Foxcroft FX-Mini User manual

1
Foxcroft FX-Mini & FX-Mini-R
Single Channel
Toxic Gas Detector
Operating Manual
Document OMFXMI I102019

2
Page intentionally left blank

3
Table of Contents
Page
Introduction, Gas Sensors Available 4
Warnings, Limitations 5
FX-Mini Gas Detector Illustration 6
Detector Component Description, Main Circuit Board, Default Wiring 7
Detector Component Description, Remote Reset, Power Supply, Battery Operation 8
Detector Component Description, 10A Relay Card 9
Gas Sensor, Test Button, Unpacking, Contents, Serial umber 10
Installation, Sensor Placement Guidelines 11
Installation, Mechanical & Dimensions 12
Installation, Wiring Connections 13
Startup 14
ormal Operation, Alarm Set Points and Conditions 15
ormal Operation and Alarm Conditions: Danger & Sensor Fail Alarms 16
ormal Operation and Alarm Conditions: Sensor Fail & Oxygen Sensors 17
Gas Sensors, Sensor Fail and Oxygen Sensors, General Information & Environmental Effects 17
Calibration 19
Calibration, Setting the Zero Point 20
Calibration, Full Calibration Toxic Gas Sensors 22
Oxygen Sensor Calibration 25
Ammonia Sensor Full Calibration 26
Sensor Response (Bump) Test 27
Gas Sensor Replacement, Electrochemical Cell Only 28
Gas Sensor with Transmitter Board Replacement 29
Gas Sensor Components 30
Technical Data, Specifications 31
Service Contact 33
Warranty 34

4
Introduction
The Foxcroft Equipment & Service Co. Inc. FX-Mini is a compact fixed point monitoring system
designed to detect and alarm to toxic gases or low oxygen levels in ambient air.
It’s available in two styles: the self contained all-in-one FX-Mini, and the remote controller style FX-Mini-
R, in which the gas sensor is mounted in a separate local enclosure placed in the potential hazard area.
Intended as a low cost device, it includes audible and LED visual alarm notification with 10A form C
relays to signal remote monitors and to actuate external alarm devices such as a beacon, horn, or
exhaust fan. o 4-20mA output or display of concentration is included.
The detector includes two alarm set points which are non-adjustable to prevent tampering or accidental
setting to an improper and possibly unsafe level. Upon sensing a gas concentration at an alarm set
point the detector will illuminate the appropriate color coded LED indicator light, sound the integral
audible buzzer (at the danger level only) and energize the appropriate relays that have been assigned.
The electrochemical gas sensing cell generates a voltage that is converted to a 4-20mA signal by an
analog circuit board. The signal is transmitted to the detector control circuit board, which correlates the
signal to parts per million (ppm) or percentage volume of oxygen level. The gas sensor and its signal
output are not controlled, manipulated or changed by the detector software.
The FX-Mini can be supplied with any of the following gas sensors, gas alarm trip levels below are fixed
to prevent tampering, non-standard ranges and trip levels are available. Values are in PPM except
Oxygen:
Gas Sensor Standard
Range
Warning Danger
Chlorine 0-10 ppm 0.5 1.0
Sulfur Dioxide 0-10 ppm 1.0 3.0
Ammonia 0-100 ppm 35 50
Oxygen 0-25% 18% 16%
Carbon Monoxide 0-500 ppm 35 50
Chlorine Dioxide 0-1 ppm 0.1 0.3
Hydrogen Sulfide 0-10 ppm
1.0
3.0
Ozone 0-2 ppm
0.05
0.1
itric Oxide 0-50 ppm
15
25
itrogen Dioxide 0-10 ppm
1.0
3.0
Hydrogen Cyanide 0-10 ppm
1.0
3.0
Hydrogen Chloride 0-10 ppm
0.7
2.0

