Campbell CR23X, Operator'S Manual

Page: 313 / 374

TD ADDENDUM—OVERVIEW
AD-OV-5
OV5.2 SAMPLE PROGRAM 2
This second example is more representative of a
real-life data collection situation. Once again the
internal temperature is measured, but it is used
as a reference temperature for the differential
voltage measurement of a type T (copper-
constantan) thermocouple; the CR10X should
have arrived with a short type T thermocouple
connected to differential channel 5.
When using a type T thermocouple, the copper
lead (blue) is connected to the high input of the
differential channel, and the constantan lead
(red) is connected to the low input.
A thermocouple produces a voltage that is
proportional to the difference in temperature
between the measurement and the reference
junctions.
To make a thermocouple (TC) temperature
measurement, the temperature of the reference
junction (in this example, the approximate panel
temperature) must be measured. The CR10X
takes the reference temperature, converts it to
the equivalent TC voltage relative to 0
°
C, adds
the measured TC voltage, and converts the
sum to temperature through a polynomial fit to
the TC output curve (Section 13.4).
The internal temperature of the CR10X is not a
suitable reference temperature for precision
thermocouple measurements
. It is used here
for the purpose of training only. To make
thermocouple measurements with the CR10X,
purchase the Campbell Scientific Thermocouple
Reference, Model CR10XCR (Section 13.4) and
make the reference temperature measurement
with Instruction 11.
Instruction 14 directs the CR10X to make a
differential TC temperature measurement. The
first parameter in Instruction 14 is the number of
times to repeat the measurement. Enter 1,
because in this example there is only one
thermocouple. If there were more than 1 TC,
they could be wired to sequential channels, and
the number of thermocouples entered for
repetitions. The CR10X would automatically
advance through the channels sequentially and
measure all of the thermocouples.
Parameter 2 is the voltage range to use when
making the measurement. The output of a type
T thermocouple is approximately 40 microvolts
per degree C difference in temperature between
the two junctions. The
±
2.5 mV scale will
provide a range of
±
2500/40 =
±
62.5
°
C (i.e.,
this scale will not overrange as long as the
measuring junction is within 62.5
°
C of the panel
temperature). The resolution of the
±
2.5 mV
range is 0.33 µV or 0.008
°
C.
Parameter 3 is the analog input channel on
which to make the first, and in this case only,
measurement. Parameter 4 is the code for the
type of thermocouple used. This information is
located on the Prompt Sheet or in the
description of Instruction 14 in Section 9. The
code for a type T (copper-constantan)
thermocouple is 1.
Parameter 5 is the Input Storage location in
which the reference temperature is stored.
Parameter 6 is the Input Storage location in
which to store the measurement (or the first
measurement; e.g., if there are 5 repetitions
and the first measurement is stored in location
3, the final measurement will be stored in
location 7). Parameters 7 and 8 are the
multiplier and offset. A multiplier of 1 and an
offset of 0 outputs the reading in degrees C. A
multiplier of 1.8 and an offset of 32 converts the
reading to degrees F.
In this example, the sensor is measured once a
minute, and the average temperature is output
every hour. Once a day the maximum and
minimum temperatures and the times they occur
will be output.
The first example described program entry one
keystroke at a time. This example does not
show the "A" key. Remember, "A" is used to
enter and/or advance (i.e., between each line in
the example below). This format is similar to
the format used in EDLOG.
It's a good idea to have both the manual and the
Prompt Sheet handy when going through this
example. You can find the program instructions
and parameters on the Prompt Sheet and can
read their complete definitions in the manual.
To obtain daily output, the Data Table
instruction is followed by the Output Instructions
to store the daily maximum and minimum
temperatures and the time each occurs.