Gas Sensors
e2v
has been leading the way in the design, development and manufacture of
a wide range of reliable gas sensors for OEMs since developing the
catalytic bead gas sensor (which became known as the Pellistor) in 1967.
As
an independent OEM supplier of gas sensors, e2v pride themselves on
providing customers with unrivalled product reliability and personal
product support via specialist engineers.
e2v gas sensors are
built to the highest standards with all pellistor and infrared gas
sensors achieving ATEX and IECEx certification, e2v gas sensors are
also UL and CSA approved.
e2v is ISO9001:2000 certified with the automotive semiconductor types also ISO/TS16949:2002 certified.
Gas Sensor Technology
e2v has provided quality sensors for
over 40 years, production of e2v’s pellistor range started in 1967, and
was later followed by the introduction of the infrared and
electrochemical sensor ranges. Over the years the product portfolio has
widened extensively, broadening the range and selectivity of detectable
gases.
These include the following:
Pellistors (Catalytic Bead)
The Pellistor Catalytic Gas Detector
Pellistors
are miniature calorimeters used to measure the energy liberated by the
burning of a combustible (flammable) gas or vapour. A pellistor
consists of a coil of small-diameter platinum wire supported in a
refractory bead on which is deposited a layer of catalytic material, on
which the gas is burnt. The coil serves two purposes. Firstly, it is
used to heat the bead electrically to its operating temperature, about
500°C, and secondly it is used to detect changes in temperature
produced by the oxidation of the flammable gas.
The earliest
forms of catalytic gas sensors consisted solely of bare coils of
platinum wire, operating at 800-1000°C. At such temperatures, platinum
wire evaporates extremely quickly causing signal drifts resulting from
a reduction in the wire diameter. The specification for such a sensor,
which is still produced commercially, requires that the sensor has a
life of 100 hours. The majority of present day devices, as stated
earlier, have the coil cloaked in a porous ceramic onto which is
deposited the precious metal catalyst.
The enhanced catalytic
activity resulting from the much larger surface area of catalyst
available permits much lower operating temperatures of around 500°C,
resulting in lower power drain and longer device lifetime.
The
concept of the pellistor is based on the fact that the most foolproof
way to determine whether a flammable gas is present in air is to test a
sample by trying to burn it!
A pellistor consists of a very
fine coil of wire suspended between two posts. The coil is embedded in
a pellet of a ceramic material, and on the surface of the pellet (or
'bead') there is a special catalyst layer.
In
operation, a current is passed through the coil, which heats up the
bead to a high temperature. When a flammable gas molecule comes into
contact with the catalyst layer, the gas 'burns'. The reaction occurs
without a flame since the level is below the Lower Explosive Limit (or
LEL) of the gas. However, just as in a burning reaction, heat is
released which increases the temperature of the bead. This rise in
temperature causes the electrical resistance of the coil to rise.
There
is another bead in the circuit which is identical to the detector bead,
but does not contain any catalyst. This bead will react to changes in
humidity, ambient temperature etc, but will not react to flammable gas.
All that is required is a comparison of the resistance of one bead
against another in a Wheatstone Bridge type circuit in order to obtain
a meaningful signal.
Infrared
Infrared Gas Sensors
Infrared
Gas Sensors exploit the property that many gases absorb radiation in
the 2-14 micron, infrared region of the spectrum. These spectral
absorbances show features which may be regarded as 'fingerprints' to
identify the gases and enable their concentrations to be deduced.
The
sensor bodies contain an infrared source and infrared detectors inside
a compact and combined gas cavity/ optical cell. The detectors have
infrared bandpass filters placed in front, which tune them to the
specific gases to be sensed.
When the specific gas enters the
cavity it is registered as a change in detector signal. The magnitude
of this change is related to the concentration of that gas via a simple
exponential formula.
By utilising different infrared filters a
range of gases can be sensed and discriminated with these devices. In
cases where spectral lines overlap, then an individual sensor may show
cross sensitivities to a gas range.
Infrared gas sensors are
very robust devices not affected by contact with a harsh chemical
environment. Any changes in ambient conditions such as temperature are
compensated for in software.
Their dimensions and power requirements are compatible with and complementary to pellistor gas sensors.
Infra-red Gas Sensors for Hydrocarbons or CO2
After
over thirty years of successful manufacture of pellistor-based
flammable gas sensors, the range of Non-Dispersive Infra-red (NDIR) gas
sensors represents the first of many diversifications into other areas
of gas sensor technology by us.
