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56.1    THEORY OF OPERATION  . . . . . . . . . . . . . .  56-1
56.2    CALCULATIONAL METHODS  . . . . . . . . . . . . .  56-2
56.2.1    Thermistor Temperature . . . . . . . . . . . .  56-2
56.2.2    Thermocouple Temperature . . . . . . . . . . .  56-2
56.3    MECHANICAL . . . . . . . . . . . . . . . . . . .  56-2
56.4    ELECTRICAL . . . . . . . . . . . . . . . . . . .  56-2
56.5    DRAWING PACKAGE NUMBER . . . . . . . . . . . . .  56-3
56.6    RESPONSIBLE ENGINEER . . . . . . . . . . . . . .  56-3

CHAPTER 56
THERMOCOUPLE REFERENCE PLANE (TRP)
The Thermocouple Reference Plane is a "cold junction" to interface  up
to  15 thermocouples to one half of a Smart Analog Monitor (SAM) using
the SLC Standard 18 pair copper conductor cable.  Up to two TRP's  may
be connected to a SAM.  The TRP, in conjunction with software support,
measures the reference plane's temperature and allows the  calculation
of the temperature of the remote TC junctions.
The temperature of the Thermocouple  Reference  Junction  is  measured
using  a  precision thermistor.  Power from the SAM is used to measure
the resistance of the thermistor, and therefore its temperature.

56.1  THEORY OF OPERATION

The  conceptually  simplest  use  of  a  thermocouple  would  be   the
following:   A  thermocouple  is  manufactured by soldering one end of
each of two pieces of pure copper wire  to  a  single  pure  piece  of
constantan wire.  One junction is immersed in oil at exactly 0 degrees
Celsius.  The two remaining ends of the copper wires are connected  to
a  perfect, high impedance voltmeter.  By using the National Bureau of
Standards (NBS) tables, the measured voltage is used to calculate  the
remote junction's temperature.
Measuring temperatures using the TRP involves the following steps:
o  The temperature of the reference  plane  must  be  calculated
from the measured resistance of the thermistor.
o  An equivalent "Error Voltage" must be calculate using the NBS
tables.   This  is  the  voltage  of  a  thermocouple  if the
reference junction is at 0 degrees K, and the remote junction
is at the temperature of the TRP.
o  A "true" TC  voltage  is  calculated  by  adding  the  "Error
Voltage" to the observed voltage across a TC.  The NBS tables
are used to calculate the TC temperature.

THERMOCOUPLE REFERENCE PLANE (TRP)                           Page 56-2

56.2  CALCULATIONAL METHODS

The calculation of a block of  16  temperatures  is  done  by  calling
TCTEMPS.   This  routine  properly  calculates  the temperatures of 15
copper constantan thermocouples, and the TRP thermistor.

56.2.1  Thermistor Temperature

The temperature of a single thermistor can be characterized as:
T = a + b * Ln(R) + c * (Ln(R))**3.
For  the  "2215"  Ohm  thermistor  used  on  the   TRP,   a=1.46161E-3
b=2.39427E-4 c=9.59358E-8.
The resistence of the thermistor can  be  calculated  by  the  voltage
across  it.  Current is supplied from a 10.240 volt reference, through
a zeros resistor of 1005110 Ohms.  (5.11 K is internal  to  the  SAM).
The current through the thermistor is:
It = (10.240-Vt) / 1005110
where Vt is the thermistor voltage.  The resistance is therefore:
Rt = Vt/It.

56.2.2  Thermocouple Temperature

The voltage as a  function  of  temperature  is  contained  in  a  NBS
publication.  The Voltage of a TC is a smooth function with a slope of
20 microvolts/degrees Celsius mean room temperature.  The curve cannot
be  accurately  characterized by fitting to a polynomial of order 5 or
less.
The temperature (voltage) of a TC  at  any  voltage  (temperature)  is
calculated  using  a parabolic interpolation of the three nearest data
points in the table of voltage/temperature.   Parabolic  interpolation
and  20  data points is sufficiently accurate for the TRP when used in
conjunction with a SAM.

56.3  MECHANICAL

The TRP is a free monitoring box.

56.4  ELECTRICAL

The TRP connects to a SAM using the SLC standard 18 pair cable.
NOTE:  It is extremely important that the SLC cable be a single  piece
of cable without any additional splices or junctions.

THERMOCOUPLE REFERENCE PLANE (TRP)                           Page 56-3

56.5  DRAWING PACKAGE NUMBER

123-622

56.6  RESPONSIBLE ENGINEER

K.  Jobe

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