Temperature has a major impact on the function and operating lifetime of most electronic devices. This is especially true for electronic pressure sensors. That’s why pressure sensor specifications typically include minimum and maximum values for operating, storage, compensated and media temperatures. Exceeding these ranges can introduce unexpected errors in the pressure readings, or in extreme cases, lead to complete failure of the instrument. Another temperature specification – calibration temperature – may also be included in the data sheet.
Operating Temperature Range for Pressure Sensors
The “operating” or “ambient” temperature range refers to the minimum and maximum temperature the transmitter body will be exposed to when in operation. For example, -20 to +80oC is a typical operating temperature range for standard pressure transmitters. If the transmitter housing temperature falls outside this range, the output signal may show an extreme drift or the transmitter may stop operating altogether. Even relatively brief exposure to temperatures outside the specified range can lead to permanent damage. That’s why it’s important to verify the environmental extremes of each application in order to select a suitable pressure transmitter. A number of other factors may impact the operating temperature of the transmitter. If the transmitter body is made of metal and is measuring warm or hot media, the heat will be conducted by the process connection into the case of the transmitter. For example, this may allow a transmitter rated at a minimum of -20oC ambient to operate in a colder environment as it will be “warmed” by heat conduction from the process media. Another factor to consider is the cable. Many cable materials become inflexible and brittle when exposed to extreme cold which may cause the cable insulation to crack if flexed in extremely cold environments.
Storage Temperature
A storage temperature specification is also usually provided. In most cases, the storage range is wider than the operating temperature range. Electronic circuits are typically less susceptible to damage from extreme temperatures when not operating. The rated storage temperature specification may actually be less than the operating temperature range if the manufacturer includes the transmitter packaging and tests to simulate transport. For example, foam packaging may deteriorate above 100 OC (212OF).
Media Temperature
The media temperature range is another important specification to consider when selecting a pressure transmitter. The media temperature directly impacts the overall performance and accuracy of the pressure sensor readings. Care must be taken to select a transmitter with a media temperature range that is greater than the expected minimum and maximum media temperatures. Exceeding the media temperature specifications can lead to significant errors or total failure of the instrument. A typical media temperature range specification is -30 to +100oC. However, many transmitters are available with significantly higher media temperature specifications, and there are a number of options available for dealing with extreme media temperatures. These include a built in cooling element with metal fins that provide for convection cooling of the media before it reaches the pressure sensor. A siphon can be used to measure the pressure of high pressure steam. Even higher temperature ranges are possible when using a diaphragm seal and capillary tube. A diaphragm seal is often used when measuring the pressure of high temperature media. The seal acts to isolate the media from the transmitter and the capillary tube, which can be several feet long, and provides isolation from the extreme temperature. Compensated Temperature Range A “compensated temperature range” specification is typically provided in most cases. The compensated temperature is the range where the internal circuitry of the sensor is designed to limit the temperature error to a defined level. For example, a pressure transmitter may have a media temperature range of -30 to +100oC. The compensated temperature range may be limited to -0 to +80oC, where the temperature error will be no greater than, say 0.2% of span for every 10 oC change within this range. The temperature error outside the compensated range will be significantly greater. As a rule of thumb, the temperature error outside the compensated range (but still below the maximum media temperature) may be up to three times the compensated range error, so in this example, it would be approximately 0.6% of span per 10oC. See Figure 1. Figure 1: Media temperature vs. % temperature error Calibration Temperature Finally, a calibration temperature is also defined in the specifications. This is the temperature the transmitter is exposed to during the calibration procedure during the manufacturing process, and is used as the starting point for calculating the error within the “compensated pressure range” described above. Some pressure transmitter manufacturers provide options for the calibration temperature for special applications. For example, a pressure transmitter is calibrated at a standard temperature of 15-25oC. If the transmitter is operated within this range, no additional temperature error is introduced. If the application is known to be at a constant temperature of 80oC, the transmitter can be factory calibrated at this higher temperature. This provides users with improved transmitter performance in high temperature applications.