Page 161 - Mechanical Engineers' Handbook (Volume 2)

150 Temperature and Flow Transducers

Industrial thermocouples are frequently installed in thermowells to isolate the sensor
from the system. Often, the thermowell is partly filled with a fluid of high thermal conduc-
tivity to assure good thermal contact between the thermocouple and the system through the
wall of the thermowell. An empty thermowell is one of the main causes of ‘‘faulty ther-
mocouple’’ reports. This type of failure can be detected by an electrical heating test known
as the loop current step response proposed by Hashemian 16 and more recently incorporated
into the Oxford University SEVA (Self Evaluation) protocol by Yang and Clarke. 17 This
procedure involves passing a current through the thermocouple to heat it above its operating
temperature and recording the cooling curve obtained when the heating current is shut off.
This procedure is executed periodically and the traces compared with a reference trace ob-
tained when the thermocouple was first installed. Any loss of thermal contact between the
thermocouple and its thermowell results in a change in the shape of the response curve in
the first few seconds of the trace.

2.13 Thermocouple Probe Designs for Gas Temperature Measurements
Thermocouples respond to their total environment, including all heat transfer modes: radia-
tion, conduction, and convection. Radiation and conduction tend to pull the thermocouple
temperature away from the gas temperature toward the temperature of the radiating source
or the conducting support structure. The thermocouple is ‘‘connected’’ to the gas temperature
by convection through the heat transfer coefficient. The equilibrium temperature is that which
results from an energy balance between these competing tendencies. In cases where chemical
reactions may be catalyzed on the surface of the thermocouple, those mechanisms must also
be considered. Any difference between the thermocouple temperature and the true gas tem-
perature is considered an error. Three types of error are recognized: radiation error, conduc-
tion error, and velocity error. Each describes the difference between the true gas temperature
and the indicated temperature due to one mechanism: radiant heat transfer, conductive heat
transfer, or high-velocity viscous dissipation effects. The heat transfer characteristics of a
thermocouple (which governs its response to its environment) is relatively well known and
cannot be changed.
Designing a thermocouple probe for accurate gas temperature measurement consists in
designing a protective envelope within which the thermocouple will be acceptably accurate,
that is, within which the sum of all three errors will be acceptably small. No thermocouple
probe can respond instantaneously to changes in gas temperature. Thermocouples (the junc-
tions themselves, not including any surrounding probe structure) act as first-order systems
in response to changes in temperature and compensation methods for first-order systems are
well understood. The general procedure for design of gas temperature probes for steady-state
accuracy and transient behavior has been described by Moffat. 18
The following citations provide examples of these principles applied to probe design.
Elmore and Watkins developed an analog compensation system that used signals from two
19
first-order thermocouples of different diameters to determine transient gas temperatures up
to 1 kHz. The fundamental approach could be implemented using high-speed digital proc-
20
essing as well. Moffat proposed a transient method for estimating the (steady) temperature
of a very hot gas stream (above the melting temperature of the thermocouple). His method
is based on briefly exposing the thermocouple to the gas stream and correcting the transient
behavior using a first-order time constant. The appropriate value of the time constant is
found by minimizing the variance in the corrected signal over the time interval of exposure
of the thermocouple to the hot gas stream.