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What is an NDIR sensor?

Alphasense IRC-A sensors use the principle of Non-Dispersive Infra-Red (NDIR) to determine gas concentration. Each sensor consists of an infrared source, optical cavity, dual channel detector and internal thermistor..

How do NDIR sensors work?

Gas diffuses into the optical cavity. Light from the infrared source passes through the optical cavity where it interacts with the gas before impinging on the detector. Certain gases absorb infrared radiation at specfic wavelengths (absorption bands). The dual channel detector is comprised of an active channel and a reference channel. The active channel is fitted with a light filter such that the only light with a wavelength that corresponds to an absoption band of the target gas is allowed to pass through. If the target gas is present in the optical cavity the intensity of light passing through the filter and hitting the active channel decreases. The reference channel of the detector is fitted with a filter that only allows wavelengths of light where there are no absorption bands to pass through. The intensity of light hitting the reference channel is not affected by the presence of gas. The use of a reference channel allows variations in the light intensity to be compensated.

For more information, see Application Note AAN 201

Is the NDIR sensor temperature and pressure dependent?

The detectors used are sensitive to the ambient temperature and the internal thermistor can be used to constantly monitor the temperature and compensate the output.

For more information, see Application Note AAN 201

Can Alphasense provide a transmitter board for the NDIR sensors?

Yes, currently we offer 4-20 mA transmitter boards for the IRC-A1 sensors only. When ordering please specify your CO2 range: 0 to 5000 ppm, 0 to 5%, 0 to 20% or 0 to 100% CO2.


What is a volatile organic compound (VOC) and which ones are sensed by PID sensors?

Volatile organic compounds, or VOCs, are organic chemical compounds whose composition makes it possible for them to evaporate under normal indoor atmospheric conditions of temperature and pressure.

PID can detect most VOCs. When light of sufficient energy is directed at a VOC, it fragments into ions. The characteristic photon energy of light causing this to happen is called the VOC’s Ionisation Potential or IP.

For more information see Application Note AAN 301

How long does the lamp and electrode stack last?

Under standard environmental conditions lamp life (lit hours) is between 2,000 and 6,000 hours. The electrode stack should last approximately the same time. Life depends on the combination of the number of lit hours and the level of contamination in the environment.

For more information see Application Note AAN 306

What voltage supply does the PID A1 and PID AH sensors require?

The underside (pin side) of the PID-A1 (or PID-AH) sensor has a small circular gold plated well that can be left open or be filled with solder. If the well is not filled with the solder, the on board regulator is enabled therefore a regulated or unregulated supply between 3.6 – 18.0 V may be supplied and the internal voltage will be regulated to 3.3 V.

If the solder well is filled with solder, the sensor’s on board regulator is disabled therefore a regulated supply of 2.8 – 3.6 V is required. This supply should be stable to within 10 mV to maintain a stable light intensity.

For more information, see Application Note AAN 302

What is the output of the PID sensor?

The sensor provides an analogue voltage output. The range is 0.0 V to Vs – 0.1 V for an externally regulated voltage supply (Vs) in the range of 3.0 to 3.6 V. When internally regulated on a supply voltage of 3.6 to 18.0 V, the output range is 0.0 to 3.2 V.

How do I confirm if the on board voltage regulator has been disabled?

If the solder well is empty or insufficiently filled by solder, the resistance between pins 1 and 3 (+V and 0 V respectively) will be around 1.6 to 2.0 MΩ and the on-board voltage regulator will be enabled. If the well has been properly filled with solder the resistance across pins 1 and 3 will be around 1.2 KΩ and the on-board voltage regulator will be disabled.

How do I test my PID sensor?

The commonly used industrial standard for testing PID sensors is exposing the sensor to Isobutylene (or Isobutene). A quick qualitative response may be obtained by briefly exposing the sensor to, for example, Acetone, Ethanol or Isopropanol.

Pellistors (flammable or combustible gas sensors)

What are Pellistors?

Pellistors, also known as flammable or combustible gas sensors, are in an explosion proof housing (certified by UL, CSA, ATEX and IECEx). They are used to detect explosive or combustible gases in air.

How do Pellistors work?

The current passing through the platinum wire heats up the catalytic material to 400-550ºC in which it is able to combust the target gas, generating thermal energy which raises the bead temperature. This type of sensor is not selective and will respond to all combustible gases and vapours.

Can Pellistors be poisoned?

Pellistors can be poisoned if the gas or vapour reacts with the catalyst causing the sensor to permanently lose its response to gas. Typical poisons are organic silicon compounds (i.e. silicones), organo-metallic compounds and organic phosphate esters.

Other compounds such as halogenated hydrocarbons and Sulfur containing compounds can cause a reversible loss in response which may recover on providing the sensor with a clean environment.

Are filters available filters for Pellistors?