5
Warnings, Limitations
WARNING: NEVER attempt to calibrate a chlorine gas sensor with bleach, pool chlorinating
chemicals, or by prod cing chlorine gas with a mixt re of bleach and an acid. Any sensor
adj stment made with s ch gases or f mes, or witho t the proper known concentration of gas
will ca se the sensor to not alarm at all or to alarm at incorrect and nsafe levels. This can
create a life threatening sit ation in which the detector may not alarm d ring a chlorine gas
leak.
Warning
If the alarm buzzer sounds a loud pulsating sound and you are not testing the gas detector, it has
sensed a gas leak. The alarm buzzer is warning of a possibly serious situation and it requires your
immediate attention.
Warning
All f nctions of this gas detector m st be checked and verified on a reg lar basis.
The ability for the gas sensor to sense toxic gas or low oxygen levels must be verified on a regular
basis. It is recommended that the gas sensor be tested by means of a certified gas standard at least
once every six months.
Warning
If the FX-Mini is altered in any way the warranty will be voided.
Example; drilling additional holes in the enclosure to mount conduits, changing operating range of the
gas sensor and altering the gas alarm trip points.
Warning
Never disconnect an electrically powered gas detector to stop n isance alarms. The source of
the problem must be corrected.
FX-Mini Gas Detectors are not foolproof. Like all other electronic devices, the FX-Mini Gas Detectors
have limitations. Detectors cannot be expected to sense a dangerous toxic gas leak or low oxygen
levels if the cavity is blocked by debris or the detector is tampered with by personnel.
The FX-Mini has a limited life. This equipment contains many parts. Just as with any other device any
one of these parts could fail at any time. Therefore, you must test your detector on a regular basis. Be
sure to have it repaired or replaced when it fails to test properly. In no case should the detector be used
for more than 10 years. All functions of this detector must be checked and verified on a regular basis.

6
FX-Mini Gas Detector Illustration
All-in-One Option
1. Main Circuit board. 2. AC-DC Power Supply 3. Gas Sensor.
4. 10A relay card. 5. Silence & Reset Button 6. Buzzer

7
FX-Mini-R Gas Detector Illustration
Remote Controller Option
In the FX-Mini-R option the gas sensor is mounted in a separate wall mount enclosure that is placed in
the potential hazard area while the controller is mounted outside of the potential hazard area. The
devices are connected by 25-feet of 22 AWG shielded cable, up to 2,000 feet of cable is available
optionally.
Wiring connections to the gas sensor are color coded, to reconnect
simply match the colors of the wires from the sensor to those of the
two conductor cable to the controller.
FX-Mini-R sensor housings do not have the red LED indicator or
circuit test push button available on our other detectors.

8
Detector Component Description
Following is a description of the system’s major components.
Main Circ it Board
The main circuit board is microprocessor based and operates on 12 VDC. It is not possible to make any
field adjustments or repairs to the circuit board. The microprocessor can be replaced with a new or
upgraded chip. The circuit board’s processor does not control the gas sensor response or signal output
in any way, it simply receives output from the sensor and energizes the alarm functions. All terminals
accept 18 to 22 AWG wire
CAUTION: the sensor printed circuit board is sensitive to Electro-Static Discharge. It can be irreparably damaged
by static electricity, causing partial or total operational failure. You must take the following precautions before
touching or making any connections to the circuit board.
Before touching the circuit board touch an earth grounded metal surface such as the detector's power supply
guard, a metal pipe or conduit to discharge any static electricity from your body, or wear an anti-static wrist strap
connected to an earth ground.
Factory Defa lt Wiring
The FX-Mini is pre-wired at the factory as follows. Field wiring to the circuit board is unnecessary
unless you are connecting a C remote reset switch. 12 & 24VDC boards are not interchangeable.
1
Power in
2
Sensor
inp t
Black to “IN”
3
Sensor inp t Red to “+12”
4
Remote reset with j mper *
5
Silence / Reset b tton
6
B zzer
7
Danger alarm relay 2
8
Sensor fail relay
9
Warning alarm relay
10
Danger alarm relay 1
1
1
Microprocessor
12
Microprocessor
“heartbeat” stat s
LED indicator
*
Do not remove j mper if a remote
reset b tton is not connected
A jumper wire is factory installed in the remote
reset terminal block; this jumper must remain
in place if a remote reset switch is not
connected.

