Page 291 - Instrumentation Reference Book 3E

Measurement techniques: thermocouples 275

7.75 mm 9.5 mm Nominal length A and enough time is allowed for the tips of the
prongs to reach the temperature of the billet, then
both junctions will be at the same temperature and
b error thermal e.m.f.s cancel. This makes a
the
simple, quick, and very inexpensive way of mea-
suring hot metal temperatures. The points of the
prongs are screwed to the main assembly and are
PVC insubted-flexible expendable. As soon as they lose their sharpness or
stranded tails 7/02 mm begin to get corroded they can be changed.

PTF E colored
14.5.3.6 Liquid tnetal thertnocozrp1t.s
When measuring the temperature of liquid metals
Nominal length A - probe, The cost of a fully protected probe would
such as steel it is desirable to use an expendable
be very high and the response time slow. For
checking the temperature of liquid steel a dipstick
probe can be used. The probe itself is robust and
constructed with a socket of thermocouple mater-
ial in the end. A disposable platinum-rhodium/
6 Nominal length 7
platinum thermocouple itself lasts in the molten
metal for a few seconds. long enough io take a
A
temperature measurement. Figure 14.39 shows
this arrangement.
14.5.3.7 Thermopiles
Where a very small temperature rise is to be
Figure 74,30 MI thermocouple terminations. Courtesy measured many thermocouples may be connected
ABB.

143.3.4 Surface contact theimocouples Mineral insulated
compensating
Thermocouples for the measurement of the sur- cable
face temperature of objects such as pipes or other
components or plant items are available. On pipes
a surface measurement makes a simple but not convolute
very accurate non-invasive Temperature measure- cardboard tube
ment. For higher temperatures or more rugged Crimped
applications thermocouples are available embed- connection
ded in a metal plate designed to be clamped or
welded to the component to be measured. For
lower temperature applications, below about
2OO”C, or for use in protected environments,
self-adhesive contact surface thermocouples are
supplied. In these probes the thermocouple is -Compensated
embedded in a small plastic pad coated on one socket
face with a suitable contact adhesive.
Fully compensated
disposable spring
14.5.3.5 Hot metal tlzernzocoilples connector
Where it IS necessary to make spot measurements -Ceramic body
of the temperature of hot metal billets. very simple
test prods are available which consist of a two-
pronged “fork.” The two prongs are made of the Fast response Steel cap protects
two thermocouple metals with sharpened points. platinum- against slag
When both prongs are in contact with the hot rhodium Silica protection
metal, two junctions are formed, metal A to the thermocouple tube
billet and the billet to metal B. If the billet is large Figure 14.39 Liquid metal thermocouple