Hydrocarbon gas detector (IR type) Infrared Type

The Infrared (IR) detection method is based upon the absorption of infrared radiation at specific wavelengths as it passes through a volume of gas. Typically two infrared light sources and an infrared light detector measures the intensity of two different wavelengths, one at the absorption wavelength and one outside the absorption wavelength. If a gas intervenes between the source and the detector, the level of radiation falling on the detector is reduced. Gas concentration is determined by comparing the relative values between the two wavelengths. This is a dual beam infrared detector.

 

Infrared gas detection is based upon the ability of some gases to absorb IR radiation. Many hydrocarbons absorb IR at approximately 3.4 micrometers and in this region H2O and CO2 are relatively transparent. As mentioned earlier, there are some hydrocarbons and other flammable gases that have poor or no response on a general purpose IR sensor. In addition to aromatics and acetylene, hydrogen, ammonia and carbon monoxide also cannot be detected using IR technology with general purpose sensors of 3.4 micron specifications.

Advantages

The major advantages of IR gas detectors:

• Immunity to contamination and poisoning.
• Consumables (source and detector) tend to outlast catalytic sensors.
• Can be calibrated less often than a catalytic detector.
• Ability to operate in the absence of oxygen or in enriched oxygen.
• Ability to operate in continuous presence of gas.
• Can perform more reliably in varying flow conditions.
• Even when flooded with gas, will continue to show high reading and sensor will not be damaged.
• Able to detect at levels above 100 % LEL.

Disadvantages

The limiting factors in IR technology:

• The initial higher cost per point. IR detectors typically are more expensive than catalytic detectors at initial purchase.
• Higher spare parts cost.
• Gases that do not absorb IR energy (such as hydrogen) are not detectable.
• High humidity, dusty and/or corrosive field environments can increase IR detector maintenance costs.
• Temperature range for detector use is limited compared to catalytic detectors.
• May not perform well where multiple gases are present.

Infrared Open Path Gas Detectors Working Principle

An alternative method of measuring gas concentration is based on absorption of infrared (IR) radiation at certain wavelengths as it passes through a volume of gas. Devices using this technology have a light source and a light detector and measure the light intensity at two specific wavelengths, one at an absorption (active) wavelength and one outside of the absorption (reference) wavelength. If a volume of gas passes between the source and detector, the amount of light in the active wavelength falling on the detector is reduced, while the amount of light in the reference wavelength remains unchanged. Much like the catalytic detectors, the gas concentration is determined from the relative difference between the two signals.

There are several key advantages to using IR-based detectors:

• Immune to all chemical poisons
• Does not need oxygen or air to detect gas
• Can work in continuous exposure gas environments
• Fail-to-safe technology
• Internal compensation virtually eliminates span drift

IR-based detectors can be either single-point or open path devices and, with the sophisticated optical designs currently being used, are factory calibrated and virtually maintenance free. This is particularly desirable when sensors must be located in inaccessible areas and cannot be easily calibrated on a periodic basis. Maintenance of IR detectors is typically limited to periodic cleaning of the optical windows and reflectors to ensure dependable performance. The current availability of reliable, low cost electronics and solid state IR detectors has reduced costs and made the technology feasible for many commercial applications. However, IR detectors cannot be used for the detection of hydrogen and certain other gases for which the catalytic method is suitable.

Infrared gas detection is based on the ability of some gases to absorb IR radiation. It is well known that almost all hydrocarbons (HC) absorb IR at approximately 3.4 µm and at this region H2O and CO are not absorbed, making the system immune to humidity and atmospheric changes. It follows therefore that a dedicated spectrometer operating at that wavelength could be used to detect hydrocarbons in air. Such a system would follow the Beer-Lambert Law which states:

T=exp (-A x C x L)

Where:
T is the transmittance of IR
A is the absorption coefficient of the particular gas molecule
C is the concentration of the gas
L is the path length of the beam through the gas

The gas concentration output for open path detectors is expressed in ppm meters (parts per million of combustible gas times the path length in meters: a highly sensitive range for detection of low level leaks) or LEL meters (a hazardous gas level).

Typical readings are as follows:

Open path IR detection offers immunity to poisons, high sensitivity gas leak detection, hazardous level gas detection, low maintenance, easy installation and fail to safe operation. However, it is not an all-encompassing answer to combustible gas detection. It offers an alternative solution to gas detection challenges and should be used in combination with point gas detection due to its limitations in targeting the specific location of gas leaks

Calibration of Gas Detectors

A gas detector is a device that detects the presence of gases in an area, often as part of a safety system. This type of equipment is used to detect a gas leak and interface with a control system so a process can be automatically shut down. A gas detector can sound an alarm to operators in the area where the leak is occurring, giving them the opportunity to leave. This type of device is important because there are many gases that can be harmful to organic life, such as humans or animals.

