TABLE OF CONTENTS Many devices are available for temperature measurement. The selection of a device will be determined by the needs of the application. A comparison of the characteristics of temperature-measuring devices is given. Thermal time constant considerations, installation calibration, and protection also are discussed.
In process control, a wide selection of temperature sensors is available [11]. However, the required range, linearity, and accuracy can limit the selection. In the final selection of a sensor, other factors may have to be taken into consideration, such as remote indication, error correction, calibration, vibration sensitivity, size, response time, longevity, maintenance requirements, and cost [12]. The choice of sensor devices in instrumentation should not be degraded from a cost standpoint. Process control is only as good as the monitoring elements.
Table 10.8 gives the temperature ranges and accuracies of temperature sensors, the accuracies shown are with minimal error correction. The ranges in some cases can be extended with the use of new materials. Table 10.9 gives a summary of temperature sensor characteristics.
| Sensor Type | Range | Accuracy (FSD) | |
|---|---|---|---|
| Expansion | Mercury in glass | ?35 to +430 C | 1% |
| Liquid in glass | ?180 to +500 C | 1% | |
| Bimetallic | ?180 to +600 C | 20% | |
| Pressure spring | Liquid filled | ?180 to +550 C | 0.5% |
| Vapor pressure | ?180 to +320 C | 2.0% | |
| Gas filled | ?180 to +320 C | 0.5% | |
| Resistance | Metal resistors | ?200 to +800 C | 5% |
