ANALOX 2000 Series User manual

ANALOX 2000 Series - Flammable Gas Sensors
User Manual
Analox Sensor Technology Ltd
15 Ellerbeck Court, Stokesley Business Park
North Yorkshire, TS9 5PT
T: [phone] F: [phone]
W: www.analox.net E: [email protected]

Analox 2000 – Flammable Gas Sensors – User Manual
Document Ref: SK0-810-04 January 2009
Page 2
CONTENTS
Introduction ....................................................................................................................................3
Principle of Operation............................................................................................................ 4
L.E.L. and U.E.L. ................................................................................................................... 5
Sensor Signal Cables ............................................................................................................6
Instrument Calibration ........................................................................................................... 7
Sensor Physical Details.........................................................................................................8
Electrical Connections ........................................................................................................... 9
Flammable Sensor Wiring Variations .................................................................................. 10
Accessories ......................................................................................................................... 11
Installation ........................................................................................................................... 11
Sensor Position ................................................................................................................... 11
Certification ......................................................................................................................... 12
Intrisically Safe System ....................................................................................................... 13
Chemical Poisoning............................................................................................................. 14
Sensor Element Life ............................................................................................................ 14
Sensor Response Time....................................................................................................... 14
Appendix ............................................................................................................................. 15
Cable Requirements....................................................................................................... 16/17
Disposal.............................................................................................................................. .18
Certificates

Analox 2000 – Flammable Gas Sensors – User Manual
Document Ref: SK0-810-04 January 2009
Page 3
ANALOX 2000 RANGE
FLAMMABLE GAS SENSORS
INTRODUCTION
The ANALOX 2000 Range of sensors are designed to detect Flammable gas, mainly Methane
(CH4) in ambient air monitoring applications. There are two models in the range:
a. Model 2000 which is of the Explosion proof type and
b. Model 2001 which is a lower cost, uncertified type, suitable for use in areas which are not
classified as being hazardous.
The Model 2000 consists of a stainless steel sensor unit certified Ex sd IIc T6, mounted on a cast
iron junction box, which is certified EEx e II T6. This model also carries an Intrinsically Safe
SYSTEM certificate Ex ias IIC. (details included later in this manual.)
The Model 2001 sensor unit is made from Acetal and contains a Ceramic sinter element. The
sensor is mounted on a IP65, mineral filled thermoplastic junction box. Both junction boxes include
screw type terminals for connection of signal cables.
The Model 2000 unit junction box includes 1 x 20mm threaded cable gland entry and 1 x 25mm
threaded cable gland entry. Certified blanking plugs are supplied with the unit.
The Model 2001 has a single 20mm cable gland fitted to the junction box.
The Pellistors used in the 2000 and 2001 sensors are of the Poison resistant variety. Both types of
sensor can be supplied with either VQ21 or VQ22 type Pellistors; Operating currents are 300 mA
and 175 mA respectively. The VQ21 has a larger active element and is slightly more resistant to
poisoning than the VQ22 but if the area to be monitored is unlikely to contain poisoning
substances then other considerations may determine which Pellistors are chosen. (See - Intrinsic
Safety applications)

Analox 2000 – Flammable Gas Sensors – User Manual
Document Ref: SK0-810-04 January 2009
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PRINCIPLE OF OPERATION
Both sensors use the same devices to actually detect the presence of a flammable gas in the air
surrounding the sensing surface. The elements used in these sensors are of the Catalytic
Pellistor type and are sensitive to most common Combustible gases and vapors. A single sensor
assembly may therefore be used to detect a wide range of these gases. The devices are made
from two coils of very fine platinum wire which are embedded in separate beads of alumina. One
device is the detector and the other is a temperature compensating element. The detecting
element is treated with a catalyst which promotes oxidation of the gas and the compensating
element is treated with an oxidation inhibiting agent. The two elements are connected to the
Instrument in a Half Bridge configuration and the excitation current passing through them raises
their temperature to about 550 Deg C. At this temperature, the gas oxidises on the Detector
element and raises the temperature of the bead even further. This alters the resistance of the
detector bead and this change in resistance is measured by the instrument and converted to
produce a reading of the gas concentration, on the Instrument display. The output of the device
is essentially linear for most gases up to high concentrations, typically 100% LEL and the
response time to 25% LEL is about 2 or 3 seconds. Any pre-filtering of the gas before it reaches
the sensor may lengthen the response time. If the sensors are required to operate in high gas
concentrations for short periods, it has been found that for periods up to about 2 minutes, 10
second bursts of 8%, 10% and 80% methane in air, produce no ill effects. Prolonged exposure
can result in zero drift which may be reversed by operating for a short period in clean air.
Exposure to 40% concentration for longer periods, will begin to destroy the detector surface,
altering the Zero point and reducing the sensitivity. Whenever a sensor is exposed to high
concentrations of Combustible gas, the calibration should be rechecked as soon as possible.
The performance of the sensors may be temporarily impaired by operation in the presence of
certain volatile substances containing halogens or sulphur. The sensors may recover after a
short period of operation in clean air. Whenever the substance produces a permanent effect on
the catalyst, resulting in a large reduction in sensitivity the sensor is said to be Poisoned. Typical
substances which can cause poisoning are silicon oils and grease, anti-knock petrol additives
and phosphate esters. Activated carbon filters will provide adequate protection from poisoning in
most cases. Notwithstanding the above comments, the Combustible gas sensors have an
inherently long life. Although the sensors respond to most Combustible gases, the signals
produced vary in magnitude depending on the actual gas to which the sensor is exposed. The
sensors should normally be calibrated using 50% LEL Methane, ie 2.5% Methane in air.
NOTE : When the sensors are used to detect a different gas than that used for calibration, then a
correction factor should be applied to the readings obtained. A table showing conversion factors
for various gases is shown in Appendix A of this manual. (See also Instrument Calibration below)
WARNING: When a pellistor type sensor is exposed to concentrations of flammable gas
greater than 100% LEL (5% Methane in air) there eventually comes a point at which the signal
output from the sensor DECREASES as a result of an INCREASING concentration of the gas.
This is caused by the flammable gas displacing Oxygen in the sample and gradually inhibiting
the normal oxidation process, at the detecting element. Most monitoring instruments provide an
OVER-RANGE indication to inform the user of this condition.(See also L.E.L. and U.E.L.)

