T775 SERIES 2000 ELECTRONIC STAND-ALONE CONTROLLERS
63-7147—3
28
Carbon Dioxide (CO
2
) Sensing – T775U
Application Description
In this application, the T775U is controlling an outside air
damper based on CO
2
sensing input, to maintain reasonable
CO
2
levels.
Sensor Designation
This device application requires one sensor.
• Sensor A is sensing CO
2
in parts per million
Operation
In this example, as the CO
2
level increases a damper is
opened to allow fresh air to enter the facility.
Many CO
2
sensors are used with a 0-2,000 ppm output range.
Because ppm units are not available on the T775, there are
two choices.
• You could set up for percent (%), and enter 0 and 100 for
the minimum and maximum sensor inputs. Then use
Table 2 to convert the percent to ppm levels.
• Alternatively, you can set the units for Pa (Pascals, a unit of
pressure not normally used in the United States) and enter
0 for a minimum and 2,000 for a maximum. The controller
will simply convert the input signal (0-10 Vdc or 4-20 mA)
into the correct ppm value and display it on the screen.
Units will still show Pa, but this may be easier than
converting using the table.
Programming Example
Program in Setup:
Press and hold the
MENU
button for 5 seconds to enter
Setup mode, and then select:
— Sensors
→
Sensor A:
Type = 0-10V
Units = %
Minimum Value = 0.0
Maximum Value = 100.0
NOTE: Sensor A is a 0-10 Vdc CO
2
sensor (C7232) with a
range of 0 to 2,000 PPM
IMPORTANT
After the desired value is selected, be sure to press
the
#
or
$
or
HOME
button in order to save that
value in the controller’s memory.
The T775U displays the sensor output as a percentage (%).
Use Table 2 to convert from % to PPM.
Alternatively you can choose Pascals (Pa) as the units and
enter the minimum and maximum PPM levels for the sensor
(0 to 2,000 for example). The controller will show the correct
CO
2
level but displays Pascals (Pa) instead of PPM.
Wiring
See Fig. 33 for wiring connections for the T775U and the CO
2
sensor.
Fig. 33. T775U Wiring - Carbon Dioxide sensing control.
Table 2. CO
2
PPM to % conversion.
CO
2
PPM
Percentage Display
0
0%
200
10%
400
20%
600
30%
800
40%
1,000
50%
1,200
60%
1,400
70%
1,600
80%
1,800
90%
2,000
100%
NOTES:
1. SHIELDED CABLE MUST BE
CONNECTED TO A SEPARATE
EARTH GROUND. HOWEVER,
DO NOT GROUND SHIELDED
CABLE AT SENSOR END.
2. TO MINIMIZE NOISE PICKUP,
MAKE SENSOR CONNECTION
FROM SHIELDED CABLE AS
CLOSE AS POSSIBLE TO
SENSOR BODY.
C7232 CO2 SENSOR
(0-10 VDC CONNECTION)
SENSOR A
+ –
SHIELDED CABLE
+ –
–
+
ANALOG
OUT
24 V
L1 (HOT)
L2
RED
BLACK
1
M24882
USE SEPARATE 24 V TRANFORMER TO POWER THE C7232 CO2 SENSOR.
PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
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