8 • 35-3010A-12 Sample-Draw Detector
Charcoal Filter
The charcoal filter is located near the top of the housing. It is held in place by a metal clip. The
charcoal filter is placed after the H
2
S sensor and before the CO sensor in the flow system. It scrubs
out interfering gases which may cause the CO sensor to respond, such as H
2
S or certain
hydrocarbons. Replace the charcoal filter when false high CO readings are noticed, especially in
the presence of H
2
S.
Pump
The pump is located to the left of the main circuit board near the bottom left of the sample-draw
detector. 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 to the sample draw detector.
Flowmeter
The flowmeter is attached to the main circuit board near the top left corner (see Figure 1). A ball in
the flowmeter column indicates the flow rate of the sample-draw detector. The flowmeter
measures the flow in the range 0.2 to 2.0 SCFH (Standard Cubic Feet per Hour). Although the
sample-draw detector will operate down to a flow of 0.6 SCFH, the optimum flow rate is 1.2
SCFH.
Bypass Valve
The bypass valve is to the left of the flowmeter. The bypass valve adjusts the flow rate to the
sensor. Use a flat-blade screwdriver to adjust the bypass valve.
NOTE:
The bypass valve allows fine adjustments of the flow rate. For a wider range of
adjustment, use the flow adjust potentiometer (see Figure 1).
Status Lights
Two status lights are above the flowmeter.
Pilot Light
The green Pilot light is on when the sample-draw detector is receiving power.
Fail Light
The red Fail light is on when the sample flow rate is below the low flow level.
NOTE:
The factory set low flow level is 0.6 SCFH (±0.2). See “Adjusting the Low Flow Setting”
on page 21 for instructions to adjust this setting.
Pressure Switch
The pressure switch is mounted to the opposite side of the main 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 relay to
interrupt the signal in the 4-20 mA line for the H
2
S and CO channels. This causes a downscale
reading at the monitor on these channels. The low flow level is factory-set at 0.6 SCFH (±0.2
SCFH).
NOTE:
There is no low flow indication for the oxygen channel.