Technical Papers

Toxic & Combustible Sensor Placement

ISA04—P268 Sallaway 5-10-04 [1]

Applications Fixed Gas Detectors

By Philip E. Sallaway

Product Manager,

25422 Trabuco Rd. #105-346

Lake Forest, CA 92630

Mobile 949-636-5286  e-mail:


Gas Detection, Toxic Gas, Gas Detectors, Safety, Flammable & Combustible Gas, Industrial Hygiene, Confined spaces, Fixed Gas Detectors


Due to liability reasons most manufacturers of Toxic and Combustible sensors will not specify the exact location for sensors. The end user is often left only with general guidelines. This paper will provide the attendee with three key pieces of information: First the importance of detector location, second guidelines on detector placement, third specific guided on specialized equipment that will help place the detector correctly. I will also discuss the limitations of sensors and how crucial proper placement is. The attendee will leave this presentation the tools to properly place a detector in their workplace. This paper contains general guidelines, each situation is unique, and every manufactures equipment is different. The manufactures recommendations should always be followed. Proper experts, engineers, and consultants should be used for proper detector placement; this paper should not be relied on as your sole source of information.

First Importance of detector Location:

 The detector or sensors must be placed where they will see gas accurately and quickly. Often time’s sensors simply get places in a “convenient location” this may lead to, in –accurate readings, low readings or no gas detection at all. When one considers the cost of purchasing, installing and maintaining a system proper location is key to getting a proper warning and function from the system proper location is a must. Simply put if the gas does not reach the detector it will not be seen no matter how much gas is present.

 Second Detector Placement:

 Placement will be predicated on several key pieces of information. The first and most obvious information needed it they type of gas or gasses that we are looking for, next we need to know the ranges and limitations of the sensor. The range and limitations vary from manufacturer to manufacturer. It is best to obtain as much information directly from the manufacturer prior to attempting to fixing the location of the sensor.

 The physical properties of the gas will have a strong influence on detector placement. Probably the key characteristic of a gas is density. Is the gas denser than air, the same density or less dense than air. This is usually presented as: Air being 1.0 and the gas as more dense being over 1.0 for example chlorine Density 2.47 or less than air Hydrogen .02 which will float. Density is determined at normal room temperature and pressure. This would appear to make placing a detector easy; however other factors come into play. Will the gas be heated, if hot enough the gas will expand and have a temporary density that is less than air, so it will float up until it cools enough then it will sink anhydrous ammonia is a good example. Also if a gas is cooled enough i.e. cryogenic gases,  during a leak it will the liquefied gas will puddle up on the floor and as the liquid slowly boils off, the cold gas may move along the floor for some distance before warming enough to float. If there is wind, air currents or other factors that can influence gas flow these need to be taken into consideration.

Common Toxic Gases                     Density Relative to Air (at the same Temp) NIOSH Guide 1994

Oxygen                                  Mixes

Carbon Monoxide                .97 Mixes

Hydrogen Sulfide                1.19

Sulfur Dioxide                      2.26

Chlorine                                2.47

Chlorine Dioxide                  2.33

Ammonia                               .60

Hydrogen Chloride              1.27

Nitric Oxide                           1.04

Nitrogen Dioxide                  2.62

Ozone                                        1.66

Please note that this density date is valid only when the Toxic gas and Air are at the same temperature. In many industrial processes gases may be heated/cooled to the point where they are more or less dense than air at room temperature. For example as a compressed gas leaks it expands and cools rapidly it may initially sink to the floor until it warms up to room temperature and when at room temperature its’ density being less it will rise. Conversely a gas (denser than air at the same temperature) which has been heated may initially rise until it cools sufficiently to reach a density greater than air and sink.

It may be necessary to deploy a wind break to prevent air currents from dispersing the gas before it reaches the sensor. Too much wind will result in erroneous readings. Electrochemical sensors work best when the wind velocity is less than 15 – 20 mph. Further continuous exposure to high humidity or very low humidity will shorten the life of electrochemical sensors, adding a strong air current will speed the sensors demise. Further placing the detector too close to incoming fresh air vents will prevent gas from reaching the sensor.

Certain gases notably Chlorine may be influences by static charges that may build up on plastic parts such as shrouds, dust covers etc. the use of conductive plastics may be required. Most manufacturers are aware of this issue and will use suitable materials. It is important that the system never be altered or modified with out the express consent of the manufacturer.

The area covered by each detector will vary greatly based on conditions. As a rule of thumb for an indoor location a detector will provide coverage of up to a 20Ft Radius, for an outdoor location the rule of thumb will provide coverage of up to 15ft Radius. If in doubt use more sensors to provide better coverage.

Typically there are a few inches of dead air next to walls, corners, floors, and the ceiling the detector should protrude well beyond these dead air spaces and be easily accessible for calibration and service.   For gases that sink Place the detector about 18 – 24 inches above the floor, to avoid dirt, dust and splash from washing. For gases that float place the detector near the ceiling. For gases that mix, place the detector at about shoulder level that is in the breathing zone.  Electrochemical Sensors should be placed facing down to avoid rain, overspray or debris accumulation. Consider the likely range of temperatures the detector will experience typical limits are -4 to +122F check with the manufacture. Avoid pressure changes greater than plus or minus 10% of atmospheric. Most equipment today is resistant to RFI/EMF however it is important to state this as a requirement.

Third specific guidance and special equipment

If precision is of the utmost importance one can use a cylinder of Calibration Gas and a portable gas detector to identify where gas from a potential lead would most likely travel. If the gas in question is not detectable via a gas detector then one can use Chem. Badges to follow the path of the gas. Another alternative is to use a surrogate gas with a similar density. If the gas has a neutral density the use of smoke tubes is recommended. Smoke tubes are glass tubes that produce a neutral density smoke via the use of a squeeze bulb. The smoke is easy to see and will move with even the smallest air current.

Consider common leak sources and place the detector accordingly:


Tank in/outlets


Reaction chambers


Storage areas

Mixing areas

Tank farms

Expansion joints

Loading and unloading areas

Sample points


Analyzer shelters


Every installation is unique and requires the careful consideration of trained engineering professionals. Always defer to the specifications and guidance of gas detector manufacturers as they know their equipment best. Proper placement will provide accurate and reliable detection of potential hazards.

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