Analox 2000 – Flammable Gas Sensors – User Manual
Document Ref: SK0-810-04 January 2009
Page 5
L.E.L. and U.E.L.
The above principle of operation is only valid where the gas mixture to be sensed contains
Oxygen. It is therefore suitable for detection of leakage’s of gasest in atmospheric
surroundings, where it is important to know the level of concentration, well before a hazardous
situation arises. Flammable gas mixtures are only ignitable between certain fairly clear, but
experimentally defined limits. These levels are known as the 'Lower Explosive Limit' (L.E.L.)
and the 'Upper Explosive Limit' (U.E.L.). Typical L.E.L. figures for some common explosive
gases are as follows :
Methane (Natural Gas) 5% + 95% Air
Hydrogen 4% + 96% Air
Ammonia 16% + 84% Air
Ethylene 3% + 97% Air
Butane 1.9%+ 98.1% Air
Propane 2.1%+ 97.9% Air
If the gas concentration is below the L.E.L. then combustion cannot take place due to
insufficient gas, and if above U.E.L. level, there will be insufficient Oxygen present to sustain
combustion.

Analox 2000 – Flammable Gas Sensors – User Manual
Document Ref: SK0-810-04 January 2009
Page 6
SENSOR SIGNAL CABLES
All the S.A.E.P. Flammable gas sensors require a Three Core cable between the sensor and
the measuring instrument. Because of the relatively heavy current demands of the Pellistor type
of sensors, the connecting cables must be chosen with care. Pellistors of the VQ21 type, which
are normally fitted in the Model 2000 and Model 2001, require an operating potential (at the
Sensor) of 2.00 volts DC at a current of 300 milliamps. When a current of this magnitude is
passed along signal cables, any significant resistance in the cables will result in a drop in
voltage along the cables. As an example : Assume a cable run of 100 metres is required
between the monitoring instrument and the sensor and the cross sectional area of the cable
chosen is 1.00mm2. Most manufacturers quote the resistance of this cable to be 19.1 Ohms
per kilometre, therefore in this case, a 100 metre length will have a resistance of 19.1 / 10 =
1.91 Ohms. Bearing in mind that as far as the loop current is concerned, the actual conductor
length is 200 metres, (100m to the sensor and 100m back again) the total cable resistance in
this case is 3.82 Ohms. Using Ohm's law to calculate the voltage drop across the entire cable:
Voltage Drop = Current x Resistance
Where Voltage is in Volts : Current is in Amps & Resistance is in Ohms
Voltage Drop = 0.3 x 3.82 = 1.15 Volts
This means that if the monitoring instrument supplies a drive voltage of 2.00 volts then the
sensor will only have 0.85 Volts applied to it. (2.00v-1.15v=0.85v). The pellistor sensor will NOT
operate correctly in this condition.
NOTE: Although the ANALOX 2000 & 2001 sensors use a 3 wire connecting cable, only two of
the wires carry the sensor excitation current. The third wire is connected to the centre tap of the
'Half Bridge' configuration. It only carries a signal voltage at very low current and may therefore
be ignored as far as resistance effects are concerned. Most monitoring instruments designed to
operate with Flammable gas detectors of the Pellistor type, have the facility for compensating,
within limits, for this voltage drop. The ANALOX range of monitors, 1300 and 1320, allow the
drive voltage to be adjusted up to about 3.8 volts. So, in the case of the above installation, the
1.15 volt drop across the cable could be compensated by adjusting the monitoring instrument
drive voltage to 3.15 volts. It is not necessary to carry out all of these calculations every time an
installation is done - correct operation of the sensor can be achieved by measuring the voltage
AT THE SENSOR JUNCTION BOX TERMINALS, as per the installation instructions in the
relevant monitoring instrument handbook. Data regarding maximum possible lengths for a
selection of commonly used cables with 3 different Pellistor sensors, is included in Appendix B,
at the end of this manual. A further point, which is often overlooked is that the resistance of
copper cable has a temperature co-efficient. The effect of this variation in resistance due to
temperature changes, can be significant on very long cable runs, particularly if the cable has a
small cross sectional area and is subject to large variations in temperature. The measuring
instrument is unable to distinguish between a change in cable resistance and a genuine gas
signal. Copper cable resistance is normally quoted at 20C and will vary by approximately 
0.4% per degree C, as the temperature changes above and below this value. Using the above
cable installation as an example, the total loop resistance was calculated to be 3.82 Ohms at
20C. If the temperature of the cable drops to 0C then the resistance will change to 3.80
Ohms. At first sight, this may not seem significant but could result in drift of the ZERO reading
on the monitoring instrument.

