Campbell Scientific TC Psychrometer PROM and Cooling Current Interface (S3497X, A3497) - Manual (Page 6)

25 PELTIER THERMOCOUPLE PSYCHROMETER
2
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 copper-
constantan (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
Wescor Merrill
CR7 Model PST-55 Model 74
Hi Red White
Lo Black Blue
Ground Blue Red