled medium. These forward-attached cooling fins have no effect at all on the accuracy of the sensor. Should the medium be extremely hot steam, however, then a siphon would instead be employed as the cooling element.

Variant C: Extremely high temperatures (up to 250 °C)

When the media temperature is extremely high, a forward-facing isolating system incorporating a cooling section can now be used. This variant, however, is quite large in size and does affect the accuracy negatively.

Variant D: Special case of a warming cabinet or climatic chamber

When pressure measurements are necessary inside a warming cabinet at ambient temperatures of up to 150 °C, the electronics of the pressure transmitter cannot be exposed to these temperatures without suffering damage. In this instance, only the measuring cell (with pressure port and stainless steel housing) is located within the cabinet, with this connected to the remote electronics outside the cabinet (also housed in a stainless steel housing) via a high-temperature FEP cable.

In summary: Consultation is king

The precision of piezoresistive pressure sensors is influenced by temperature conditions. The temperatures acting on the pressure port can be compensated for passively or actively so that the pressure sensor used meets the requirements upon accuracy over the anticipated temperature range. Furthermore, the influence of ambient temperature on the mechanical components of the measuring instrument must also be taken into consideration. Using forward-mounted cooling elements and heat-resistant materials, this can also be brought under control. Users should thus always rely on the comprehensive advice offered by the manufacturer and ensure that the pressure transducers available can be optimized to their very own specific applications.