Campbell Scientific S3497X TC Psychrometer PROM and Cooling Current Interface - Page 6

25 PELTIER THERMOCOUPLE PSYCHROMETER If zero delay between wet-bulb readings is specified in Parameter 14, the time between recorded readings for a given TCP is equal to tr. The time interval between initiating the first measurement and completing the fifth measurement for a given TCP is approximated by ti = (4) (60) +45(4Np+1) = 285+180Np where ti is in milliseconds. 3. TCP BASE TEMPERATURE MEASUREMENT Three wire psychrometers such as those manufactured by J.R.D. Merrill Specialty Equipment* or Wescor, Inc.** provide for a TCP base temperature measurement. The base measurement made by Instruction 25 is identical to the standard CR7 Single-ended Thermocouple Measurement Instruction 13 using an input range of 15 mV, a "slow" integration time (16.6 ms), and a copperconstantan (type T) TC. Since the base temperature TCs are connected to every other single-ended input channel, Parameter 4 may be used to specify whether the first measurement begins on the high or low input. The base temperature measurement requires a reference junction temperature in order to compute an absolute temperature. This value is obtained using the Panel Temperature Instruction 17. The Input Location Number of the reference temperature is entered in parameter 3. If 0 is entered for Parameter 3, no base temperature measurements are made, and no Input Storage is allocated for these measurements. This option allows for situations where the base temperature TCs are not copper-constantan, and the measurements must be made using Instruction 13. 4. INPUT AND A3497 INTERFACE CONNECTIONS TCPs manufactured by both Merrill Specialty Equipment and Wescor, Inc. are wired identically. Figure 1 shows the TCP connections to both the A3497 and CR7. Table 1 gives color coding for both a Merrill and Wescor screened psychrometer. Note that: TCP temperatures are lower than the base temperature (e.g., at wet-bulb) result in positive readings. Connect low side of each TCP to the A3497 with a wire inserted into the low input terminal for the respective TCP. Instruction 25 automatically advances to the next excitation channel each time a new A3497 is encountered within a Loop. Use the next available excitation channel for each additional A3497 wired to the CR7. NOTE: All A3497s measured within a loop must excite the same number of TCPs. * J.R.D. Merrill Specialty Equipment R.F.D. Box 140A Logan, UT 84321 (801) 752-8403 ** Wescor, Inc. 459 South Main Street Logan, UT 84321 (801) 752-6011 Table 1. TCP Color Coding for Wescor and Merrill Psychrometers CR7 Hi Lo Ground Wescor Model PST-55 Red Black Blue Merrill Model 74 White Blue Red 2