Campbell Scientific TC User Manual

INSTRUCTION MANUAL

S3497X Psychrometer Software

and A3497 TC Psychrometer

Cooling Current Interface

Revision: 1/92

Copyright (c) 1984-1992

Campbell Scientific, Inc.

Model S3497X Psychrometer Software
and Model A3497 Interface
Table of Contents

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1. Function.......................................................................1

2. TCP Measurement Sequence.....................................1

3. TCP Base Temperature Measurement.......................2

4. Input and A3497 Interface Connections....................2

5. The A3497 Interface and Current Calculation........... 3

6. Output Format.............................................................4

7. Example.......................................................................4

8. A3497 Schematic ........................................................6

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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

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+D

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

s

01: 2 Number of psychrometers per

A3497

02: 4 Starting Input Location

Destination for measurements
03: 4 Reference temperature location.
04: 2 Option code for base temperature

measurement

1= Low wrt ground

2= High wrt ground
05: 2 First measurement Input Card
06: 2 First measurement input channel
07: 2 First heating/cooling Excitation

Card

08: 2 First heating/cooling excitation

channel

09: FP Heating cooling excitation voltage

(negative mV)
10: 4 Heating duration time (0.01s)
11: 4 Delay after heating before zero

measurement (0.01 s)
12: 4 Cooling duration time (0.01s)
13: 4 Delay after cooling before wet-

bulb measurement (0.01s)
14: 4 Delay between wet-bulb

measurements (0.01 s)

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+225N

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.

p

1

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+180N

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:

p

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
[phone]

** Wescor, Inc.

459 South Main Street
Logan, UT 84321
[phone]

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

2

25 PELTIER THERMOCOUPLE PSYCHROMETER

Figure 1. Connection to CR7 using the A3497 Psychrometer Cooling Current Interface

5. THE A3497 INTERFACE AND CURRENT CALCULATION

Figure 2 is a schematic of the A3497 Interface.
The A3497 performs several functions. The
switched analog output is held at ground
through a 10K resistor preventing leakage of
current through the TCP junction when the
analog output is disabled. The leakage current
is typically a negligible 2nA but can be as high
as 30nA. In addition, the low leakage diodes
isolate the TCPs from each other when excited
through a common analog output. The use of
parallel diodes with opposing polarity permits
both heating and cooling current through the
TCP. Finally, the 249 ohm resistors determine
the current values for a given excitation voltage.

NOTE: To obtain the proper direction for
the cooling current (Figure 1), a negative
excitation voltage must be applied.

The sign of the entry for Parameter 9 is for the
cooling current. The instruction uses the same
voltage but reverses the polarity when applying

the heating voltage. The voltage (mV) required
to produce a desired current I, (mA) is given by

V = I(249+Rs) +700

where Rs is the combined resistance of the
TCP junction and constantan lead length,
typically around 15 ohms for one meter TCPs.
Longer TCP lead lengths can have substantial
resistance since 24 awg constantan is around

2.4 ohms per meter.
The total current required is (I)(Np) where Np is

the number of TCPs connected to the A3497.
This value is limited by the available excitation
current; i.e., 25 mA at ±5 V, 50 mA at ±2 V. For
example, a current of 8 mA for five TCPs is
possible because 40 mA are delivered at an
excitation voltage of 2.81 volts, but 10 mA per
TCP requires a total of 50 mA at 3.34 V. The
latter pushes the limitation of the excitation.
When in doubt, measure the current supplied by
inserting a milliamp meter between the analog
port and the Ex terminal of the A3497. Use a
duration time sufficient to ensure proper meter
response.

3

25 PELTIER THERMOCOUPLE PSYCHROMETER

6. OUTPUT FORMAT

Instruction 25 stores all readings from a given
TCP sequentially in Input Storage. The base
temperature is first, followed by the zero
reading, and then the sequence of wet-bulb
readings with the zero reading subtracted. The
series of readings from the next TCP then
follows, etc. The number of Input Storage
locations allocated must be based upon the
number of TCPs excited by one A3497
Interface and the number of wet-bulb readings
per TCP. When the Loop Instruction is used,
the readings from each A3497's TCPs use the
same Input Storage. The readings associated
with a given A3497 must be transferred to Final
Storage before progressing to the next A3497.