Electrochemical
Electrochemical
sensors work on a different principle from the pellistor and IR
devices. With electrochemical sensors the target gas undergoes a
chemical reaction, producing a current that is directly proportional to
the concentration of gas present. The sensors use very little power and
show good responses to various gas concentrations over a wide range of
ambient conditions.
Two sizes of electrochemical
sensors are available for the measurement of Carbon Monoxide and
Hydrogen Sulfide. These sensors are drop-in replacements for the
current 20mm and 32mm diameter sensors.
Our latest addition to
the range of e2v electrochemical sensors is the EC410, which is a RoHS
compliant Oxygen sensor. This unique sensor contains no lead, has a
greatly increased life span and is significantly lower mass compared
to other sensors.
Thermal Conductivity
Thermal Conductivity Gas Sensors
Because
pellistors measure the flammability of a gas, they cannot be used to
measure levels of gas above the Lower Explosive Limit (LEL), since the
reducing level of oxygen will result in a fall-off of signal. However,
a similar device can be used to monitor these high levels of gas.
We
have a range of thermal conductivity sensors, which are designed to
complement the pellistor range in terms of electrical characteristics,
so that they can be used in the same Wheatstone Bridge circuits. They
are supplied with a compensator bead which is in a sealed enclosure of
air. This enclosure acts as the thermal conductivity reference in
exactly the same way as it acts as the reference for a pellistor.
Because thermal conductivity measurements do not rely on the
flammability of the gas, the technique can be used to analyse a whole
range of gas mixtures, provided that there are only two gases present
and that the two gases have significantly different thermal
conductivities. Examples include:
- 0 - 100% Hydrogen in Air
- 0 - 100% Methane in Air
- 0 - 100% Carbon Dioxide in Air
- 0 - 100% Carbon Dioxide in Methane
- 0 - 100% Helium in Air
Thermal conductivity cannot be used for gas mixtures where the thermal
conductivities of the two gases are similar. The best example of this
is oxygen levels in air, as the thermal conductivities of oxygen and
nitrogen are too close to give a meaningful signal.
Fixed Gas Detection Heads
Our
pellistors and thermal conductivity sensors can be obtained already
packaged as complete, flameproof gas detection heads for use in fixed
gas detection systems. The VQ4250 gas detection head has CSA and
BASEEFA approval and is just one example of our success in packaging
sensors. The VQ600 is the latest product in this range and shows our
commitment to continual development of device performance whilst
reducing costs.
Metal oxide semiconductor
e2v is able to offer a range of semiconductor based gas sensors and sensing modules:
Sensors
follow the link on the main left hand menu to access datasheets
| Product name | Gas to be detected | Concentration range | Package | Cap | Dual chip | Options |
| 2610 | Ozone | 10ppb - 1000ppb | TO39, 4 pins | 160u metal mesh | No | Plastic cap or special filter |
| 5521 | CO, HC | 1 - 1000 ppm CO | TO39, 4 pins | 160u metal mesh | No | No |
| 5524 | CO, HC | 1 - 1000 ppm CO | SMD 7x5 mm | Plastic cap | No | No |
| 5135 | Ethanol, VOC
Low cost | 20 - 500 ppm ethanol | TO39, 4 pins | 160u metal mesh | No | No |
| 4514 | CO/NOx | 1 - 1000 ppm CO
0.05 - 5ppm NO2 | SMD 7x5 mm | Plastic cap | Yes | No |
Please contact us
if you are looking for sensors to detect methane or hydrogen, or if
your requirement does not match one of the gases listed above.
Sensor package options
Sensors can be supplied with different packaging:
-With or without protection grid
-With hydrophobic teflon membrane
-With active charcoal filter
-TO or SMD packages
Sensor notes
2610 ozone sensor
Ozone
is used, because of its oxidizing properties, in a variety of
applications to clean and decontaminate air and water. These
applications include water treatment, air purification in smoky
environments, and home air cleaners. Since ozone is hazardous to human
health, its concentration needs to be monitored so that it does not
exceed around 0.1 ppm in livable spaces. Mics 2610 is highly responsive
to ozone and robust enough to be used in industrial devices.
5135 Ethanol
Measurement
of the blood alcohol content (BAC) by correlation with the alcohol in
breath is common practice by the police to determine if a person is
driving under the influence of alcohol. Using the 5135 alcohol sensor,
a personal alcohol tester can be built at low cost, so that individuals
can test their own approximate BAC level by simply blowing on the
sensor. Such devices should not be used as a license to drive after
drinking.
5524 CO/HC
This sensor measures CO and hydrocarbons. Its SMD package makes integration on a PCB very cost-effective.
