The docking stations for GX-2009, GX-2012, and 03 Series instruments all use the same software to communicate and upload data from the instrument to a data file. This software is called SDM-GX Docking Station PC Controller. You can have one centralized data file on a network location where multiple users can access and review data received from any of these instrument docking stations. As long as the docking stations are connected to a PC (up to 10 docking stations can be connected to one PC) and the PC is connected to the same network where the data file is located, then all instruments can automatically transmit their alarm events, calibration and bump test data to one data file. Anyone who is on the same network and has the SDM-GX software installed on their workstation will have access to all the data coming from these docking stations.
Below is a link to a step by step guide for setting up a network location for a SDM-GX Docking Station PC Controller data file.
RKI offers a compact 11” notebook PC with Ethernet and the SDM-GX Docking Station PC Controller software pre-installed as an available accessory to our line of calibration stations. This will allow up to 10 docking stations to be managed.

By definition a confined space is any space that is large enough and so configured that an employee can bodily enter and perform assigned work, has limited or restricted means for entry or exit, and is not designed for continuous employee occupancy. These spaces may include, but are not limited to, underground vaults, tanks, storage bins, pits and diked areas, vessels, sewers, and silos. In addition, this space may not have adequate ventilation or air movement allowing gases to form pockets or stratify within the confined space adding to the danger.

When testing confined spaces prior to entry it is necessary to test at all levels looking for dangerous gases. This may include gases that are lighter than air and may collect at the top of a confined space, such as methane, heavier than air gas that may settle at the bottom of a confined space such as hydrogen sulfide, carbon monoxide which has about the same density as air and oxygen content. Using a sample drawing portable gas monitor for this application makes this task extremely easy to perform. Confined space monitors can be provided with an internal motorized sample pump or an attachable sample pump. An attached pump, either motorized or hand aspirated, can turn a personal portable diffusion monitor into a sample drawing instrument allowing for greater versatility.

For example, if a worker is required to enter a confined space such as a manhole, this individual would need to test the atmosphere around the top of the manhole cover before removing the lid. A confined space safety gas monitor with a sample pump will allow the user to easily “sniff” around the lid for gas. If the manhole lid has pick hole openings, the sample probe can be used to test under the lid for explosive, toxic gas, and oxygen content. Once the lid is removed, the sample probe can then be lowered into the confined space starting at the top and sampling all levels until the probe reaches the bottom.

With a non-sample drawing instrument the sensor block or the monitor itself is lowered into the confined space. The dangers of using a monitor in this fashion is that it can be dropped into liquids destroying the sensors or the instrument. Also, if the monitor is lowered into the confined space, the user would be unable to see the actual gas readings at the various levels. Monitors provided with sample pumps include hoses that can be purchased in various lengths to accommodate a variety of confined spaces. In addition, each monitor can include a probe with water-blocking filter to prevent damage to the instrument in the event that the probe is dropped into liquids.

In summary, choosing a monitor with either internal motorized pump or a diffusion monitor with attachable pump will allow the instrument to be used in a variety of different applications including confined space entry where accurate sampling of the atmosphere is essential to worker safety.

Did you know leak detection of hydrocarbons is required at many injection, disposal, and storage wells, compressor sites, and around brine pits? The Railroad Commission of Texas (RRC) statewide rules 95, 96, and 97 outline the requirements for installation and operation of LEL leak detectors. Fixed Systems

The RKI M2 is an ideal solution for meeting these requirements. The M2 transmitter can operate as an independent, stand-alone detector head or with a RKI controller. A digital display of the gas concentration, as well as alarm and status lights, can be viewed through the front window. The M2 also has two levels of alarms with relays, plus a fail alarm with relay.

The RRC rules state the detector needs to be tested twice a calendar year. Calibration of the M2 can be performed by one person utilizing the magnetic wand. Since the housing does not need to be opened, it is unnecessary to declassify the area for maintenance.

Depending on the size of the site, it may be necessary to install multiple LEL detector heads. With our Beacon controller line we can accommodate 1, 2, 4 or 8 points of detection. Or the M2 can operate as a standalone either 24VDC or 12 VD powered.

