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Daviteq PID Gas Sensor
LoRaWAN
Sigfox
Sub-GHz
NB-IoT

Daviteq PID Gas Sensor

PID

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1. Giới thiệu

Overview

Daviteq PID Gas Sensor is designed for detecting VOCs over the widest dynamic range on the market without compromising performance. It is also the only PID sensor with a lamp out diagnostic. It is a simple plug-and-play sensor able to deliver a dynamic and dependable response to thousands of volatile organic compounds (VOCs) across many diverse applications.

  • Patented ‘fence’ electrode for excellent humidity resistance (from iOnScience)

  • Anti-contamination design

  • Reliable lamp ignition – illuminates at low temperatures

  • Superior lamp life – 10.6 and 10.0 eV => 10,000 hours

  • User-replaceable electrode stack in event of corrosive or mechanical damage

  • Lamp out error detection (for range 0∼4000 ppm only)


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


Applications
  • Industrial hygiene & safety monitoring

  • Soil contamination and remediation

  • Hazmat sites and spills

  • Leak detection

  • EPA Method 21 and emissions monitoring

  • Arson investigation

  • Indoor air quality monitoring

  • Outdoor air quality monitoring

2. Nguyên tắc hoạt động

How PID Sensor works?

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 amplified 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


Environment:
  • 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. Hiệu chuẩn

The Daviteq PID 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).


Calibration guidelines

PID sensor naturally produces ozone in air, which over time acts to remove organic detritus from within the PID cavity. In many domestic and light industrial environments, the PID is self-cleaning. Re-calibration is then only needed to adjust for any decay in the PID lamp output on which the photoionisation measurement depends. In a fixed instrument, typically this will be every one to two months of cumulative sensor operation. However, when first deploying a PID instrument in a new environment, end users should be encouraged to bump test and re-calibrate as necessary. Since portable PID instruments are exposed to unknown environments, their calibration may be required more often.


Calibration of instruments containing PID usually demand measurements of ‘zero gas’, containing near zero concentrations of VOCs, and a span gas, used to calibrate the PID sensor in its linear range. Both gases are usually prepared with a ‘balance’ gas of artificial air, comprising ~80% nitrogen and ~20% oxygen. Do not use pure nitrogen as a balance gas as this delivers up to 20% more responsivity than air.


Appropriate end user calibration gases for the various MiniPID 2 sensors are identified below:

Zero gas of <0.1 ppm VOC is usually provided by ultrahigh purity (UHP) air. To enable frequent calibration, purified gas systems providing the same gas purity can be used. Lubricating oils in compressed air lines should be avoided as they will foul PIDs if exposed to the gas stream for extended times.


Some gases absorb UV light without causing any PID response (e.g. methane, ethane). In ambient atmospheres where these gases are present the measured concentration of target gas will be less than is present. Methane absorbs UV strongly, so for accurate measurements in methane containing atmospheres, calibrate with a calibration gas containing the expected methane concentration.


The HS sensor requires particular care in handling as identified in the technical article TA-14. Since air may contain semi-volatiles which give rise to a significant and slowly moving sensor response, calibration of a high sensitivity VOC detector must always be undertaken after burn-in using clean air. This will provide the zero point calibration reading.


How to calibrate the PID Gas sensor?

Please follow below steps to attach the calibration cap onto the sensor module and connect the sensor to standard gas cyclinder for zero calibration and span calibration


Step 1. Prepare the calibration cap



Step 2. Attach the calibration cap to the sensor head and fix the cap by turning clockwise the T head bolt


Step 3. Install the Gas regulator to the Standard Gas cylinder


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


Step 4. Attach the tube to the regulator and calibration cap



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


With the 2-point calibration method, the user input the Zerro value and Span value on local display and then the device will calculate the A and B factors automatically. Please find below the steps of calibration.


Step 1. Zero calibration

- Power ON the device and wait at least 90 minutes for stable measurement

- Connect 0.1 ppm Isobutylene standard gas cylinder to the device. Detail of how to implement the connection as described above

- Access ZERO CALIBRATION menu on local display and input value of 0.1 by virtual keyboard on local display without input OK by virtual key). Then open regulator slightly to supply Isobutylene standard gas of 0.1 ppm to the device and wait for 3 minutes for stable flow and measurement, then input OK on virtual keyboard to complete the zero calibration process. Please refer below video for more detail of zero calibration


Note: DON'T use Nitrogen as standard gas for zero calibration


Step 2. Span calibration


Note: Keep the sensor Power ON all the time


- Connect 10 ppm Isobutylene standard gas cylinder to the device. Detail of how to implement the connection as described above

- Access SPAN CALIBRATION menu on local display and input value of 10 by virtual keyboard on local display (without input OK by virtual key). Then open regulator slightly to supply Isobutylene standard gas of 10 ppm to the device and wait for 3 minutes for stable flow and measurement, then input OK on virtual keyboard to complete the span calibration process. Please refer below video for more detail of zero calibration


Note: 

Standard gas for span calibration MUST NOT has balance gas of Nitrogen. This gas should have balance gas of air.

Above is the span calibration for 10 ppm Isobutylene standard gas. In real condition, the span calibration could be implemented by Isobutylene with other range such as 20 ppm, 30 ppm... based on availability of Isobutylene standard gas and sensor measurement range.


- 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.



The instructions of the zero calibration and span calibration in below video:





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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

In a fixed instrument, typically this will be every one to two months of cumulative sensor operation. However, when first deploying a PID instrument in a new environment, end users should be encouraged to bump test and re-calibrate as necessary.


Routine Maintenance


The electronics in the PID sensor are not accessible and designed to be maintenance-free.

The electrode stack is easily replaced and inexpensive. The PID stack will operate for years in most non-corrosive environments. It is recommended that end users of instruments containing PID carry at least one electrode stack in stock. The electrode stack is not toxic. Due procedures should be considered for safe disposal in the event of the stack being exposed to toxic environments.


Unscheduled Maintenance

The sensor have some error code for maintenance indicator for unscheduled maintanance as below table:


The PID should be dismantled and stack and lamp inspected in the following circumstances:

  • On exposure of the sensor to very humid, acidic (sour) and salty environments. This may cause inorganic salts to accumulate on PID enclosure walls, which ultimately compromises the screening potential of the PID fence electrode. This is often indicated by a moisture sensitive signal.

  • Visual indications of liquid ingress into the electrode stack.

  • A sensor error 4 indicates a failure of contact has developed between the sensor pins and the stack pads. The wings on the PID stack may fail after repeated dis- and re-assembly. It may also be caused by failure of the stack to engage fully with the sensor body, which may be corrected by refitting the lamp and stack. The PID lamp should be cleaned and replaced as necessary if the responsivity of the sensor, as measured in bump tests or during calibration, decreases unexpectedly. Note that exposure of the sensor to amines is liable to temporarily contaminate the PID lamp. Instrument design should cater for this.


Removing the electrode stack and lamp


CAUTION:

Only use the electrode stack removal tool. Any other tools (for example screwdrivers) may damage your PID body and will invalidate your warranty.


  1. Wear gloves. Carefully remove the sensor from instrumentation.

  2. Locate electrode stack removal tool in the side slots of the PID and squeeze together until electrode stack and lamp are released.

  3. Lift carefully the PID body away from the electrode stack and lamp.

  4. Occasionally the lamp may be temporarily lodged in the sensor body and will need to be freed carefully with tweezers. Occasionally the small spring behind the lamp will come out when the lamp is removed from the sensor. Simply replace it into the sensor body.