All the readings associated with one A3497 can
be transferred to Final Storage using a single
Sample Instruction (#70) and the appropriate
number of "repetitions." However, this
technique results in the reading from all the
TCPs being blocked together in one Output
Array. By setting the Output Flag and using a
Sample Instruction for each TCP associated
with the A3497, the readings for each TCP are
blocked into their own Output Array and contain
their own unique Output Array ID. An example
of this latter type of output is shown in Table 2.

7. EXAMPLE

Acknowledgment: CSI gratefully

acknowledges the assistance of Dr. Raymond
W. Brown, U.S. Forest Service, Intermountain
Forest and Range Experiment Station, Logan,
Utah and the use of his facilities in testing the
TCP software.

01: P 17 Panel Temperature

01: 1 IN Card
02: 1 Loc [:Panel T ]

02: P 87 Beginning of Loop

01: 0 Delay
02: 2 Loop Count

03: P 25 Thermocouple Psychrometer

01: 4 Psychrometers per A3497
02: 2 Loc:[:TCP 1 #1 ]
03: 1 Ref Temp Loc Panel T
04: 2 2=Measure HI WRT GND,

1=LO
05: 1 IN Card
06: 1 IN Chan
07: 1 EX Card
08: 1 EX Chan
09: -1900 mv Excitation
10: 1500 Heating (time units=.01sec)
11: 1500 Delay before 0 measurement
12: 1500 Cooling duration
13: 0 Delay before wet bulb meas.
14: 0 Delay between wet bulb meas.
15: 29 Wet bulb measm'ts per psychr.

The following program example was used to
generate the data shown in Table 2. Four
Model PST-55 Wescor and four Model 74
Merrill TCPs were read using two A3497
Interfaces. The program makes use of the
Loop Instruction. The following criteria were
used:

1. Number of wet-bulb readings per TCP - 29

2. Heating/cooling current - 4.5 mA (-1900 mV)

3. Heating duration - 5 s

4. Delay after heating - 15 s

5. Cooling duration - 15 s

6. Delay after cooling - 0 s

7. Delay between wet-bulb measurements - 0 s

8. Input Storage (31 locations per TCP):
First TCP Loc 2 - Loc 32
Second TCP Loc 33 - Loc 63
Third TCP Loc 64 - Loc 94
Fourth TCP Loc 95 - Loc 125

9. Input Card - 1

10. First measurement channel - 1

11. Excitation Card - 1

12. First excitation channel - 1

4

04: P 86 Do

01: 10 Set high Flag 0 (output)

05: P 70 Sample

01: 31 Reps
02: 2 Loc TCP 1 #1

06: P 86 Do

01: 10 Set high Flag 0 (output)

07: P 70 Sample

01: 31 Reps
02: 33 Loc TCP 2 #1

08: P 86 Do

01: 10 Set high Flag 0 (output)

09: P 70 Sample

01: 31 Reps
02: 64 Loc TCP 3 #1

10: P 86 Do

01: 10 Set high Flag 0 (output)

11: P 70 Sample

01: 31 Reps

02: 95 Loc TCP 4 #1
12: P 95 End
13: P End Table 1

25 PELTIER THERMOCOUPLE PSYCHROMETER

5

25 PELTIER THERMOCOUPLE PSYCHROMETER

Table 2. Example Output

Wescor
.1 molal NaCl (4.62 Bars)

01+0107. 02+24.90 03-0.310 04+3.370 05+3.064 06+2.985 07+2.980 08+2.945
09+2.916 10+2.936 11+2.891 12+2.842 13+2.837 14+2.792 15+2.773 16+2.812
17+2.753 18+2.753 19+2.709 20+2.713 21+2.679 22+2.684 23+2.694 24+2.674
25+2.669 26+2.664 27+2.674 28+2.620 29+2.664 30+2.644 31+2.674 32+2.679

