6 • 35-3001-06H-DIL Hydrogen/Oxygen Sample-Draw Detector
half of what the flowmeter indicates. The flowmeter measures the flow in the range 0.2 to 2.0
SCFH (Standard Cubic Feet per Hour). The optimum flow rate is 0.8 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 detectors. 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 1.0 SCFH (±0.1 SCFH). See
“Adjusting the Low Flow Setting” on page 17 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.
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 upper 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.
Dilution Fitting
The catalytic hydrogen sensor requires oxygen to operate. In environments where there is not
enough oxygen to operate the hydrogen sensor, the dilution fitting adds sufficient oxygen by
blending ambient air with the incoming sample. The standard dilution fitting dilutes at a ratio of
approximately 1:1 (one part air to one part sample).
Hydrogen Sensor
The catalytic hydrogen sensor detects hydrogen in the %LEL range. It uses a catalytic element for
detection. The reaction of gas with oxygen on the catalyst causes a change in the resistance of the
element which changes the current flowing through it. The current is proportional to the detection
range of the sample-draw detector.
The hydrogen sensor is installed in the 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. An
