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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 The TRP is powered by the SAM.
56.5 DRAWING PACKAGE NUMBER 123-622
56.6 RESPONSIBLE ENGINEER K. Jobe