9
Detector Component Description
Terminal connections are provided for both +12VDC and +5VDC power input to the relays. 12VDC coils
are standard. The 5VDC terminals are not used as standard and should be ignored.
Remote Reset
A terminal (supplied with a factory installed jumper) is provided on the main circuit board to connect a
normally closed ( C) remote reset switch. Remove the jumper only if connecting a remote switch.
Power S pply
The detector uses a switching AC to 12VDC power supply (for all gases except those stated below) that
accepts universal voltage input of 88-264 VAC 47-63Hz. Rating: 6W max output power; output current
0.5A; short circuit, over voltage, overload protection; 2 pole USA plug, 4-ft cord, Class II power, Agency
Approvals: CE/CUL/CB/FC
The power cord plugs into a standard EMA 5-15R receptacle.
24VDC NOTE: Detectors for Ammonia, Hydrogen Cyanide and Ozone se an AC to 24VDC
power adapter that accepts niversal voltage inp t of 88-264 VAC 47-63Hz.
The board circ itry for 24VDC powered sensors is different than 12VDC powered nits; as s ch
they are not interchangeable.
WARNING: Applying 120V AC line power directly to the circ it board will destroy the detector
and void the warranty
Battery Power Operation
For battery powered operation the detector requires (1) high quality 12 volt DC battery. The detector
power consumption is about 100mA. Using a 50 amp/hour rated battery the expected battery life would
be 4-6 months.
10VDC is the minimum voltage required to energize the 10A alarm relays.
A DC volt meter may be installed to monitor battery output.

10
Detector Component Description
10 Amp Relay Card
The FX-Mini gas detector includes a separate card containing (4) 10A SPDT Form C dry contact relays.
The 10A relays are unpowered when the detector is not in alarm state; they energize upon reaching an
alarm set point.
The relays are rated as follows for all gases except Ammonia, Hydrogen Cyanide and Ozone: form
C, 12VDC coil, resistive load 10A @125 VAC, 7A @ 24VDC, 7A @ 240VAC
For Ammonia, Hydrogen Cyanide and Ozone detectors: form C, 24VDC coil, resistive load 10A @125
VAC, 7A @ 24VDC, 7A @ 240VAC
WARNING: The 10A relays are not fused. Any external load connected to the relay must have a
current limiting device installed to limit current to the relay to less than 10 amps.
Relay #1 = Danger
Relay #2 = Danger
Relay #3 = Warning
Relay #4 = Sensor Fail
O = Relay red LED
O = Relay card power on LED
Relay test buttons - yellow highlight
Relays #1 & #2 by default are not adjustable in
any way; they trip at the Danger level and are
latching (they must be reset or de-energized
manually). Relay #3 is non-latching and
assigned to the Warning alarm level; relay #4 is
non-latching and assigned to Sensor Fail.
To add additional danger relays: Either of the
relays assigned to Warning or Sensor Fail can
be made to function at the Danger alarm level
by re-wiring. Take the wire from the desired
Warning or Sensor Fail relay terminal on the main circuit board and connect it to the “Coil” terminal on a
Danger relay terminal block on the main circuit board. This relay will then actuate at the Danger level
and be latching.

11
Detector Component Description
Gas Sensor
The detector includes one electrochemical gas sensor that transmits a 4-20mA signal to the detector
electronics. The calibration nose fitting is used only for calibration, not for normal operation.
Test B tton
Unlike the previous model FX-1, the FX-Mini does not incl de a test b tton. In all Foxcroft gas
detectors the test button has only tested electrical continuity and functions of components such as the
buzzer, LED’s, and relays.
Contrary to popular belief by those who did not read the operating manual, the test button has never
tested the ability of the gas sensor to respond to gas.
We believe it is safer to require full functional testing using calibration gas rather than include a test
button that can provide a false sense of security in an electrically functional gas detector that may have
a non-functional gas sensor.
Contents, Serial umber
Unpacking
The detector is shipped with a piece of cardboard that covers and protects the gas sensor,
remove this prior to installation.
After unpacking, it is recommended to save the shipping carton and packing materials if the instrument
must be stored or re-shipped. Inspect the equipment and packing materials for signs of shipping
damage. If there is any evidence of damage, notify the delivery carrier immediately.
The shipping container consists of the following:
1 - FX-Mini Single Channel Toxic Gas Detector
1 – AC to 12VDC power supply with 4-ft cord prewired to main circuit board
WARNING:
• The gas sensor has a 6 month shelf life if stored npowered.
1 Bag containing:
1 - Calibration nose fitting with O-ring and (2) sealing caps
1 – Sensor blue millivolt test connector with wire leads (see photo page 21)
1 - Set of Four Mounting Feet. #10 mo nting screws are not provided.
1 - Instruction Manual with calibration instructions
Serial N mber
The detector serial number label is located both inside and outside of the detector enclosure. The gas
sensor has a separate serial number on a label on the sensor itself.

