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The CTS-M20 transmitter/sensor is an industry leader providing a Carbon Dioxide concentration measurement for Indoor Air Quality and ventilation control in commercial office buildings, arenas, gymnasiums, conference centers, greenhouses and other facilities with variable occupancy. The QEL proprietary designed Infra-Red sensing element is the result of extensive research and development by QEL engineers. The NDIR technology is accurate to +/-2% with minimal zero drift. The M20 offers improved performance and standard features that are unmatched by others. A digital display (LCD), push-button programming and onboard meter jacks are all standard. The transmitter provides an analog output of 4-20mA DC or 2-10 VDC, linear to the measured gas range, for transmission to the Building Automation System (BAS) or controller supplied by QEL to others. The signal is fully user assignable over the gas range and can be configured for rising or falling gas concentration. Input voltages of 24 VDC or 24 VAC with wide tolerances are acceptable. Relay setpoints may be set as increasing or decreasing actuation simply by adjusting the deadband relationship. Set the deactuation point above the actuation setpoint and the M20 will automatically alarm on decreasing concentrations. It also offers "user selectable" time delay settings. Surface mount technology has been used for the circuitry resulting in reduction of overall size and total number of components. Fewer components increase the mean time between potential failure. Enclosure styles include the office mount for occupied rooms and the duct mount for return or fresh air intake ducts.


An air sample is introduced into the air column within the sensor block by diffusion. At one end of the sensor block by cycling tungsten light source emits Infra-red energy through the air column. Energy of a specific wavelength is absorbed by any carbon dioxide present. A thermopile element, at the opposite end, senses the remaining Infra-Red energy and provides a DC millivolt signal.

The electronics measures the difference in the thermopile output between the off and the on light source cycle. This signal differential is proportional to the concentration of carbon dioxide. Energy measurement during the off cycle of the lamp provides an absolute reference for compensation of environmental conditions experienced by the block. The measured signal is linearized and converted to a 4-20 mA or 2-10 volt DC output. Zero and span adjustments are provided for accurate calibration.


Carbon dioxide (CO2) is the most abundant greenhouse gas. Major sources of CO2 include the combustion of fossil fuels and the respiration of human beings. Human exhaled breath contains a concentration of about 40,000 ppm (4%) CO2, while trees and plants consume CO2 and produce oxygen. The global average background CO2 level is, at present, approximately 375 ppm and this average has been rising for decades. The background concentration in any one area may vary depending on the environmental conditions. The center of a city will have a much higher level, while the background level in a forested area will be lower. ASHRAE number 62-1989 requires that commercial buildings have an adequate exchange system to ensure that CO2 concentrations do not exceed 1000 ppm.


Input Power: 24 Volts AC Floating
24 Volts AC One side grounded
24 Volts DC
Enclosure Materials: Polycarbonate/ABS blend
Fire Retardant
Temperature: 5 C to 40 C
Humidity: 0 to 99% RH, non-condensing
Pressure: Atmospheric 10%
Response Time: Less than 45 seconds for 90% of step change
Accuracy: 2% of range
Repeatability: 10 ppm
User Programmable

0 to 2500 ppm
0 to 5000 ppm
Approval: (pending)
Sensor Type: NDIR - Non-dispersive infra-red Thermo compensated aluminum block
Output Signal: 4 - 20 mA or 2 - 10 VDC
Display: Alphanumeric - 2 line X 8 digit LCD
Relay Output: One Single pole double throw (SPDT),
Form C, 1 amp dry contact
CSA 1500 V FCC Part 68
Time Delays: Actuation - 0 to 60 minutes in
5 minute increments
De-Actuation - 0 to 60 minutes in
5 minute increments
Mounting: Screw mounts to a standard 2" X 4" electrical junction box.

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Last Updated 03/18/10