Common Locations:

• Compressor Sites
• Disposal Wells
• Injection Wells
• Storage Wells
• Brine Pits

In environments with combustible gas hazards, it is important to know long before the gas concentration reaches the LEL. Typical safety standards require that a gas detection unit give warnings at 10 – 20% of the LEL. Do not confuse the alarm level with the volume of gas required to reach the LEL. For example: Methane has an LEL of 5% by volume in air. For a gas detector to give an alarm at 10% of the LEL, it must trigger when it detects 0.5% by volume. The detector for this application would most likely be calibrated for the range from 0% to 5% gas by volume, but display the reading as 0 – 100% LEL

You can receive a calibration certificate with any instrument order, if you request it at the time you place your order. Calibration certificates are $10 each. Just reference part # 90-CALCERT as a separate line item of your order.

As an ISO 9001 company, shipping quality products is a major priority for RKI. Each RKI product is put through a detailed quality assurance check-list prior to shipping. RKI includes a Statement of Quality and Conformance card with each instrument to verify this quality process.

For customers who require more detailed calibration information for their instruments, RKI offers a Calibration Certificate. These certificates include the instrument’s model number, part number, and serial number as well as the specific readings during it’s pre-shipment calibration. The certificate also includes traceability information for the gas used.

Deciding on the appropriate and most efficient solution for a fixed system application involves evaluations of the environment and decisions about the different ways to monitor that environment. Typical components of a fixed system include a Controller and a Detector. One decision to make is whether to use a Direct Connect sensor or a Sensor/Transmitter for the detector part of the fixed system. Both Direct Connect Sensors and Sensor/Transmitters have advantages that should be understood before deciding on which detector to select.

Direct Connect Sensors

• Lower Cost
• Requires an RKI Controller*
• Remote range up to 1,000 feet
• Non-intrusive calibration

Direct connect sensors offer the advantage of having a lower cost than a sensor/transmitter style, because a direct connect sensor does not utilize a transmitter in its design. A direct connect style detector can only be used with an RKI controller. The signal-processing circuitry normally found in the transmitter is located in the RKI controller for further signal treatment. In a direct connect configuration, the calibration adjustments are performed at the controller, with the test gas being applied to the sensor. RKI’s controllers utilize unique operating software that allows direct connect sensor calibration to be done quickly and easily by only one person. The distances that a direct connect sensor can be located from the RKI controller are appreciable, ranging up to 1,000 ft. with appropriate sized wire. One other advantage of a direct connect sensor is that it can be calibrated without opening the detector enclosure (also known as ‘non-intrusive calibration’), this can be a big cost savings for detectors located in hazardous locations, as a hot work permit is not required.

Sensor / Transmitters

• Longer remote range, up to 1 mile
• Calibrate at the sensor/transmitter location
• Compatible with 3rd party controllers, PLC/DCS systems

Detector/transmitters send a feedback signal, usually an industry standard 4-20 mA, with 4 mA being the output at zero (gas free atmosphere) and 20 mA, the output at full scale gas concentration. A mA is a unit of measure for electrical current, one thousandth of an amp. Other digital outputs are also available in various formats, for example ModBus, DeviceNet, Lonworks, etc. There are three primary advantages for choosing a detector/transmitter as supplied by RKI. They are:

1. Allows for long distance between the detector assembly and controller (up to a mile with the appropriate wiring size).
2. Allows for the calibration to take place at the detector assembly without adjustments having to be made at the controller.
3. Allows use with generic PLC/DCS systems, when output parameters and voltage requirements are compatible with one another for proper operation to be achieved.

It is important to remember that RKI’s Direct Connect detectors are designed and intended to work only with RKI controllers. Also, most of RKI’s controllers can accept either direct connect style or detector transmitter styles. These controllers include the models Beacon 110, Beacon 200 and Beacon 410. The Beacon 800 is designed to operate only with detector/transmitter styles. Also, not every gas detector offering from RKI is available in a direct connect configuration. Please consult with RKI regarding your specific applications.

We hope that the above summary of information helps to clarify the major distinctions between Direct Connect Sensors and Sensor/Transmitters.

