JET PROPULSION LABORATORY INTEROFFICE MEMORANDUM TO: Distribution S & I DFM 96-014 FROM: Tim Schofield 18th December 1996 SUBJECT: AIP data conversion expressions - Revision B. REFERENCE(S): (1) AIP data conversion expressions, S & I DFM 96-013, 12th November 1996. This memo corrects reference (1) for mis-assignments of W identifier number in the Aeroshell Instrumentation Package (AIP) platinum resistance thermometer (PRT) temperature sensors. The cause of these mis-assignments is still not understood. They were originally noted during ASI/MET electrical interface testing on 20th June 1996, and these results were verified by the spacecraft temperature distribution observed during the first ASI/MET cruise health check on 16th December 1996. The assignment changes are as follows: Identifier Was Is W - 2044 PRT1 (Backshell) PRT2 (Heatshield, Ref. for TC1-6) W - 2045 PRT2 (Heatshield, Ref. for TC1-6) PRT3 (BIP, Ref. for TC7-9) W - 2046 PRT3 (BIP, Ref. for TC7-9) PRT1 (Backshell) where BIP stands for backshell interface plate. The following GDS processing changes must be made to correct for scrambling noted above. 1. Rename W - 2044, W - 2045, and W - 2046, mnemonics in DMD displays. 2. Reassign W identifiers used in thermocouple engineering unit conversions. 3. The engineering conversions associated with each W identifier do not change. The remainder of this memo is a corrected duplicate of reference (1). Count to ADC Voltage conversion. -------------------------------- John Genofsky's conversions express physical parameters in terms of the ASI-MET 14 bit analog to digital converter (ADC) input voltage range of +/- 3 Volts for each channel. Nominally, the relationship between counts and this voltage is given by the following expressions. Volts = Counts x 6/2**14 (Counts <= 8191) Volts = Counts x 6/2**14 - 6 (Counts >= 8192) Counts must therefore be converted first into voltage and then into physical parameters using the expressions tabulated on the following page. Parameter ADC Voltage to physical parameter conversion. ------------------------------------------------------- These expressions convert ADC input volts (VADC) to physical parameters for each ASI-MET channel. Parameter 1 - TC1 - Heatshield Thermocouple 1 (W-2035) ------------------------------------------------------ The conversion from ADC volts to temperature for the thermocouples proceeds in four stages. a. Convert ADC input volts to thermocouple hot minus cold junction output voltage (VH-C). VH-C (millivolts) = 21.328 - 11.50 VADC (Volts) b. Calculate thermocouple cold junction voltage (VC) corresponding to TPRT2, the temperature in degC measured by PRT2 (Parameter 10 - W-2044), using a polynomial fit to the nominal Type K (Chromel/Alumel) thermocouple temperature versus voltage curve. VC (millivolts) = 2.91846x10**-2 + 3.93105x10**-2 TPRT2 + 5.97095x10**-6 TPRT2**2 - 4.02608x10**-9 TPRT2**3 c. Calculate thermocouple hot junction voltage VH by adding the measured voltage to VC. VH (millivolts) = VC + VH-C d. Calculate hot junction temperature corresponding to VH, using a polynomial fit to the nominal Type K thermocouple temperature versus voltage curve. TH (degC) = -8.39166x10**-1 + 2.55089x10**1 VH - 9.45586x10**-2 VH**2 + 1.55364x10**-3 VH**3 Convert this temperature from degC to K by adding 273.15. Parameter 2 - TC2 - Heatshield Thermocouple 2 (W-2036) ------------------------------------------------------ Identical to Parameter 1. Parameter 3 - TC3 - Heatshield Thermocouple 3 (W-2037) ------------------------------------------------------ Identical to Parameter 1. Parameter 4 - TC4 - Heatshield Thermocouple 4 (W-2038) ------------------------------------------------------ Identical to Parameter 1. Parameter 5 - TC5 - Heatshield Thermocouple 5 (W-2039) ------------------------------------------------------ Identical to Parameter 1. Parameter 6 - TC6 - Heatshield Thermocouple 6 (W-2040) ------------------------------------------------------ Identical to Parameter 1. Parameter 7 - TC7 - BIP Thermocouple 1 (W-2041) ----------------------------------------------- Identical to Parameter 1, but PRT3 (Parameter 11 - W-2045) substituted for PRT2 (Parameter 10 - W-2044) in step 1b. Parameter 8 - TC8 - BIP Thermocouple 2 (W-2042). ------------------------------------------------ Identical to parameter 7. Parameter 9 - TC9 - BIP Thermocouple 3 (W-2043). ------------------------------------------------ Identical to parameter 7. Parameter 10 - PRT2 - Heatshield PRT (W-2044). ---------------------------------------------- Two stage conversion; a. volts to sensor resistance, b. sensor resistance to temperature. R(Ohms) = 699.3007 + 93.0233 VADC Temperature (degC) = - 238.9485 + 0.44648 R + 7.45434x10**-5 R**2 - 2.34165x10**-8 R**3 The conversion includes nominal amplifier gain and offset plus a cubic fit to the PRT temperature vs. resistance calibration curves. Currently, the same calibration curve is used for all PRTs. Temperature should be converted to K by adding 273.15. Parameter 11 - PRT 3 - BIP PRT (W-2045). ---------------------------------------- Two stage conversion; a. volts to sensor resistance, b. sensor resistance to temperature. R(Ohms) = 606.0606 + 63.2911 VADC Temperature (degC) = - 238.9485 + 0.44648 R + 7.45434x10**-5 R**2 - 2.34165x10**-8 R**3 The conversion includes nominal amplifier gain and offset plus a cubic fit to the PRT temperature vs. resistance calibration curves. Currently, the same calibration curve is used for all PRTs. Temperature should be converted to K by adding 273.15. Parameter 12 - PRT1 - Backshell PRT (W-2046). --------------------------------------------- Identical to parameter 11, as variations between PRTs are currently being ignored. Distribution: ------------- Michele Katti Rob Manning Keith Novak Kathy Sturdevant