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16.2 Sensor characteristics 221
Use of a thermowell or a bypass line installation causes a slower time response
following a coolant temperature change than would occur with a bare sensor inserted
in the fluid. Regulations require verification of suitable sensor time response. A
method for in-situ response time measurement of RTDs is available [5].
16.2.2.4 Advanced temperature sensors
Pyrometry is potentially useful for temperature measurement in high-temperature
conditions. Pyrometers measure the heat radiation from an object. Pyrometers could
be used to measure pipe wall temperature or fluid temperature by focusing into wells
immersed in the fluid.
A new temperature measuring system is based on monitoring of Johnson noise in
the sensing element. Johnson noise is naturally occurring temperature-dependent
fluctuations in material. Johnson noise thermometry systems do not decalibrate
and they are suitable for high temperature measurements.
16.2.3 Pressure sensors
Reactor pressure sensors have an elastic element (either a diaphragm, bellows or a
Bourdon tube) that expands or contracts when experiencing a pressure difference
across the element. See Fig. 16.10.
Pressure is determined by measuring the displacement of the elastic element or by
measuring the force required to push the element back to its rest position (called a
force-balance sensor).
FIG. 16.10
Elastic components for pressure sensors.
