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Sensor Technology

Catalytic Bead Sensors [combustible gases]
Electrochemical Sensors [toxic gases & oxygen]
Semiconductor Sensors [hydrogen sulfide & combustible gases]
Infrared Sensors [combustible gases]
Paper Tape Detectors [non reactive gases]

Where to Locate Sensors

CATALYTIC BEAD SENSORS
The most popular sensing method for Combustible Hydrocarbon (CHC) gases. Not used for toxic gas monitoring. This is the best all around sensor for ambient CHC monitoring. It is simple, reliable, and inexpensive. The disadvantages apply mainly in atypical applications.

Principle of Operation. A coil of wire is coated with glass or ceramic material which is, in turn, coated with a catalyst. The coil is electrically heated to a temperature that will allow it to burn (catalyze) the CHC being monitored. When burned, all CHCs liberate heat proportional to the concentration of the CHC present. This heat increases the temperature of the wire coil, increasing its resistance. The increase in resistance is measured electrically and is the source of the signal.

Advantages of Catalytic Sensors: Sensors are small, can be made mechanically rugged, signal is generated directly by actually burning the gas which is the property of the gas being measured; specific - only measures combustible gases; inexpensive; long life - usually 2 to 4 years; simple to calibrate and maintain.

Disadvantages of Catalytic Sensors: Span loss is undetectable except during calibration; can be poisoned by trace amounts of silicone, halocarbons and metallo-organic substances unless special methods of prevention are used; small signal-to-noise ratio; signal can go back to zero in CHC concentrations above their stoichiometric mixtures; some sensors can be permanently damaged when exposed to high or continuous concentrations of gases being measured (Delphian offers poison resistant and high temperature catalytic sensors).

Delphian [Combustible] Catalytic Sensors


SEMICONDUCTOR SENSORS
Semiconductor sensors are used for H2S gas measurements but as well as in certain CHC measuring applications. The quality of sensors varies widely from manufacturer to manufacturer with substantial variations in performance often found from a single manufacturer. Semiconductor sensors are one of the best sensors for H2S gas monitoring where sensitivity to low concentrations (ppm range) is required. CHC monitoring can be a problem because of interferences due to non-specificity.

Principle of Operation: A semiconducting material is applied to a non-conducting substrate between two electrodes. The substrate is heated to a temperature such that the gas being monitored can cause a reversible change in the conductivity of the semiconducting material. Under zero gas conditions, it is thought that O2 molecules tie up free electrons in the semiconductor material by absorbing to its surface, thereby inhibiting electrical flow. As H2S or CHC gas or vapor molecules are introduced, they replace the O2, releasing the free electrons and decreasing the resistance between the electrodes. This change in resistance is measured electrically and is proportional to the concentration of the gas being measured.

Types of Semiconductor Sensors

 

Advantages

Disadvantages

Sintered bulk semiconductor
composed of tin dioxide
deposited on a ceramic
tubular former (CHC)
Small
Mechanically Rugged
ppm sensitive
Does not flood
Inexpensive
Sensitive to Humidity
Sensitive to Temperature
Non specific to gases & vapors
Solid state thin film metal
oxide semiconductor deposited
on a ceramic substrate
(H2S)
Small
Somewhat rugged
ppm sensitive
Can be specific
Wide Temperature Range
Needs linearizing output
"Spiking" or "going to sleep"
Response may slow on aged sensor
Needs temperature controlled heater
Expensive
Separation in high vibration applications

Gas Detector using SolidState Tin Oxide Sensor


ELECTROCHEMICAL SENSORS
The most popular sensing method for toxic gases and oxygen monitoring. Not used for combustible gas monitoring. This is the best all around sensor for ambient toxic gas monitoring. It is simple, reliable, and inexpensive. The disadvantages apply mainly in atypical applications.

Principle of Operation: The electrochemical sensor is a self powered micro fuel cell. The cell consists of a casing containing a gel or electrolyte and two active electrodes: the working electrode (anode) and the counter-electrode (cathode). The top of the casing has a membrane which can be permeated by the gas sample. Oxidization takes place at the anode and reduction at the cathode. A current is created as the positive ions flow to the cathode and the negative ions flow to the anode. Gases such as oxygen, nitrogen oxides and chlorine which are electrochemically reducible are sensed at the cathode while those which are electrochemically oxidizable such as carbon monoxide, nitrogen dioxide and hydrogen sulfide are sensed at the anode.

Advantages of Electrochemical Sensors: They can be specific to a particular gas or vapor. They are typically very accurate. They do not get poisoned. They monitor at ppm levels.

Disadvantages of Electrochemical Sensors: They have a narrow temperature range. They have a short shelf life. They are subject to several interfering gases such as hydrogen. Sensor lifetime will be shortened in very dry and very hot areas.