Analox 2000 – Flammable Gas Sensors – User Manual
Document Ref: SK0-810-04 January 2009
Page 7
INSTRUMENT CALIBRATION
There is some confusion over what is meant by the Sensitivity of instruments used to measure
the concentrations of combustible gas. Ideally, all units employing catalytic combustion as the
measuring principle should be calibrated in terms of percentage of potential combustibility,
where 100% scale reading represents an ignitable mixture, for a particular Gas/Air mixture. It is
unfortunate that the sensing elements are not equally responsive to all combustible gas/air
mixtures and although the sensitivity to a wide range of gases is similar to within a few percent
of L.E.L. and for all practical purposes, the inaccuracies may be ignored, it may be desirable in
certain cases to make allowances for these variations. It is therefore common practice to
calibrate units of this type over the range 0 - 100% L.E.L. where 100% scale is equal to the
actual percent concentration of the calibration gas, at the Lower Explosive Limit. e.g. An
instrument calibrated 0-100% L.E.L. Methane would effectively read 5% v/v at 100% scale and
one calibrated for Butane would indicate 100% scale at 1.9% gas concentration. Cross
sensitivities between a number of gases may be derived from the selection of data and
formulae given in Appendix A of this manual.

Analox 2000 – Flammable Gas Sensors – User Manual
Document Ref: SK0-810-04 January 2009
Page 8
SENSOR PHYSICAL DETAILS

Analox 2000 – Flammable Gas Sensors – User Manual
Document Ref: SK0-810-04 January 2009
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ELECTRICAL CONNECTIONS
ANALOX 2000 SENSOR
ANALOX 2001 SENSOR

Analox 2000 – Flammable Gas Sensors – User Manual
Document Ref: SK0-810-04 January 2009
Page 10
FLAMMABLE SENSOR WIRING VARIATIONS
There are currently three different types of Sensor Head available from Analox and the Wiring
colours differ on each sensor. Details of the wiring colours are given below
a. EEV Stainless Steel, VQ4250 Exds Sensor has wiring colours :
BLUE, GREEN/YELLOW, BROWN
Where Blue is the Compensator Lead, which connects to the 'A' Terminal of the 1300.
Green/Yellow is the Signal output Lead which connects to the 'B' Terminal of the 1300.
Brown is the Detector Lead, which connects to the 'C' Terminal of the 1300
Due to possible confusion with European mains wiring colours (Blue, Green/Yellow, Brown)
and in the interests of safety, the manufacturer decided to change the colours of the wiring on
all new issues of this sensor.
The wiring colours on the latest issue of the Stainless Steel VQ4250 sensor are as follows :
RED, YELLOW, BLUE
Where Red is the Compensator Lead, which connects to the 'A' Terminal of the 1300.
Yellow is the Signal output Lead, which connects to the 'B' Terminal of the 1300.
Blue is the Detector Lead, which connects to the 'C' Terminal of the 1300.
b. GROVELY 210 Mod FLAM Stainless Steel Exds Sensor has wiring colours :
RED, GREEN, BLUE
Where Red is the Compensator Lead, which connects to the 'A' Terminal of the 1300.
Green is the Signal output Lead, which connects to the 'B' Terminal of the 1300.
Blue is the Detector Lead, which connects to the 'C' Terminal of the 1300.
c. ANALOX Black Acetal Sensor has the following wiring colours :
RED, GREEN, BLACK
Where Red is the Compensator Lead, which connects to the 'A' Terminal of the 1300.
Green is the Signal output Lead, which connects to the 'B' Terminal of the 1300.
Black is the Detector Lead, which connects to the 'C' Terminal of the 1300.
Sensors a. and b. are normally supplied fitted to an EExe Cast Iron Junction Box and together,
form the ANALOX 2000 Flammable gas sensor unit, suitable for use in Hazardous areas.
Sensor c. , the ANALOX 2001, is a less expensive alternative which can be used in areas
which are not classified as Hazardous.
IMPORTANT: IF THE SENSORS ARE INCORRECTLY CONNECTED, PERMANENT
DAMAGE COULD BE SUSTAINED WHEN POWER IS APPLIED.