9
A100g
JULY 2017
SETTING ADJUSTMENTS OF SENSORS
There are several selectable features included in the
Hansen Gas Sensor. Refer to pages 7 and 8 for the
adjustable features. Note the adjustment screws on
the circuit board for the electrochemical sensor have
a different layout then the semiconductor sensor. The
jumpers may be moved without shutting off the power.
With the use of a 12-24V AC/DC power source, these
sensors can be set to the customers specifications
prior to mounting the gas sensor in the engine room or
refrigerated area.
Relay and Horn Set Point
The adjustment screw A sets the trip point for the relay
and the alarm horn. A 0-5V scale measures the full
sensing range of the Gas Sensor. The trip point voltage
can be measured at points ØV and REF1 located on the
board. Refer to pages 7 and 8 for location of the reference
pin REF1. A 100 PPM range gas sensor reading 2.5 volts
at the reference pin would be equivalent to half the range
or 50 PPM.
Default factory setting is 50% of range.
Note that the set point of the relay is independent
of the set points to the Gas Detection Monitor.
To
assist in selecting the correct voltage for the desired
PPM set point, refer to
Table 2
below.
VOLTAGE
PPM RANGE
5
100
250
500
3000
10,000
4
80
200
400
2400
8000
3
60
150
300
1800
6000
2
40
100
200
1200
4000
1
20
50
100
600
2000
.5
10
25
50
300
1000
TABLE 2
Setting Minimum or Maximum Relay Set Point
There is a practical limit for setting the minimum or
maximum relay set point. There is the tolerance of the
electronic board and the test meter. It is recommended
to keep the relay set point between 10% and 95% of
the range.
Time Delay
A time delay for the operation of the relay and alarm horn
can be selected using jumpers JP5 and JP6.
Default
factory setting is zero.
Alarm Horn
Alarm horn is 40dB at 2 feet for Basic Sensor; all other
models not audible with cover closed. The alarm horn
can be disabled using jumper JP2.
Default factory
setting is enabled.
Adjustment of the Detection Range
Adjustment Screw Z sets the zero (0 PPM) and Adjustment
Screw S sets the span. (full range = 1000 PPM, for
instance). The output of the sensor circuit can be
monitored via test points ØV and VS. The full range equals
5 volts DC. See
Calibration Procedures
for further details.
Note:
The unit is calibrated in factory there is no need
for adjustment of Span in regular circumstance. Span
needs to be adjusted only when the unit is calibrated,
e.g. calibration gas applied to the sensor. After initial
installation of the unit adjustment of Zero might be needed
as the ambient conditions are different between factory
and install location.
GAS SENSOR OPERATION
On power up the internal relay will energize switching
from the normally closed position to the normally open
position. In addition, the sensor will go through an
initial warm-up delay for five minutes. At this time, the
I and V outputs are 0. This is indicated by the green
LED flashing approximately every second. After five
minutes, the green LED stays on continuously, indicating
power to the gas sensor and ready for service. The gas
sensor also sends a milliamp output signal to the gas
detection monitor, PLC, or computer. This output signal
is proportional to the amount of the specific gas being
monitored in the air around the gas sensor. If signal
reads 0.5mA, it is an indication the sensing element has
failed or disconnected. Verify proper installation and
refer to testing and calibration procedures.
Fail Safe Operation
Hansen Gas Sensors feature a one amp SPDT alarm
relay for fail safe operation. In the event of a sensor
element failure, power loss or gas leak detection, the
internal relay will be de-energized. This relay can be
used to trigger an external alarm.
Start Up Test
Once the installation of the gas sensors and monitors
is complete, the entire gas sensor system should be
tested. Normally, two people are required—one person
using a small amount of the target gas to “bump” each
sensor in turn, a second person at the monitor location
to verify the sensor is connected to the correct input,
responds to the target gas and causes the lights and
alarms to respond.
Gas Sensor Testing
Field testing of the gas sensor is normally done for three
reasons. The first is to determine if the gas sensor is
responding to the specific gas. This test is sometimes
called a “bump” test. The frequency of the test is usually
stated in the facilities Process Safety Management (PSM)
document, but not less than every six months.
The second reason for gas sensor testing is to be sure
the gas sensor is properly calibrated. Depending on the
sensing element type, the sensor sensitivity will change
with time. In particular, electrochemical sensors use a
material that is depleted with time. The length of time
varies depending on the amount of exposure to the target
gas, the ambient temperature and humidity, and changes
to temperature and humidity range. Atmospheres that
are very dry or very humid will shorten the life of a
sensing element.
The third reason is to be sure the gas sensor output
triggers the alarms on the monitor, PLC, Gas Alert System,
horn or lights. All gas sensors are recommended to be
recalibrated at least every 6 months. All sensor types
should be recalibrated immediately after exposure to
a large concentration of the target gas.
Note:
Insurance companies, local and state agencies
may require more frequent testing and calibration; refer
to insurance carrier mandates as well as local and state
codes.
