Stand-Alone Carbon Monoxide Control

While many parking garage applications require a carbon monoxide transmitter with a current or voltage signal to a Building Management System (BMS) controller, there are applications for smaller garages/ service stations for the carbon monoxide sensor to provide relay contacts, to control the fan or damper operation directly, without the BMS controller. These systems are considered stand-alone systems. The Dwyer Series GSTA and Series CMT200 transmitters can meet the need of the applications that just require a current or voltage output. For stand-alone systems, these transmitters can be combined with a SCD Series process controller.


The SCD Series controller accepts the signal input from the carbon monoxide transmitter and allows the user to adjust the set point and differential for the fan/damper operation, typically around 25 PPM of carbon monoxide. In addition to the set point, the user can also set a high gas concentration alarm, normally set around 100 PPM. For systems that need to communicate with a BMS or other online management system, the Series SCD has Modbus® RS-485 serial communications as a standard offering to give feedback of the current gas concentration, set points, and output status.


For additional energy savings, the SCD series controller has PID capabilities that can be used to drive a VFD to have better ventilation control. Both the transmitters and the SCD can be powered using a common 24 VDC power supply such as our model SCD-PS. For applications that require additional sensors, pluggable slave modules are available that allow the controller to be expanded to have multiple carbon monoxide sensor loops.


Programming Guide for the SCD Series in this Application

All programming will be configured either at the factory or using the communications port. For the communication port the programmer will need a model MN-1, RS-485 to USB converter, laptop, and SCD-SW configuration software.


Series SCD Parameter Settings

Initial Setting Parameters:

  • Input: 4 to 20 mA.
  • Units: None.
  • Scale High: 200.
  • Scale Low: 0.
  • Control Mode: On-Off (PID if using with VFD).
  • Heat/Cool Selection: Cool.
  • Alarm 1 Type: 11-high alaram with hysteresis.
  • Communication Data: Set to match BMS settings.

Operating Setting Parameters:

  • Set Point: 25 (or different carbon monoxide PPM value depending on local requirements)
  • Alarm 1 Set Point: 100 (or different carbon monoxide PPM value depending on local requirements)

Regulation Setting Parameters

  • Cooling Hysteresis (Differential): 5 (or different carbon monoxide PPM setting depending on local code)
  • Process Input Offset: Only if there is any adjustment required due to sensor drift or error due wire resistance.