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Daviteq Ex d ATH Gas Sensor

Daviteq Ex d ATH Gas Sensor


New replacement products

Replaced SKU

Satellite IoT


1. Introduction


Daviteq Ex d ATH Sensor is an Exd-approved ambient Humidity and Temperature sensor. it utilizes a Digital capacitance humidity sensor to deliver high accuracy and stable measurement.

It has low power consumption, which allows it to be integrated with wireless devices such as Sub-GHz transmitters, Sigfox transmitters, LoRaWAN transmitters, RS485 output transmitters, etc.

  • Ambient humidity in an Explosive environment;

  • Ambient humidity in the Petrochemical warehouse;

  • Ambient humidity in Oil & Gas warehouse;

  • Ambient humidity in Soya bean warehouse;

2. Principle of Operation

How does the ATH-EXD Sensor work?

The PID sensor measures volatile organic compounds (VOCs) in air by photoionisation detection (PID). The sensing mechanism is shown schematically below. Test gas (1) is presented to the external face of a porous membrane, through which it freely diffuses, into and out of a gaseous enclosure, (shown by double headed arrows). From the opposite face of the enclosure, (2) an illuminated lamp emits photons of high energy UV light, transmitted through a crystal lamp window (wavy arrows). Photoionisation occurs in the enclosure when a photon collides with a photoionisable molecule (3a) to generate two electrically charged fragments or ions, one positively charged, X+, and one negatively charged, Y- (3b). These are separated at, oppositely charged metal electrodes, being a cathode and anode, generating a tiny electric current. The current is amplifiAd in an electric circuit (not shown) and presented as a sensor voltage output which depends on the concentration of photoionisable gas. The Mini PID 2 includes a third fence electrode which ensures that the amplified current does not include significant contributions due to other current sources such as electrolytic salt films on the chamber walls.


Volatile organic compounds (VOCs) sensed by PID sensor

Most VOCs can be detected by PID sensor. Notable exceptions are low molecular weight hydrocarbons.

Every VOC is characterized by an Ionisation Energy (IE). This is the minimum energy required to break the VOC into charged fragments or ions. Volatiles and gases in air are photo-ionized, and hence detected, when exposed to light of photon energy greater than their IE. PID sensor is provided with a light source of three different photon energies: 10.0 eV, 10.6 eV or 11.7 eV.

Standard PID sensors (PPM, PPB and HS) engage an unfiltered krypton light source, which delivers 10.6 eV UV light. The sensors respond to about 95% of volatiles, notable exceptions being most volatiles of one carbon atom, acetylene, ethane, propane and saturated (H) CFC’s.

The PID 11.7 eV, which employs an argon lamp light source, responds to almost all VOCs: the few exceptions are methane, ethane and saturated fluorocarbons. 11.7 eV PID is less selective but particularly of interest in measuring formaldehyde, methanol and the lighter hydrocarbons, for which scant other sensing technology is available.

Finally, PID 10.0 eV, which engages a krypton light source and a crystal filter, responds to more limited range of VOC’s. Aromatics and most other unsaturated molecules are most readily detectable with this lamp, whereas most saturated hydrocarbons, with which they often occur, are sensed more weakly or not at all.

For detection of a volatile compound, it must be sufficiently volatile. A fairly large molecule such as alpha-pinene, (a constituent of turpentine), saturates in air at about 5000 ppm at 20℃; this is the maximum concentration of the alpha-pinene that can be measured at 20℃. Some compounds, e.g. machine oils and plasticisers, generate a fraction of a ppm of vapor at ambient temperatures. Because the diffusion of such large molecules is also very slow, in most scenarios they are not detectable. Organic compounds of boiling points 275 to 300℃ (at 1 atm.) are considered to be semi-volatiles and marginally detectable. Compounds of boiling point > 300℃ are considered non-volatile and undetectable.