Gas detectors can be used to detect combustible, flammable and toxic gases, and oxygen depletion. This type of device is used widely in industry and can be found in locations, such as on oil rigs, to monitor manufacture processes and emerging technologies such as photovoltaic. They may be used in firefighting.

Gas leak detection is the process of identifying potentially hazardous gas leaks by sensors. These sensors usually employ an audible alarm to alert people when a dangerous gas has been detected. Common sensors include infrared point sensors, ultrasonic sensors, electrochemical gas sensors, and semiconductor sensors. More recently, infrared imaging sensors have come into use. All of these sensors are used for a wide range of applications and can be found in industrial plants, refineries, waste-water treatment facilities, vehicles, and homes.

Calibration of Gas Detectors:  SC111/112 General Monitors

When Gas detector Model SC111/112 General Monitors power-up, it will take an initial power-up period of 1 minute approximately, and is observed by the SC111 during which it display “PU”. This is to allow the sensor to stabilize, then the display should read ‘0’ if there is no gas present at the sensor. If doesn’t occur then refer to manual book, in section troubleshoot.

Calibration of SC111/112 General Monitors:

1. Ensure that the SC111 has stabilized for at least 1 hour, and there is no combustible gas present at the sensor. A true zero reading will be obtained when the reading stabilized at the lower value.
2. Place the magnet over the General Monitors logo at the surface body. ‘–‘ will appear on the display first to indicate that the magnet has been positioned correctly. Then the display will begin to flash. After a total of 9 second ‘AC’ will be displayed, indicating that the unit is in the auto-calibration, then remove the magnet.
3. Use a General Monitors Portable Purge or Calibration Chamber to apply gas at 50% LEL (+/-5%) to the sensor. When the SC111 detects this gas it will display ‘CP’ , this mean ‘CP’ = CALIBRATION IN PROGRESS
4. Wait until ‘CC’ is displayed before removing gas. This will normally take less than 2 minutes, ‘CC’ = CALIBRATION COMPLETE
5. When the gas disperses from the sensor the SC111 will leave CALIBRATION MODE and return to a normal monitoring condition. The display should read ‘0’ when the gas has dispersed.
6. If the above does not occur as describe and a different code is displayed..go to the TROUBLESHOOTING section.

Calibration Check on SC111/112 General Monitors

After perform calibration on Gas detector SC111/112 then will continue to perform calibration check:

1. Position the magnet over the General Monitors logo, The symbol ‘–‘ will be displayed for three second and then will begin to flash
2. Remove the magnet and the display will now flash the gas concentration at the sensor. The analog output will be held at 1.5mA regardless of the gas concentration at the sensor.
3. If gas is not applied within 6 minutes the analog output will fall to 0mA and the display will read ‘F2’. To recover from this position, replace the magnet over the General Monitors Logo, repeat step 1 and 2 then proceed to step 4 within the timeout period
4. Apply gas at 50% LEL to the sensor. Observe that the gas reading settles at 50+/-5%. Should the final response fall outside this limit, a full calibration is required.
5. Note : The sensor should be exposed to clean air conditions for at least two minutes prior to entering calibration mode.
6. The display will continued to flash and the analogue output will remain at 1.5mA until the gas has been removed and the level at the sensor drops below 3% LEL approx. Normal monitoring will then be resumed (i.e the display will give a steady reading and the analogue output will follow the gas concentration at the sensor)

Calibration of Gas Detectors: BW Killark Gas Point

Calibration of BW Gas Point can be executed at any time during normal operation except the self test period (from 10 minutes before the self test until the self test is complete) BW technologies recommends a premium grade calibration gas, Gases with NIST (National Institute of standards and Technology)

Start Calibration:

1. Press and hold the external button down while the LCD displays the alarm setpoints, continue to hold the button until the display reads CAL ( approx. 5 seconds) and then release the button.

– The 4-20 mA output will be 3 mA throughout calibration. Calibration the Gas Point will not cause false alarms at the controller

– on this step: first LCD monitor will display low and high alarm (approx. 8 sec), next the CAL icon lights for 3 sec, gas type is constantly displayed and backlight is activated.

Auto Zero:

2. The Gas Point will then take a zero level reading, combustible and toxic sensors; if background gas is present, apply zero the sensor, restart the calibration sequence will take 30 to 60 seconds.

– On display, numeric display will read 00, auto zero advice flashes, gas monitor constantly displayed.

Auto Span:

3. Insert Cal cap and apply gas to sensor for approx. 2 minutes (ammonia 5 minutes)
4. When the countdown (300 to 00) begins, span is complete, disconnect the gas

– If span fails: check calibration gas  cylinder used and concentration expected. Replace the cylinder and/or change the Cal gas expected value, if required, re-calibrate

– Oxygen sensor, use pure air calibration gas (20.9% O2) in case of deficient to enriched atmosphere.

– on display : numeric display will show calibration gas value expected, gas cylinder icon flashes, span advice lights, gas type is constantly displayed, after a successful calibration, gas point automatically returns to normal operation and displays the current reading (ppm or %) present.