Unless otherwise specified, the detector is shipped from the factory with the selection jumpers
set in the 4-20 mA current loop mode. To change the mode to one of the voltage modes, the
JP2 selection jumper must be moved to the “Volts” pins. After this, the JP3 jumper may be
moved to the desired output. To return to the 4-20 mA mode, replace the JP3 jumper on the
“mA” pins and the JP2 jumper to the “4-20 mA” pins. Ensure that power to the detector is off
before attempting to change these settings.
The output signal is connected using a two-conductor shielded cable. The resistance of the
total cable length must be less than 250 Ω if the 4-20 mA current loop mode is selected. For
the voltage modes, the input impedance of the controller must be greater than 100 kΩ.
Ground the shield of the signal cable at the controller earth grounding point only to avoid noise
conditions created by ground loops. Although the positive signal lead can be shorted to
ground without causing damage, this condition should be avoided. Also, do not route the
signal cable in conduit containing other wiring to avoid unwanted noise pickup.
The linear outputs produce a signal based on the full scale response of the detector. See the
graphs in figure 14 on page 45 of this manual.
As an example, a Brasch Carbon Monoxide Detector has a full scale output of 200 PPM CO.
For the 4-20 mA output mode, 0 PPM CO would equal 4 mA while 200 PPM CO would equal
20 mA. Follow the instructions supplied with your controller to adjust the controller’s input for
the proper scaling of the signal. Consider a 0 mA or 0 VDC output as a failed sensor condition.
Obtaining the Best Operation
Carbon Monoxide and/or Nitrogen Dioxide Detectors
These detectors are designed to control the ventilation system in response to a rising
concentration of the target gas. No two installations will be exactly the same. Different
ventilation components, the number of gas producing sources, air flow patterns inside the
room, the total room volume, and the exact location of the detector(s) influence how effective
the system is in controlling the target gas concentration. The ideal operation occurs at
detector settings that remove the most target gas without unnecessarily operating the
ventilation system.
The detector has two variables that can be adjusted to obtain the best performance under the
conditions it is operating – Low Alert threshold concentration and delay time. Each of these
are adjustable in 8 steps using the corresponding rotary switches located on the control board.
The threshold sets a desired maximum concentration while the delay assures that transient
levels of gas do not cause the fans to operate for unnecessarily short periods of time. This
delay for any target gas is adjustable from 0 to 7 minutes in 8 steps of one minute each. The
IOM01
Rev 1.0 – December 22, 2020 28
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