Alphasense offers disposable Hydrogen Sulfide and Chlorine filters for ‘’A’’ and ‘’D’’ type pellistors. The filter should last for several years but if the filter changes colour from white to black it must be replaced.

Toxic sensors

What materials are used in Alphasense toxic gas sensors?

Alphasense electrochemical toxic gas sensors are sealed units containing an aqueous solution of Sulfuric Acid (H2SO4) or Propylene Carbonate, Polytetrafluoroethylene (PTFE), Polycarbonate (PC), Noryl Polymer and small quantities of Carbon (C), Platinum (Pt) and other precious metals.

How do toxic gas sensors work?

Alphasense toxic gas sensors are electrochemical cells that operate in the amperometric mode. They generate a current that is linearly proportional to the fractional volume of the toxic gas.

For more information, see Application Note AAN 104

Do toxic gas sensors require Oxygen?

Yes. For most gas sensors, good operation depends on reduction of Oxygen at the internal counter electrode; the counter electrode will do whatever it must to keep up with the working electrode. If there is no Oxygen, it will use protons and although the sensor is operating, the electrochemical balance is changed and the reading may be incorrect.

Is a shorting FET recommended in portable gas detectors?

It is normal practice to add a shorting FET for unbiased sensors This FET ensures that the working electrode is maintained at the same potential as the reference electrode when the circuit is switched off. The shorting FET is normally open circuit when power is applied.

Biased sensors (NO is the most common) must not be shorted when switched off, rather the bias voltage must be maintained when the unit is powered off, usually by a back-up battery.

For more information, see Application note AAN 105

‘’+’’ or ‘’-‘’ Bias voltage for NO sensors?

The potential of the working electrode must be increased to +300 mV above the reference electrode to ensure the NO is oxidised.

For more information, see Application note AAN 105

What is the stabilisation time for Toxic sensors when first plugged in?



New sensor or after long period of removal


After brief removal e.g for replacement

(Minutes – unless stated)












12 Hours












12 Hours






















How does temperature affect toxic gas sensors?

Electrochemical gas sensors are sensitive to ambient temperature. Both sensitivity (expressed as nA/ppm) and the zero current (expressed as equivalent ppm or nA) change with temperature. The individual technical data sheets specify the tolerance of temperature dependence at -20ºC and +50ºC, so bear this in mind when setting your software corrections.

For more information see Application Note AAN 110

How does humidity affect toxic gas sensors?

Humidity transients cause current spikes, which decay in about 10 minutes. Note that the spikes are first positive then negative with a humidity decrease, and first negative then positive with a humidity increase.

For more information see Application Note AAN 110

How does pressure affect toxic gas sensors?

When exposed to a positive pressure change, toxic sensors show a rapid positive current spike, then settle quickly to a constant output.

For more information see Application Note AAN 110

Is it possible to use Alphasense Oxygen and toxic sensors with 4-20mA Transmitter Boards?

Yes, we offer 4-20mA analogue transmitter boards for Oxygen sensors and both analogue and digital transmitter boards for toxic gas sensors

What is the difference between 3 and 4 electrode toxic gas sensors?

B4 and A4 sensors are specifically designed for low gas concentration detection: parts per billion (ppb). As well as the normal Working, Reference and Counter electrodes, B4 and A4 sensors include a 4th auxiliary electrode, which is used to correct for zero current changes.


Oxygen sensors

What materials are used in Alphasense Oxygen sensors?

Alphasense electrochemical Oxygen gas sensors are sealed units containing an aqueous solution of Potassium Acetate (KC2H3O2) and small quantities of Platinum (Pt), Carbon (C), Lead (Pb) and Lead Oxide (PbO), with trace amount of Antimony (Sb). Housing is ABS and dust filter is PTFE.

How do Oxygen sensors work?

Alphasense Oxygen sensors operate like a metal/ air battery. Oxygen is reduced at the cathode to hydroxyl ions, with a balancing reaction of lead oxidation at the anode. Alphasense mass flow Oxygen sensors use a very small capillary to restrict the flow of gas to the cathode. Mass flow Oxygen sensors are the technology of choice for industrial safety gas detectors.

For more information, see Application Note AAN 009

What is the easiest way of taking the readings from Oxygen sensor?

Oxygen sensors generate a current which is proportional to the rate of Oxygen consumption. This current is easily measured by placing a load resistor between the cathode and the anode (the 2 pins on an Oxygen sensor) and measuring the resultant voltage drop. This load resistor should be between 10 and 100Ω (we recommend 22Ω or 47Ω).

What are the minimum and maximum detection limits for Alphasense Oxygen sensors?

0.5% (5,000 ppm) O2 is the recommended minimum detection level for which a sensor will give stable readings. Oxygen sensors will operate repeatably and reliably up to 95% Oxygen, but sensor lifetime will decrease when the Oxygen concentration is above 20%.