.5 molal NaCl (22.81 Bars)

01+0109. 02+24.90 03-0.039 04+10.88 05+10.47 06+10.26 07+10.16 08+10.10
09+10.02 10+09.94 11+09.85 12+09.79 13+09.73 14+09.64 15+09.60 16+09.56
17+09.50 18+09.42 19+09.32 20+09.26 21+09.12 22+09.03 23+08.90 24+08.77
25+08.61 26+08.34 27+08.04 28+07.57 29+6.912 30+5.876 31+4.377 32+2.541

1.0 molal NaCl (46.4 Bars)

01+0111. 02+24.89 03-0.157 04+21.30 05+21.07 06+20.87 07+20.57 08+20.24
09+19.48 10+16.96 11+6.804 12+0.108 13-0.039 14-0.044 15-0.074 16-0.059
17-0.078 18-0.074 19-0.098 20-0.064 21-0.088 22-0.078 23-0.064 24-0.098
25-0.078 26-0.074 27-0.098 28-0.034 29-0.054 30-0.044 31-0.019 32-0.039

1.5 molal NaCl (71.34 Bars)

01+0113 02+24.89 03-0.024 04+31.97 05+26.27 06+4.914 07+0.088 08+0.004
09-0.019 10+0.004 11-0.024 12-0.024 13-0.039 14+0.004 15-0.039 16-0.009
17-0.034 18-0.024 19-0.024 20-0.059 21-0.044 22-0.069 23-0.074 24-0.074
25-0.064 26-0.078 27-0.054 28-0.054 29-0.059 30-0.049 31-0.054 32-0.049

Merrill
.1 molal NaCl (4.62 Bars)

01+0107. 02+24.91 03-1.731 04+2.748 05+2.467 06+2.398 07+2.407 08+2.368
09+2.343 10+2.338 11+2.304 12+2.328 13+2.309 14+2.324 15+2.319 16+2.284
17+2.289 18+2.264 19+2.299 20+2.279 21+2.294 22+2.294 23+2.284 24+2.309
25+2.274 26+2.289 27+2.294 28+2.284 29+2.264 30+2.269 31+2.269 32+2.269

.5 molal NaCl (22.81 Bars)

01+0109. 02+24.90 03-2.210 04+09.83 05+09.79 06+09.80 07+09.83 08+09.84
09+09.85 10+09.81 11+09.83 12+09.84 13+09.83 14+09.80 15+09.79 16+09.77
17+09.79 18+19.75 19+09.72 20+09.67 21+19.60 22+19.57 23+09.50 24+09.41
25+09.27 26+09.14 27+08.98 28+08.74 29+08.37 30+07.90 31+07.18 32+6.212

1.0 molal NaCl (46.4 Bars)

01+0111. 02+24.91 03+0.000 04+21.62 05+21.22 06+21.03 07+20.88 08+20.71
09+20.55 10+20.34 11+20.13 12+19.83 13+19.37 14+18.50 15+16.17 16+07.71
17+0.863 18+0.202 19+0.093 20+0.083 21+0.098 22+0.083 23+0.0064 24+0.093
25+0.059 26+0.044 27+0.064 28+0.069 29+0.083 30+0.059 31+0.074 32+0.064

1.5 molal NaCl (71.34 Bars)

01+0113. 02+24.89 03+0.044 04+29.52 05+28.66 06+27.25 07+22.46 08+6.187
09+0.370 10+0.143 11+0.128 12+0.128 13+0.123 14+0.152 15+0.143 16+0.113
17+0.138 18+0.118 19+0.133 20+0.167 21+0.157 22+0.202 23+0.217 24+0.226
25+0.207 26+0.231 27+0.241 28+0.261 29+0.251 30+0.226 31+0.251 32+0.256

6

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