12
Installation, Mechanical
Sensor Placement Guidelines
The FX-Mini Gas Detector is a fixed, single point type that can only sense toxic gas or oxygen that
diffuses into the sensing chamber of the gas sensor.
Proper placement of the sensor in the potential hazard area is essential to the operation of the gas
detector. Improper location can cause alarm delay or failure to warn of hazardous conditions. There are
various factors to consider in placing the gas sensor.
One of the main factors is the weight of the gas being monitored relative to the weight of ambient air.
Generally, gases that are lighter than air such as ammonia should be located near the ceiling, or in
some applications in the breathing zone 4-6 feet from the floor. Gases that are heavier than air such as
chlorine or sulfur dioxide should generally be placed 6”-12” from the floor.
Air flow patterns and the size of the room to be monitored must be considered in addition to the vertical
placement of the sensor. It’s possible that air flow patterns or lack of ventilation may carry heavier
gases such as chlorine to a higher level such as the breathing zone. It’s possible that one sensor may
not provide sufficient protection.
• Locate the sensor with the sensing chamber opening pointing down toward the floor near the
gas source where the normal air flow pattern will carry the gas to the sensor.
• Do not install in dusty or dirty areas. Dust and dirt can build up in the sensing chamber, making
it overly sensitive. Dirt can block openings to the sensing chamber and keep the gas detector
from sensing toxic gas or low oxygen levels.
• Do not install in areas of wash down. The gas detector should not be washed with water or
mounted in areas where water is used for cleaning.
• Take into account ventilation air flow patterns, the gas sensor may not detect a gas leak in the
room in which it’s mounted due to air flow patterns.
• Do not mount the sensor in a “dead air” space such as a corner.
• Do not install near fresh air vents, next to doors or drafty areas, failure to detect gas and
excessive sensor drift can occur.
• If installing in an outdoor area, the sensor should generally be placed downwind of the potential
gas source. More than one system may be needed to efficiently protect some outdoor locations.
• Do not install in insect-infested areas. If insects enter a sensing chamber, they may cause a
nuisance alarm. Also they can block the sensing chamber and prevent toxic gas or oxygen from
entering the chamber.
• ever mount the FX-Mini Gas Detector in direct sunlight.
• Place on a surface or wall that is not subject to vibration or shock.

Installation, Mechanical
, Dimensions
1.
Attach the wall mounting feet to the rear of the
Mounting screws a
re not included
2. Mount the detector
near the potential leak source
leak,
weight of the target gas and air
from the potential leak source.
3. Remove the
cardboard protecting the gas sensor.
Local Sensor Housing Dimensions
13
, Dimensions
Attach the wall mounting feet to the rear of the
main enclosure using #
10 diameter
re not included
.
near the potential leak source
; taking into account the
location of a potential
weight of the target gas and air
flow patterns.
For remote styles locate in a safe area away
from the potential leak source.
cardboard protecting the gas sensor.
Local Sensor Housing Dimensions
10 diameter
screws.
location of a potential
For remote styles locate in a safe area away

14
Installation, Wiring
WARNING: The alarm relays are not fuse protected. You must install a current limiting device to limit
current to the relays to less than their resistive current rating.
WARNING: Make all connections before yo apply power to the detector.
1. Relay Wiring: Install a watertight conduit connector into the 7/8” dia. Hole in the left side of the
enclosure. Route cables for the alarm relays through the connector and wire to the 10A relays
as either normally open or normally closed as required. Relays #1 & #2 are factory set as
Danger, latching manual reset and are non-adjustable. By default relay #3 is configured as non-
latching auto reset to alarm at the Warning level. Relay #4 is configured as non-latching to
alarm at the Sensor Fail level.
Tip: Test your wiring by pushing the test buttons on the relay card.
Tip: If you think the warning alarm level may trigger nuisance alarms, do not assign a relay
to this level.
Tip: If you need an additional danger alarm relay, take the wire from the desired Warning or
Sensor Fail relay terminal on the main circuit board and connect it to the “Coil” terminal on a
Danger relay terminal block on the main circuit board. This relay will then actuate at the
Danger level and be latching.
2. Remote Reset Switch: To connect a remote reset switch first remove the gray jumper wire
from the remote reset terminal on the main circuit board. Connect the switch to this terminal. A
C switch is recommended so that a break in the circuitry can be easily detected.
3. Power: Simply plug the supplied power cord of the power adapter into a standard EMA 5-15R
120VAC wall receptacle.

