FAQs

General

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.

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.

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

  1. Gas flow should be always across/ parallel to the sensor face, not onto/ perpendicular to the sensor face.
  2. If you need to restrict flow, then place the restrictor upstream of the sensor.
  3. Pumps create oscillatory pressure changes which can lead to high readings and should be upstream of the sensor if possible.
  4. Sensor outlet should ideally exhaust to ambient to minimise pressure drops/ variations.
  5. 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.
  6. 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.
  7. In-line gas filters should be considered to reduce cross-sensitivity effects.

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.

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

NDIR

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.

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 specific 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 absorption 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

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

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..

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.

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

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.

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

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.

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

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

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.

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.

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).

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

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.

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

Pellistors (flammable or combustible gas sensors)

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.

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.

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.

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.

PIDs

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.

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.

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

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

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.

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

Toxic sensors

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.

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

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

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

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

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

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

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.

 

GAS New sensor or after long period of removal

(Hours)

After brief removal e.g for replacement

(Minutes – unless stated)

H2S 2 10
CO 2 10
SO2 2 10
NO 12 12 Hours
NO2 2 10
Cl2 2 10
HCl 12 240
ETO 12 12 Hours
HBr 12 10
HCN 12 10
PH3 2 10
NH3 12 240
O3 2 10
Br2 2 10
H2 2 10

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.