For more information, see Application Note AAN 003

What is the stabilisation time for Oxygen sensors when first connected?

Recommended stabilisation time for as-received sensors is two hours after first installation.

Which carrier gases might change the Oxygen sensor reading?

Helium (He), Argon (Ar), Carbon Dioxide (CO2) and Hydrogen H2 all increase diffusion of O2, hence increasing output current.

What are the minimum and maximum usage temperatures for an Oxygen sensor?

We warrant performance to -20oC and lower temperature limit of use is -30oC. We specify performance to 55oC, but spikes to 60oC will not harm the sensor.

How does temperature affect Oxygen sensors?

Temperature dependence is due primarily to the change of viscosity of the gas. Rapid changes in temperature will create transient peaks. Alphasense sensors show very good temperature repeatability.

For more information, see Application Note AAN 005.

How does humidity affect Oxygen sensors?

Ambient Oxygen concentration decreases slightly at higher humidities due to dilution of Oxygen by water vapour. Rapid humidity changes can cause transient performance: Alphasense Oxygen sensors are designed to minimise humidity transient.

For more information, see Application Note AAN 008.

How does pressure affect Oxygen sensors?

Mass flow controlled Oxygen sensors show transient behaviour to pressure pulses. Positive pressure steps force more air into the sensor (following Fick’s Law), increasing the measured current. Negative pressure steps reduce air flow and hence reduce the measured current, giving a negative spike. Alphasense Oxygen sensors have a unique design that minimises pressure spikes.

For more information, see Application Note AAN 004

Is vacuum or very low absolute pressures a problem?

Alphasense do not recommend use of our sensors in vacuum pressure. Very low pressures will deplete the liquid electrolyte through evaporation

Can acetone or MEK damage Oxygen sensors?

Yes, they will react with the sensor body, which is made from ABS (Acrylonitrile Butadiene Styrene), causing solvent-induced crazing. This crazing and even solvation of the polymer is very fast and irreversible. Generally if the solvent attacks ABS then it will damage the sensor.

What is the difference between Galvanic Lead and Lead-free 3-electrode Oxygen sensors?

Traditional mass-flow Oxygen sensors respond rapidly, require the simplest of circuits to measure the sensor, they require no power and are very stable over time. However, eventually the internal lead is completely oxidised and the sensor then needs to replaced after one, two or three years, depending on the sensor.

The lead-free Oxygen sensor does not have a fixed, limited lifetime so 5 years or more are quoted for its lifetime, which theoretically is unlimited. However, this advantage is countered by the need to power the sensor continuously, even when the gas detector is off, requiring a permanent power supply.


Where I can find technical information regarding Alphasense sensors.

Individual product datasheets can be downloaded direct from our website at Downloads.

Alphasense Application Notes, which are easily accessible at www.alphasense.com provide detailed technical information. You can also contact us direct if something is needs further explanation: email sensors@alphasense.com or telephone +44 (0)1376 556700

Does Alphasense test 100% of its products?

Yes, the performance of every sensor is tested before leaving the factory. We ensure that all sensors perform to the specifications in our individual product data sheets, which are easily accessible on our web site www.alphasense.com. Alphasense can trace all test results for individual sensors using the bar code or serial number on the sensor label.

How often should I check sensor calibration?

Calibration interval depends on the application, sensor technology, industry-required performance and legal requirements. As good practice, sensor calibration should be checked on receipt and then about 30 days after installation. Once readings are stable, the calibration check period can be extended to 3, 6 or even 12 months, depending on your application.

For more information, see Application Note AAN 105

What is recommended flow rate for Alphasense sensors?

The recommended flow rate is 500sccm = 0.5L/ min. Lower flow rates may not provide adequate gas to the sensor and higher flow rates may cause pressure errors, leading to high readings.

What are the design rules when constructing a gas sampling train?

a. Gas flow should be always across/ parallel to the sensor face, not onto/ perpendicular to the sensor face.

b. If you need to restrict flow, then place the restrictor upstream of the sensor.

c. Pumps create oscillatory pressure changes which can lead to high readings and should be upstream of the sensor if possible.

d. Sensor outlet should ideally exhaust to ambient to minimise pressure drops/ variations.

e. Include a particulate filter on your inlet if sampling in dirty environments. If measuring reactive gases such as NO2, H2S, O3, Cl2 then use filters constructed from fluorinated substrates if possible.

f. Avoid condensation in your gas train. The sensor temperature should be at least 5oC above the dewpoint temperature. Nafion dryer systems are frequently used to regulate the humidity of the sampled gas.

g. In-line gas filters should be considered to reduce cross-sensitivity effects.

Can I purchase calibration accessories from Alphasense for development testing?

We do offer accessories such as gas hoods and suitable tubing however we cannot provide calibration gases. For more information please contact Alphasense, email sensors@alphasense.com or telephone +44(0)1376 556700.

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