Campbell Scientific CR7 Measurement and Control System - Manual (Page 20)

CR7 MEASUREMENT AND CONTROL SYSTEM OVERVIEW
OV-10
Tables OV3-1 and OV3-2 summarize the
Keyboard Commands and Control Modes used
to program the CR7, monitor Input and Final
Storage and control data output to peripherals.
The instructions, and their associated
parameters, are the CR7's programming steps
and are used to build the CR7's program. It is
not necessary to understand all the commands
to proceed with this programming exercise. It is
helpful to find the example's instructions on the
CR7 Prompt Sheet provided with this manual.
As you become familiar with programming the
CR7, you will find that the Prompt Sheet or the
PC208 program EDLOG has all the information
you need to write your program. By following
along on the Prompt Sheet as you proceed with
this exercise, you will learn how to use it to write
your own programs.
OV4.1 MEASUREMENT
To make a thermocouple temperature
measurement, the CR7 must know the
temperature of the reference junction. The CR7
takes the reference temperature, converts it to
the equivalent TC voltage, adds the measured
TC voltage and converts the sum to
temperature through a polynomial fit to the TC
output curve. In this example, the reference
junction is at the Analog Input Card. Its
temperature is measured with Instruction 17,
Panel Temperature. If you have an Analog
Input Card with RTD, check to see which
number is assigned to it. A tag labeled RTD is
on the left hand side and the card number is on
the right hand side of the Analog Input Card. If
the RTD card is not card 1, you must enter the
correct card number as Parameter 1 of
Instruction 17. If you do not have an Analog
Input Card with RTD, you will omit Instruction 17
from the Program and enter a "dummy"
reference temperature after the Program is
compiled.
The thermocouple temperature measurement is
made using Instruction 14 (differential voltage
measurement of TC) on differential channel 5.
When using a copper-constantan
thermocouple, the copper lead is connected to
the high input of a differential channel and the
constantan lead is connected to the low side.
The channel numbering printed on the Analog
Input Cards refers only to differential channels.
Either the high or low side of a differential
channel may be used for single ended
measurements. (Each side is counted when
assigning single ended channel numbers; e.g.,
the high side of differential channel 8 is single
ended channel 15 and the low side is single
ended channel 16).
The first parameter in Instruction 14 is the
number of times to repeat the measurement: 1
is entered because only one thermocouple is
measured. If more thermocouple
measurements were desired, the copper leads
would be connected to the high sides of
consecutive differential channels, the
constantan leads to the low sides and the
number of repetitions entered in Parameter 1
would equal the number of thermocouples.
Parameter 2 is the voltage range to use when
making the measurement. The output of a
copper-constantan thermocouple is
approximately 40 microvolts per oC difference
in temperature between the two junctions. The
+5000 uV scale will provide a range of +5000/40
= +125 oC (i.e., this scale will not overrange as
long as the measuring junction is within 125 oC
of the panel temperature). The resolution of the
+5000 uV range is 166 nV or 0.004 oC.
Parameter 3 is the Input Card number and
Parameter 4 is the channel on which to make
the first measurement. If more than one
thermocouple is measured, the CR7 will
automatically advance through the channels
and on to the next card if necessary. Similarly,
Parameter 7 is the Input Storage Location in
which to store the first measurement; e.g., if
there are five repetitions and the first
measurement is stored in location 3, the final
measurement will be stored in location 7.
Parameter 6 is the Input Storage location in
which the reference temperature is stored, and
Parameters 8 and 9 are the multiplier and offset
to apply to the temperature value. A multiplier of
1 and an offset of 0 give the result in oC, a
multiplier of 1.8 and an offset of 32 give the
result in oF.
Now that you have some idea of what you are
telling the CR7 by entering the parameters, we
will proceed with programming the CR7.