Page 143 - Mechanical Engineers' Handbook (Volume 2)

132 Temperature and Flow Transducers

1 INTRODUCTION
There are hundreds of different transducers for temperature and flow measurements. The
most common types will be discussed in this chapter; some others will be mentioned only
briefly.
In flow measurements, only closed-channel flow measurement techniques are considered
here, and only for ‘‘clean’’ fluids. Slurries and liquids carrying large objects are not treated.
In temperature measurements, current interests range from cryogenics (a few kelvins)
to plasmas (upward of 10,000 K). Most applications, however, are in the range from room
temperature to 2000 K, and that is where the bulk of this chapter will be concentrated.
The accuracy of a temperature or flow measurement depends not only on the sensor
characteristics but also on the interaction between the sensor and the system being instru-
mented. There are two primary classes of interactions: system disturbance errors (i.e.,
changes in the behavior of the system caused by the presence of the sensor) and system/
sensor interactions (the sensor responding to more than one parameter of the system).
High-temperature measurements are subject to installation errors caused by heat transfer
between the system and the transducer. The term error is defined as the difference between
the observed value and the true value of the intended measurand. The output of a temperature
sensor describes its own temperature, the achieved temperature, but the objective is usually
to measure the temperature at a particular point in the solid, liquid, or gas into which the
sensor is installed—the available temperature. There is often a significant difference between
the available value and the achieved value because the sensor exchanges heat with its entire
surroundings, not just with the immediate region around the sensor. It is not uncommon in
high-temperature gas temperature measurements with unshielded sensors, for example, to
have errors of several hundreds of degrees caused by radiation error, velocity error, or con-
duction error effects on the sensor. These errors cannot be accounted for by calibration of
the sensor, nor can corrections be applied with any degree of certainty. The sensor must be
protected by appropriate shielding. In most applications at high temperature, the installation
errors are far larger than the calibration error of the sensor; hence, sensor accuracy does not
mean the same as measurement accuracy.
Individual transducers are discussed in the following sections.

2 THERMOCOUPLES*
Thermocouples are the most commonly used electrical output transducers that measure tem-
perature. They are inexpensive, small in size, and remarkably accurate when their peculiar-
ities are understood.

2.1 Types and Ranges
Any pair of thermoelectrically dissimilar wires can be used as a thermocouple. The wires
need only be joined together at one end (the measuring junction) and connected to a voltage-
measuring instrument at the other end (the reference junction) to form a usable system.
Whenever the measuring junction is at a different temperature than the reference junction, a
voltage will be developed, which is related to the temperature difference between the two
junctions. Several metallic materials are listed in Table 1 in order of thermoelectric polarity;

*Materials in this section is substantially derived from Ref. 2 with permission, except where otherwise
referenced.