Delphian Electrochemical Sensors

INFRARED SENSORS
Light Absorption sensors are becoming more affordable and more reliable. They are used for optically absorbing gases that fall in the infrared (IR), visible, or ultraviolet (UV) spectral ranges. Some gases reliably detected by optical sensors are: carbon dioxide, methanol, hydrogen cyanide, chlorine, refrigerant gases and naphthalene.

Principle of Operation: The essential components of an IR system are: a source of IR radiation, a detector capable of seeing the IR radiation, and a path between the detector and the source open to the gas to be detected. The detector electronics measures the difference between the dark (no light hitting the detector) and light (full energy hitting the detector). When gas in the path absorbs energy from the source, the detector receives less radiation than it normally would. This reduction in radiation is used to measure the gas concentration.

Advantages of Infrared Sensors: They can be made specific to a particular gas, they require less calibration than other sensors, there is no contact with the gas, no minimum level of oxygen is necessary and they are relatively maintenance free.

Disadvantages of Infrared Sensors: They cannot monitor all gases (only nonlinear molecules), they can be affected by humidity and water, they can be expensive and dust and dirt can coat the optics and impair response.

Delphian Infrared System

PAPER TAPE DETECTORS
Delphian does not offer a paper tape detector. This is older technology still in use in larger installations for testing for toxic gases.

Principle of Operation: A roll of treated paper tape is mechanically fed through an instrument. When exposed to gas, the tape changes color and the color difference is detected through a photocell, and a result displayed.

Advantages of Paper Tape Detectors: Very high sensitivity to non-reactive gases, physical evidence of gas detection event, not prone to interferences.

Disadvantages of Paper Tape Detectors: Expensive to install and maintain, mechanical devices are prone to failure, slow response time.

Color Change Badges for
Hydrogen Sulfide, Chlorine, Hydrazine, Carbon Monoxide, Ozone

Where to Locate Sensors
(This section was written specifically for catalytic bead sensors.  If you are using other sensing technologies, be sure the advice given here pertains to your sensor).

This information is included for informational purposes only and is intended to be used only as a guide to some of the important considerations. It is not an exhaustive review of all considerations. It is not a substitute for common sense and good engineering judgment. Because there are so many variables that must be taken into account, there are no hard and fast rules.

CAUTION: A sensor is a point monitor, which means it can monitor gas only at a single point. It does not cover an area. If gas from a leak does not reach the sensor, it will not be detected. The selection of the number of sensors to install and the site for each sensor is one of the most critical factors to be considered for overall system effectiveness. PLACING THE SENSOR IN THE WRONG LOCATION WILL DEFEAT THE PURPOSE FOR WHICH IT IS INTENDED. One of the most important concepts to keep in mind is that the sensor should be placed between the potential leak and the ignition source for combustibles or between the leak and the likely location of people for toxic gases. For complicated installations, it is good practice to prepare drawings showing all potential leak sites. The locations should be graded in terms of their potential for harm.

Vapor Density of Gases to Be Monitored
Sensors should be located near the ground for gases or vapors which are heavier than air. You should consider not installing them closer than 18 inches above the ground so they will not be as likely to collect mud and water. To detect heavy gases some companies do not allow sensors to be installed higher than 36 inches. Sensors should be located near the ceiling or roof or exit fan to detect lighter than air gases. You can not rely, however, on heavier or lighter gases always behaving in a predictable manner. Even inside, air currents can create anomalies. Be especially aware of areas which could become potential gas pockets.

Air Currents
Locate the sensor where prevailing air currents would be likely to contain the maximum concentration of the gas being monitored. Consider the possibility of changes in wind direction at different times of the day or during different seasons. Your local weather information center should have data on the direction and velocity of prevailing winds during the yearly cycle.

Dispersion of Gas/Vapor
Generally sensors should be located close to any potential leak source. Liquids of low volatility, in particular, may require the location of the sensor in the immediate area of the vapor source. Liquids with high flash points or slow rates of dispersion take a long time to produce readings if the sensors are any distance from the spill or leak.

Temperature Limitations
All sensors and electronics have ambient temperature limitations. The installation of the sensor must be within the operating range of each. If the area is too hot for the electronics, a sensor extension kit is available to separate the sensor from the transmitter/remote calibration modules. If the gas temperature is too hot for the sensor, a gas reclamation adapter is available to draw air samples to the sensor. By connecting metal coils to the inlet side of this adapter, the gas can be cooled down. If any preconditioning system is to be used, make sure that the vapors will not condense in the piping.

Vibration
Vibration can be damaging to the sensor and may void the warranty. Anchor the sensor to a wall or firm base rather than to a vibration source such as a motor housing. A length of flexible conduit used between the sensor and the pipe conduit can also give vibration protection.