Selectable Ranges (Isobutylene equivalent) & Performances
  • Range: > 10,000 ppm. Minimum detection limit: 500 ppb (10.6 eV Lamp). Response time in diffusion mode (T90) < 3s

  • Range: 0 to 4000 ppm. Minimum detection limit: 100 ppb (10.6 eV Lamp). Response time in diffusion mode (T90) < 3s

  • Range: > 200 ppm. Minimum detection limit: 20 ppb (10.6 eV Lamp). Response time in diffusion mode (T90) < 8s

  • Range: 0 to > 100 ppm. Minimum detection limit: 5 ppb (10.0 eV Lamp). Response time in diffusion mode (T90) < 8s

  • Range: 0 to > 100 ppm. Minimum detection limit: 100 ppb (11.7 eV Lamp). Response time in diffusion mode (T90) < 8s 

  • Range: 0 to > 40 ppm. Minimum detection limit: 1 ppb (10.6 eV Lamp). Response time in diffusion mode (T90) < 8s

  • Range: 0 to 3 ppm. Minimum detection limit: 0.5 ppb (10.6 eV Lamp). Response time in diffusion mode (T90) < 12s

  • Relative humidity range: 0∼99% RH, non-condensing;

  • Operating temp range: -40℃∼55℃ ( except 0∼40℃ for range 3 ppm sensor)

Response Factors for other Gases:

Our PIDs are calibrated using isobutylene, but PID is a broadband detection method with a variable sensitivity to each VOC. The relative sensitivity to each compound also varies significantly with PID photon energy (10, 10.6 or 11.7 eV). It varies much less with PID design and lamp output.

Response Factors (RFs) provide an indication of the relative sensitivity of PID to specific VOCs, relative to isobutylene. The RF of a VOC is used to convert the calibrated response of the sensor with isobutylene into a concentration of the target VOC.

Example: Toluene

  • A PID 10.6 eV sensor is calibrated with isobutylene and found to have a sensitivity of 1 mV.ppm⁻¹.

  • If the sensor is exposed to 100 ppm isobutylene the output will be 100 mV.

  • The response factor for toluene using 10.6 eV is listed as 0.56.

  • If the sensor is exposed to 56 ppm toluene then the displayed uncorrected concentration will be 100 ppm.

A complete list of response factors is shown in this link.

If response factors are programmed into an instrument, it is possible for target VOC to be specified, and the instrument can then display and record a concentration for that target volatile.

The Notes column below identifies the following:
  • S: Slow. PID requires at least 30s for a stable response.

  • V: Variable response. The response is susceptible to small changes in ambient conditions, particularly humidity.

  • X: Temporarily contaminating. PID responsivity may be suppressed for at least 30 min after 100 ppm-min exposure.

  • W!: Expected to cause PID lamp window fouling. May require regular bump tests and window cleaning.

3. Calibration

The Daviteq GHC Gas Sensor must be connected to a reading device, it usually is a wireless transmitter like Sub-GHz, Sigfox, or LoRaWAN.

Why do we need to calibrate the gas sensor? There are some reasons:

- The output value of a sensor is different from the other sensor. It is not the same value for all sensors after manufacturing.

- The output value of a sensor will be changed over time.

Therefore, users need to calibrate the sensor before use or in a pre-defined interval time (30 months for example).

How to calibrate the GHC Flammable Gas sensor?

Note: The calibration can only be done in the safe zone!!!

Please follow steps for Instruction to attach the calibration cap onto the sensor module to get Zero or Span values:

Step 1. Remove the Filter and prepare the calibration cap

Step 2. Attach the calibration cap to the sensor head

Step 3. Installed the Regulator to the Gas cylinder

Step 4. Attach the tube to the regulator

Please select the flow regulator with a flow rate of 2.5 LPM or 5.0 LPM.

With the 2-point calibration method, the user can define the A and B factors. Please find below the steps of calibration.

Step 1. Get the Zero value.

- Power ON the device;

- Place the device in a clean-air environment (the target value is nearly zero) at a temperature from 20∼30℃, in at least 60 minutes.

