10 • 35-3001A-10 Oxygen/Carbon Monoxide Sample-Draw Detector
Flowmeter Circuit Board
The flowmeter circuit board is mounted to the left side of the main circuit board using standoffs. It
includes the flowmeter, sensor flow control valve, status LEDs, pressure switch, and pump reset
switch.
Flowmeter
The flowmeter is mounted to the right side of the flowmeter circuit board. You can see it through the
window in the door. A ball in the flowmeter column indicates the flow rate to the sensor. The
flowmeter measures the flow in the range 0.2 to 2.0 SCFH (Standard Cubic Feet per Hour). The
optimum flow rate is 1.0 SCFH.
Sensor Flow Control Valve
The sensor flow control valve is mounted to the flowmeter circuit board above the flowmeter. The
sensor flow control valve adjusts the flow rate to the detector. Turn the valve’s knob clockwise to
increase the flow and counterclockwise to decrease the flow.
Status LEDs
Two status LEDs are above the flowmeter. They are also visible through the window in the housing
door. The green Pilot LED is on when the sample-draw detector is receiving power from the controller.
The red Fail LED is on when the sample flow rate is below the low flow level.
Pressure Switch
The pressure switch is mounted to the back of the flowmeter circuit board. The pressure switch
monitors the flow rate of the incoming gas sample.
If the flow rate falls below the preset low flow level, the pressure switch causes the Fail LED to turn on
and interrupts the signal from the detector. The interrupted detector signal causes a fail condition at the
controller. The low flow level is factory-set at 0.6 SCFH (±0.1 SCFH). See page 23 for instructions to
adjust this setting.
Pump Reset Switch
The pump reset switch is located to the left of the status LEDs. When a low flow condition occurs, the
pump will be shut off. To reset the low flow condition and start the pump again, press and hold the
pump reset switch for about 2 seconds, then release.
Hydrophobic Filter
The hydrophobic filter is located toward the bottom left of the main circuit board. The filter prevents
particulates and water in the incoming gas sample from damaging the flow and detection systems.
Replace the filter when it appears dirty, discolored, or clogged.
Pump
The pump is mounted to the right side of the main circuit board. The pump pulls the gas sample into
the sample-draw detector. The pump operates on 24 VAC, which is generated from the 24 VDC
supplied by the controller.
Oxygen Sensor
The oxygen sensor is installed in a flow block and the flow block is mounted to the middle of the main
circuit board. It is the lower of the two flow blocks mounted to the main circuit board. The oxygen
sensor is retained in the flow block by a bracket with two screws.
The oxygen cell is protected within the sensor assembly. Through a series of chemical and electronic
reactions, the oxygen cell produces a millivolt output that is proportional to the detection range of the
sample-draw detector. The leads extending from the sensor terminate in lugs that connect to the
oxygen terminal strip.