Air Speed
The sensor operates on a diffusion principle. If the air speed past the sensor is too great it may disrupt the normal flow of air in and out of the housing. In a catalytic bead sensor, it may also cause turbulence inside, causing the reference and active sensing elements to be cooled differentially. Delphian has technical notes on air speed considerations which you should request if you are installing the sensor in a duct or any high air flow area. You should be concerned if the normal air flow past the sensor is greater than 5 miles per hour, 7.3 feet per second or 437 feet per minute. If you are placing the sensor outside where it will be subject to occasional strong winds, these winds could cause a false alarm. Using the foam splash guard has been shown to reduce these false alarms.

Wiring
Sensor wiring should be separate from other high current AC or DC wiring. If EMI or RFI is a problem, the transmitter has an earth ground screw. This is the only earth ground on the system (the Flexirack chassis has an earth ground to avoid shocks, but this ground is not used by the system’s circuitry). If it is used it should be well grounded and separate from other grounds. Connectors on the remote calibration and transmitter modules should be tight and free of corrosion. High resistance wire splices can cause an unstable zero. If possible, splices should be avoided. Good instrument wiring practices are an essential to good sensor installation. Review the wire size and allowable distances described in the Owners Manual before making a final location commitment. Before deciding how far to locate the sensor from the transmitter, review the allowable distance. Use 18 or 16 gauge wire up to a maximum of 100 feet. For installations requiring the sensor a considerable distance from the transmitter, Delphian supplies a sensor extension kit. For shorter distances, up to 72 inches, use Delphian extension cord.

Moisture Protection
Sensors should be installed where they are protected from immersion or direct contact with water,i.e. where the floor is hosed down, steam cleaning is done, or water levels/table rises in a drainage ditch. In the case of water contact, steam or hosing, or blowing rain, the foam splash guard will help protect the sensor. In the case of rising water levels, the water protection adapter will protect the sensor from damage for moderate rises in water level.

Accessibility
Sensors that need periodic calibration should be installed in a location permitting reasonable access and with sufficient room to allow the calibration adapter and calibration apparatus to be connected easily. If the sensor is to be placed in a remote location, i.e. near the ceiling, the Delphian ported adapter should be considered.

Sensor Orientation
Some sensors are orientation sensitive.  The catalytic sensor must be installed in a vertical position, with the sensing elements pointed at the floor. If the sensor is not mounted this way, the sensor may not work properly. In addition, the sensing elements may get wet and fail, and the flame arrestor will more easily become clogged.

Conduit, Seals, And Drain Plugs
It is mandatory that good conduit installation practices are observed. To comply with the requirements for equipment installed in Class I, Division 1 areas, an EYS seal is required within a specified distance of a junction box. Once the wiring has been pulled, the seals are potted. This prevents a flame front from travelling down the conduit when the cover is removed from the conduit box.

Perimeter Monitoring
To monitor an area, you should consider placing the sensors no further than 50 feet from the leak sources and not place the sensors farther than 50 feet apart from each other. Some companies require sensor spacing to be between 30-40 feet.
To monitor an outside leak location of a heavier than air gas, 4 sensors are a minimum. Each sensor being placed at ninety degrees from the other. A wind shift of 45 degrees could still mean that no sensor would see the leak. To have good assurance, at least 8 sensors would be required. Consider placing them no more than 5-10 feet from the leak source.

Dust Protection
Sensor dust covers should be used if sensors are mounted in dirty or dusty environments.

Obstructions
Even small structures, such as piping and equipment, between the possible leak source and the proposed sensor location can change the normal flow of air. All obstructions should be evaluated carefully.

Interior
Where coverage of an entire area is desired, pay particular attention to grade, floor or operating levels as well as to air flow from heating/ventilation systems. Pay attention not only to possible leak sites but to all penetrations and other points of entry where gas can be introduced into a closed area. Some companies have designated 400 square feet as the maximum area to be covered by one sensor. This number may not be appropriate for you.
For installations where coverage of specific equipment is desired, consider placing sensors no closer than 12 inches nor more than 5 feet away from the anticipated leak source. Sensitivity of the sensor can be controlled, to a degree, by moving the head closer to the source for more sensitivity. A sensor between two vessels that are close together can often serve to monitor both.
For locations in fresh air ducts or in or near exhaust systems, possible areas of concern are high or variable air flow, possible sensor poisons or contaminants and high levels of humidity or temperature.

Codes
Local, state and federal codes for electrical installation and regulations should be checked.

Other Exposures
Besides protecting workers in an environment, consideration should be given to other areas which may need protection, such as roads, housing, adjacent plants, public buildings, community activities, etc. In addition, you may wish to consider protection from hazards which might be caused by adjacent plants.

Glossary of Gas Detection Terms  |  How to Specify a Delphian Product

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