- After 60 minutes, force the device to send data, read and record the Raw_value, so now you got the Zero_value = Raw_GHC value.

Recommendation: Record many Raw_GHC values ​​at least 10 minutes apart (10 values). Zero value is the average of the recorded Raw values.

Note: The Raw_GHC values can be positive or negative; Its value is usually 7 (% LEL)

Step 2. Get the Span value

Note: Keep the sensor Power ON all the time

- Use the standard gas cylinder with a known concentration (for example Eythylene Air  1.35% is equivalent to 50% LEL ) to supply the gas to the sensor;

- Use the calibration cap as above pictures to attach to the sensor and connect the tubing to the gas cylinder;

- Open the valve on the Cylinder slowly and make sure the gas has reached the sensor. The flow regulator should be 2.5 LPM or 5.0 LPM.

Notes: The tube length is short as possible to reduce the gas loss.

- Press a timer to start counting the time;

- After 2 minutes, force the device to send data once every minute, and stop forcing at 5 minutes;

- The highest Raw_value is the Span value.

Note: Just get one value for Span.

- After that, immediately turn OFF the valve to save the gas;

- Remove the calibration cap from the sensor;

- Place the sensor in clean air again.

Note: Always keep the sensor Power ON all the time.

Step 3. Calculate the new A and B

The calculation of new A, B value based on basic linear formula:

y = A * x + B


A, B is calibration coefficients

x is the sensor process value (example gas level in ppm) read on reading device such as on application server/network server, on offline tool. The process value is the RAW_VALUE in the payload

y is the correct value. y is the value of standard gas/standard condition

Which condition of Zero value: y₀ = A * x₀ + B

Which condition of Span value: yₛ = A * xₛ + B

From the two formulas, the calculation of A, B as below

A = (y₀ - yₛ) / (x₀ - xₛ)

B = (yₛ * x₀ - y₀ * xₛ) / (x₀ - xₛ)

Example of A, B calculation for LoraWAN Ammonia Gas sensor (item code WSLRW-G4-NH3-100-01):

* With condition of clean-air environment at a temperature from 20∼30℃, there is no ammonia gas (y₀ = 0); while the NH₃ level on reading device (RAW_VALUE in the payload) is -0.25 (x₀ = -0.25)

* When the sensor is connected to standard gas cylinder having ammonia level of 25 ppm (yₛ = 25); while the NH₃ level on reading device (RAW_VALUE in the payload) is 18.66 (xₛ = 18.66)

The calculation of A, B for the Ammonia gas sensor:          

A = (0 - 25) / (-0.25 - 18.66) = 1.32205

B = (yₛ * x₀ - y₀ * xₛ) / (x₀ - xₛ) = (25 * (-0.25) - 0 * 18.66) / (-0.25 - 18.66) = 0.33051

The factory default A = 1 and default B = 0

The RAW_VALUE in the payload is used for calibration

Step 4. Configure the new A and B into the device

- User can use the off-line tool or downlink to write the values of A and B;

- Writing the new A and B successfully meant you had done the calibration process. Congratulation!

4. Application Notes

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5. Installation Notes

  • If the PID is deployed in a wall-mountable detector, the sensor is ideally located in the instrument to be as far from the wall mountings as possible, to minimize condensation phenomena caused by the air vs wall temperature disparity (consider the accumulation of particulates and heavy volatiles on shelves and kitchen units).

  • The sensor should be pointing downwards or sideways, to avoid slow accumulation of volatile in the sensor cavity, and dust.

6. Troubleshooting

7. Maintenance

Sensor replacement instructions:

Please remove the batteries before doing the following steps.

Step 1. Unscrew the gas housing

Step 2. Remove the filter housing

Step 3. Unplug gently the sensor module

Step 4. Plug gently and firmly the new sensor module into the PCB

Step 5. Insert the batteries and start calibration as per Section 3