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Number 1138
Application Note Switching in
Series Multipoint Testing
create measurement errors of several degrees; steps must be
Switching in Multipoint Testing taken to recognize, control, and minimize these error sources.
Selection of suitable switching is an extremely important but In addition, a thermocouple produces an EMF proportional
sometimes under-emphasized function in the process of measure- to the difference in temperature from end to end. Therefore, the
ment system design. Switch selection is, in fact, no less critical temperature to be measured cannot be determined unless the
than the selection of system instrumentation or the design of the temperature is known at the cold junction. For this reason, a tem-
test interface. perature reference must be provided.
The objective of any system design is to transfer the accu- The reference must be held at or as near as practical to the
racies and specified performance of system instrumentation to temperature of the thermocouple cold junction. This is generally
the Unit Under Test (UUT). These specifications are guaranteed accomplished by mounting the terminal connector and the refer-
only at the instrument terminals. To complete the test procedure, ence device in one thermally conductive block.
it may be necessary to insert meters of cable, complex switching,
and a unique test interface between the system instrumentation The System Solution
and the UUT. The system designer must deal with errors created
There are at least three ways to use thermocouples for multipoint
by cabling, connections, measurement techniques, and system
temperature measurements. The method chosen depends on the
switching. Add the need to accommodate the designer's worst-
"acceptable measurement uncertainty."
case measurement situation, and achievement of the test objec-
tive becomes extremely difficult. 1. Direct without Compensation. The method shown in Figure
1 is simple and can provide adequate accuracy in monitoring
This application note describes the use of currently avail-
high temperatures (typically above 300