15
Startup
1. Remove the cardboard covering the gas sensor opening.
a. NOTE: You must never operate the detector with the calibration nose installed, it is used
only for calibration.
2. Power Up: Turn on power to the detector by plugging the power adapter into a standard
120VAC eletrical outlet. The detector will take 60 seconds to warm up; none of the alarm
indicator LED’s will illuminate during the warm up period. The main circuit board power on
green LED and relay card power on green LED will be illuminated.
3. Sensor Warm Up & Acclimation: The gas sensor requires a warm up and acclimation period
during which the sensor may drift and set off false alarms until it stabilizes. Allow the sensor to
warm up for about 20 minutes. Allow 1 to 12 hours depending on conditions and the sensor type
for the sensor to fully stabilize and acclimate to the environment before calibrating or putting the
detector into service.
4. Calibration Check: Although every gas sensor is factory tested and calibrated before shipment,
differences in air pressure, temperature and relative humidity at the installation site may cause
the zero point and consequently the alarm trip points to shift slightly.
5. The sensor after acclimating should be fully calibrated to your local conditions by adjusting the
zero point if necessary and applying a low level target calibration gas that matches the range of
your detector and adjusting the sensor as required. See the calibration section for the procedure
for instructions to bump test and calibrate.
6. Test Sensor and Alarm F nctions. It is best to use a known concentration calibration gas to
bump test the detector and check its functions. Do not test detector f nction with a signal
generator.
a. You may also use a mixture of 1 part (teaspoon) vinegar and 2 parts (teaspoons) of
household bleach in a squeeze bottle held under the sensor opening. Warning: doing so
risks exposing the gas sensor to excessively high concentrations that can damage the gas
sensor if not done carefully. Squeeze the bottle gently to emit only a brief modest amount of
vapor. Do not allow liquid to contact the sensor membrane and do not overload the sensor
with chlorine fumes.

16
Startup
7. The LED’s on the detector should illuminate as alarm trip levels are reached, the buzzer should
sound at the Danger alarm level, the relays should energize upon reaching alarm levels (the LED’s
next to each relay should illuminate red upon actuation) and external devices connected to the
relays should function. The silence button should turn off the alarm buzzer, but pushing the button a
second time will not reset the detector or relays until the sensor output falls below the danger alarm
level. If any of the alarm notifications do not occur during the test return the detector to the factory.
8. After calibration and testing the detector is operational.
ormal Operation and Alarm Conditions
Normal Operation
NOTE: The gas sensor and its signal o tp t are not controlled, manip lated or changed by the
detector software or hardware.
Under normal operating conditions you will see the green “power on” LED illuminated on the face of the
detector, the red “heartbeat” LED indicator flashing on the main circuit board and a green power on
LED illuminated on the relay card.
Alarm Set Points and Conditions
The FX-Mini includes two gas alarm set points and a sensor fail alarm set point:
Warning: an alarm set point set to OSHA, IOSH and ACGIH recommendations.
Danger: an alarm set point set to OSHA, IOSH and ACGIH recommendations.
Sensor Fail: indicates signal output from the sensor is at 3.84mA or less. Sensor signal output in
ambient air with no gas present is 4mA.
NOTE: Alarm notifications cannot be ca sed by a software malf nction. An alarm signals a
potentially dangero s condition. Yo m st determine the ca se of the alarm and take
appropriate action. Attempting to reset the gas detector by powering off and on will not stop or
reset an alarm notification or correct a dangero s condition or component fail re.
Warning Alarm
When this alarm set point is reached the LED indicator will illuminate orange and the relay assigned to
alarm at this level will energize, the red LED below the relay will illuminate. The buzzer will not sound.
By default Warning relay #3 will auto reset, or de-energize, when the gas concentration falls below the
alarm set point.

