Sensoray 518 - Manual (Page 46)

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CALL Send518Byte (192 + 5) ‘ declare channel 5 linearization coefficients:
CALL Send518Real (.19) ‘ A
CALL Send518Real (24.1) ‘ B
CALL Send518Real (7.2) ‘ C
After defining the channel 5 sensor type and coefficients, invoking the Read Sensor Data
command for channel 5 will return a value with data scaled to 1 mm/bit. The following fragment
will fetch channel 5 data from the coprocessor and display in the desired format:
Send518byte (BasePort%, 5) ‘ send READ DATA command for channel 5
ChanData% = Read518word%(BasePort%) ‘ read channel 5 data from 518
PRINT USING “###.#”; ChanData% / 10.0 ‘ display in centimeter form
Thermocouples
A thermocouple typically consists of a wire pair whose wires are made of two dissimilar metals.
At one end of the pair, the two wires are electrically shorted together (by soldering, welding,
etc.). This “end” of the thermocouple— commonly referred to as the “hot-junction”— is
thermally attached to the point to be measured. The other end — the “cold-junction” — is
connected to a measuring device (in this case, a 7409TC).
Thermocouples generate an open-circuit voltage that is proportional to the temperature gradient
between hot- and cold-junctions (Re: the Seebeck effect). Since this voltage is a function of the
temperature difference between junctions, it is necessary to know the temperature at the cold-
junction in order to accurately determine the temperature at the hot-junction.
An integrated circuit temperature transducer resides on the Sensoray 7409TC termination board.
This transducer, because of its close physical proximity to the thermocouple terminations,
represents an accurate measurement of the thermocouple cold-junctions. Periodically, the 518’s
CPU measures the I.C. transducer signal and computes the corresponding thermocouple voltage,
known as the “cold-junction compensation voltage”.
When a thermocouple channel is measured, the 518’s CPU adds the thermocouple voltage to the
cold-junction compensation voltage to form the “corrected thermocouple voltage”. This is the
voltage one would measure if the cold- junction were maintained at zero degrees C. Finally, the
corrected thermocouple voltage is converted into the appropriate temperature units via a non-
linear mapping function.