Campbell Scientific S3497X TC Psychrometer PROM and Cooling Current Interface - Page 5

25 PELTIER THERMOCOUPLE PSYCHROMETER 1. FUNCTION Instruction 25 measures up to five thermocouple psychrometers (TCP) excited through Campbell Scientific's Model A3497 Psychrometer Cooling Current Interface. Groups of TCPs may be read by using an A3497 for each group of five TCPs and programming Instruction 25 within a Loop. An option is provided for drying the TCP junction by heating before the cooling current is applied. The heating and cooling current is selectable within the limitation of the CR7 Excitation Card by specifying the excitation voltage. The results of the measurements: TCP base temperature (°C), the TCP zero (dry-bulb) reading (µV), and a sequence of TCP wet-bulb readings (µV) are stored in Input Storage. The wet-bulb microvolt readins have the zero (dry bulb) reading subtracted (i.e., the wet-bulb depression is stored). PAR. DATA NO. TYPE DESCRIPTION 01: 2 02: 4 03: 4 04: 2 05: 2 06: 2 07: 2 08: 2 09: FP 10: 4 11: 4 12: 4 13: 4 14: 4 Number of psychrometers per A3497 Starting Input Location Destination for measurements Reference temperature location. Option code for base temperature measurement 1= Low wrt ground 2= High wrt ground First measurement Input Card First measurement input channel First heating/cooling Excitation Card First heating/cooling excitation channel Heating cooling excitation voltage (negative mV) Heating duration time (0.01s) Delay after heating before zero measurement (0.01 s) Cooling duration time (0.01s) Delay after cooling before wetbulb measurement (0.01s) Delay between wet-bulb measurements (0.01 s) 15: 4 Number of wet-bulb measurements per psychrometer Input Locations Altered: Np (Nm +2) Intermediate Storage: 7 Execution Time (ms): 80+60Np + (Nm+1)(300 + 225Np) + NmDm+Ds Np - Number of TCPs Nm - Number of wet-bulb measurements per TCP Dm - Parameter 14 in ms Ds - Parameters 10+11+12+13 in ms The excitation voltage for the heating current is specified in Parameter 9. The heating duration (Parameter 10), delay before the zero measurement (Parameter 11), cooling duration (Parameter 12), and the delay before the first wet-bulb measurement (Parameter 13) are all selectable in units of hundredths of a second. The number of wet-bulb readings in the sequence and the time between each reading is specified by Parameters 15 and 14, respectively. 2. TCP MEASUREMENT SEQUENCE The zero reading and each individual wet-bulb reading are the numerical average of five slow integration differential measurements made on the ± 1.5 mV full scale range. The resolution is 50 nV with an RMS input noise level of 30 nV. Averaging the five measurements reduces the noise level in the reading to 15 nV. All the TCPs connected to one A3497 Interface are measured sequentially at a rate of approximately 45 ms per TCP; this process is repeated five times, the average formed, the zero reading subtracted, and the result stored in Input Storage. Approximately 60 ms is required for self calibration before each of the five measurement sequences. The time required to complete this five measurement per TCP reading tr, is thus: tr = 5(60+45Np) = 300+225Np where tr is in milliseconds and Np is the number of TCPs measured (maximum of five). Thus, five TCPs are read in about 1.4 s while three TCPs require about 1 s. 1