17
ormal Operation and Alarm Conditions
Danger Alarm
When the gas concentration reaches the Warning level its alarm LED will illuminate orange. When the
Danger alarm set point is reached the Warning indicator will go out, and the red LED indicator on the
face of the detector will illuminate, manual reset latching relays #1 and #2 will energize and their LED’s
will illuminate, any additional relay configured to alarm at this level will energize and the piezo buzzer
will sound.
If the sensor fails during a danger alarm condition the sensor fail blue LED will illuminate, the Danger
alarm LED will continue to illuminate and the Danger relays will remain energized until the unit is reset
after and only after the gas concentration falls below the Danger alarm threshold.
The latching relay can only be reset after the measured concentration falls below the designated alarm
set point. If you touch the reset button while the concentration is at or above the alarm set point the
detector will remain in alarm and relays will remain energized until safe levels are measured.
Attempting to reset the gas detector by powering off and on will not stop or reset an alarm
notification; the detector will ret rn to the danger alarm state pon power restore or system
reset if gas concentrations are in fact detected as being at or above the alarm level.
If a sensor fail alarm occurs during a danger alarm, the detector will continue to display the danger
alarm notification as well as the sensor fail notification.
Note: After a Danger alarm has occurred and the gas concentration falls to zero, the gas
sensor output may momentarily fall below the 3.85mA limit before stabilizing. As sensor output
passes through this threshold the blue LED will illuminate and you will hear the sensor fail relay
energize or “chatter” while the sensor reaches its normal state. This is normal.
Note: In some cases of sensor failure the current output can raise up to maximum, in which
case the danger alarm will trip even though no gas is present.
Sensor Fail Alarm
The sensor fail alarm indicates that signal output from the sensor has fallen below the limit of 3.85mA
either temporarily or permanently. Signal loss can be caused by sensor failure, breakage in the
normally closed sensor circuitry, or temporary signal loss caused by normal sensor drift.
Note: Sensor signal loss is not produced or controlled by the detector electronics or software.
During normal operation the zero point of an electrochemical gas sensor can drift up or down over time
and in response to changes in temperature, humidity or air pressure. Drift can also occur if the sensor
is directly exposed to wind or air flow.
The sensor output must be at or below 3.84mA before the sensor fail alarm will trip. In this mode the
Sensor Fail LED will illuminate blue and the 10A alarm relay will energize. The buzzer will not sound. If
sensor output rises back up to 4mA during a sensor fail alarm relay will automatically reset and the LED
will go out. The sensor fail alarm cannot be reset manually until sensor output reaches 4 mA.

18
ormal Operation and Alarm Conditions
If a sensor fail alarm occurs during a danger alarm, the detector will continue to display the danger
alarm notification as well as the sensor fail notification.
Note: While the detector is in Sensor Fail mode its alarms and relays remain fully functional in
the event that the sensor fail alarm is tripped due to a brief disruption and not an actual sensor
failure.
Sensor Fail Alarm with Oxygen Sensors
The Sensor Fail alarm is not available with Oxygen sensors because the Oxygen and Sensor Fail
alarms both trip in the same sensor output range. Given the possibility of being unable to positively
distinguish between the two alarm types due to sensor drift or environmental factors, it’s prudent to
assume a sensor output of 4 mA or less signifies an Oxygen deficiency. If testing with a separate
instrument demonstrates safe Oxygen levels then the sensor functionality can be tested and the sensor
replaced if needed.
Gas Sensors
General Information & Environmental Effects
The responsiveness of electrochemical sensors will vary with environmental conditions. Sensor
response may be higher or lower depending on actual environmental conditions.
Gas Expos re
The electrochemical gas sensors used in FX-Mini gas detectors are strictly designed and intended for
occasional intermittent exposure to the target gas. Under no circumstances will the sensor survive
continuous exposure to target gas. The only exception to this is Oxygen sensors.
Sensor Serial N mber and Date Code
Each gas sensor has an eight digit serial number followed by a 3 digit date code. The first two digits of
the date code signify the month, the third is the year of shipment from the factory.
H midity
Gas sensors are relatively unaffected by humidity if conditions are not condensing within a range of
15% to 90% RH. The sensor will show a transient response to rapid changes in humidity which should
go away after 30 seconds.
The gas sensor includes an aqueous electrolyte and a porous diffusion barrier. The means the sensor
can both absorb water from the atmosphere and dry out. At continuous operation at high temperatures
and 90%-100% RH the sensor can become prone to leakage as the free space in the sensor slowly fills

