Ambient temperatures have a major influence on the functionality and accuracy of pressure transmitters, with Arctic temperatures representing a particular challenge here.
In pressure measurement on a piezoresistive basis, semiconductors diffused onto a silicon membrane serve as strain gauges. When pressure acts on the membrane, these strain gauges deform and a change in resistance occurs. It is this change that ultimately gives the determined pressure. These resistors, however, are also temperature-sensitive. The sensitivity of pressure sensors is therefore decreased with sinking temperature. The pressure transducer is thus no longer as precise as at room temperature.
Because of this property, manufacturers of pressure transmitters always indicate the behavior of their products under certain temperature conditions. To achieve the most linear behavior possible, pressure transmitters are nowadays electrically compensated over a relatively wide temperature range (temperature compensation). This implies that temperature errors are automatically calculated. As a result, piezoresistive pressure transmitters can provide precise measurements over a relatively wide temperature range. Temperature effects, however, cannot be completely eliminated. For this reason, the manufacturer datasheets are generally specified with accuracy values for different temperature ranges.
Extreme cold: Pressure transmitters without O-rings
The cold not only affects the resistances in the semiconductors employed. There are four other factors that should be considered when looking for a suitable measuring instrument for outdoor applications in cold regions. Among these is the use of sealing rings. Temperatures below -20 degrees Celsius cause the sealing materials between the pressure port and the membrane to become brittle. Leakage will then render the sensor useless. No pressure transmitters with O-rings should therefore be used in regions of extreme cold. A compact pressure sensor in which the pressure port and measuring cell are directly fused together would be the right choice here.
Icing: Beware of overload pressure
Freezing over can also affect the functionality of a sensor. If we take, for example, natural gas drilling in Arctic regions, then water can also be present in the gas-conducting pipes. When this water freezes, the pressure acting on the pressure transmitter may increase to a degree for which it has not been constructed. The consequence here can be tearing of the membrane. If there is a risk that the sensor is iced up, then a corresponding overload pressure must be watched out for.
In piezoresistive pressure measurement, the pressure is applied indirectly to the silicon membrane via a transfer medium. This usually consists of an oil. As the temperature drops, the viscosity of this oil will increase. Depending upon the oil and the actual temperature, it can gel or even harden up. This change also adversely affects the functionality of the pressure transducer.
Also to be considered is the resistance to condensation: If there is damp air in the housing of the pressure sensor, condensation will form at cold ambient temperatures, which can damage the electronics and destroy the sensor.
Summary
Users employing pressure sensors in cold temperatures should ensure that the individual components are directly fused without O-rings and are also resistant to condensation. It is also to be evaluated whether the pressure transmitter can freeze over if, for example, it comes into contact with water. In this case, a pressure transmitter with a corresponding overload pressure should be selected. As with any application, the pressure transducer should, of course, be compensated for the expected temperature range.