19
Gas Sensors
General Information & Environmental Effects
with water. The sensor can gradually be restored to balance without permanent damage by exposure to
lower relative humidity.
Likewise, continuous operation at 0-15%RH will cause the sensor to dry out, which can cause the acid
electrolyte to attack the seals. This occurs if the volume of electrolyte decreases by more than 40%. If
not left in this condition too long the sensor can be restored by exposing the sensor to RH humidity
above 15%.
Temperat re
Both the baseline (zero point) and output span are affected by temperature. The baseline
approximately doubles with every 10°C increase in temperature. The output span will increase slightly
up to about 10% before leveling off with gradual increase in temperature. A transient spike can occur
with rapid changes in temperature; this should go away after 30 seconds.
Maxim m Overload
The gas sensor maximum overload rating, for chlorine it is 250 ppm, is specified in terms of maintaining
a linear response over a 10-minute exposure and recovering quickly. At higher levels the sensor will
progressively become more non-linear and take increasingly longer to recover as the sensing electrode
is unable to consume all the gas diffusing to it.
If the gas level is increased even further gas will build up inside the sensor and diffuse into the internal
spaces where it may interact with the reference electrode, altering its potential. If this happens the
sensor may take several days to recover once placed in clean air.
Sensor Zero Point Drift
Over time all gas sensors will experience a shift of the zero, or reference point. When this happens the
gas concentration readings will also shift accordingly, producing inaccurate readings.
ote that sensor responsiveness will vary with environmental conditions.
Typically gas exposure mentioned throughout this document refers to the target gas, however exposure
to interfering gases can cause similar effects as target gas exposure.
Causes of gas sensor zero point drift include:
• Chemical degradation of the sensor over time.
• Temperature, affecting both the span and zero point
• Repeated use in extreme high or low temperature or humidity conditions, or environments with
high levels of airborne particles. Temperatures above the rated limit stress the seals, which will
cause electrolyte leakage.

20
Gas Sensors
Sensor Zero Point Drift (cont.)
• Exposure to high levels of gas, or exposure to concentrations that exceed the range of the
sensor.
• Low level gas exposure for extended periods of time.
• Continuous exposure to solvent vapors or highly corrosive gases.
• Rough handling or jolting of the sensor and electronics
Exposure to these conditions may damage the sensor to the point that it can no longer operate
properly over its full range or be able to calibrate properly.
Calibration
Calibration Definitions
B mp Test: The Industrial Safety Equipment Association (ISEA) defines a bump test as: "A
qualitative function test in which a challenge gas is passed over the sensor(s) at a concentration
and exposure time sufficient to activate all alarm indicators to present at least their lower alarm
setting.
"The purpose of this test it to confirm that gas can get to the sensor(s) and that all the alarms
present are functional.
"This is typically dependent on the response time of the sensor(s) or a minimum level of response
achieved, such as 80% of gas concentration applied. ote this check is not intended to provide a
measure of calibration accuracy".
Calibration check: is defined by ISEA as: "A quantitative test using a known traceable
concentration of test gas to demonstrate that the sensor(s) and alarms respond to the gas within
manufacturer's acceptable limits. This is typically ±10% to ±20% of the test gas concentration
applied unless otherwise specified by the manufacturer".
Full Calibration:
Full Calibration:Full Calibration:
Full Calibration: ISEA defines this as: "The adjustment of an instrument's response to match a
desired value compared to a known traceable concentration of test gas. This should be done in
accordance with the manufacturer's instructions".
Calibration Freq ency
The only way to guarantee that an instrument will detect gas accurately and reliably is to test it with a
known concentration of gas. Regular testing and calibration is the only way to be certain that a detector
is fully functional. A calibration or gas sensor can only be as accurate as the test gas used to perform
the calibration.