*Capable of accepting Direct Connect Sensors

Electrical equipment sometimes must be installed in areas where combustible vapors and gases are used or may be present. These are commonly referred to as “hazardous locations”, and are defined by the National Electrical Code (NEC) in the US, or the Canadian Electrical Code (CEC) in Canada. When equipment must be installed in hazardous locations, there are strict requirements for the construction of the installation, including materials and design requirements. To prevent inadvertent ignition of flammable gases and vapors by electrical equipment, the two most common methods of protection are “Explosion Proof” and “Intrinsically Safe”. We will discuss these methods as they relate to gas detection equipment.

Explosion Proof
Generally speaking, “explosion proof” is the more commonly used method for detector/sensor assemblies for fixed gas detection systems, where higher voltages and power requirements may be encountered, and the installation is permanent. Intrinsically safe method can also be used for permanent installations where the detector/sensors are relatively low power devices. Almost all portable instruments use the “intrinsically safe” method.

An “explosion proof “ classification for a sensor/transmitter means that the housing has been engineered and constructed to contain a flash or explosion. Such housings are usually made of cast aluminum or stainless steel and are of sufficient mass and strength to safely contain an explosion should flammable gases or vapors penetrate the housing and the internal electronics or wiring cause an ignition. The design must prevent any surface temperatures that could exceed the ignition temperature of the gases or vapors covered by its Group rating (see below). If the sensing element is a high-temperature device (e.g. Catalytic bead or “pellistor”), it may be protected by a flame arrestor to prevent the propagation of high temperature gases to the ambient atmosphere.

Intrinsically Safe
An “intrinsically safe” classification and design means that an electronic circuit and it’s wiring will not cause any sparking or arcing and cannot store sufficient energy to ignite a flammable gas or vapor, and cannot produce a surface temperature high enough to cause ignition. Such a design is not explosion proof, nor does it need to be. For permanent installations, such an installation may include “intrinsically safe barriers” that are located outside the hazardous location, and limit the amount of energy available to the device located in the hazardous area.

The North American classifications for hazardous locations as related to flammable gases and vapors:

Class I: Gases and vapors

Division 1: Gases or vapors are usually present and/or may be present at any time in sufficient concentrations for an explosion hazard.

Division 2: Gases or vapors are not normally present and are present only in the event of a leak in some kind of containment vessel or piping, again in potentially hazardous concentrations.

Groups A, B, C, D: Groups of atmospheres categorized by the volatility and/or ignition temperatures. “A” is the most hazardous and “D” is the least hazardous group for gases and vapors.

• Group A: Atmospheres containing acetylene.
• Group B: Atmospheres containing hydrogen or gases or vapors of equivalent hazard.
• Group C: Atmospheres containing ethyl-ether vapors, ethylene, or cyclo-propane.
• Group D: Atmospheres containing gasoline, hexane, naptha, benzene, butane, propane, alcohol, acetone, benzol, lacquer solvent vapors, or natural gas (methane).

A typical dilution fitting is a plumbing device that is attached to a gas detection instrument sample inlet port, and then the sample hose is attached to the dilution fitting. When used, the sample flow going into the instrument passes through the dilution fitting. The dilution fitting has 2 small holes; one is in the sample gas stream path, and the second is through the side of the fitting and causes the instrument to take in ambient air. Essentially, the dilution fitting creates a calibrated “leak” into the incoming sample, and dilutes the sample with fresh air. If the dilution fitting is calibrated to be 1 to 1, then when used it will dilute the sample gas stream with an equal amount of ambient air.When is a dilution fitting needed?
There are at least two situations where a dilution fitting is needed. The first common usage is when a catalytic LEL sensor is used to test a space that is inerted (contains no oxygen). Since a catalytic sensor requires oxygen in order to operate, a 1 to 1 dilution fitting blends enough fresh air with the sample to provide enough oxygen for the sensor to properly detect flammable gases if they are present. The second common reason for using a dilution fitting is to extend the range of the gas monitor.

Interpretation
When a dilution fitting is used, it reduces the reading of the gas monitor. If the gas monitor is calibrated to read correctly without the dilution fitting used, then when the fitting is used the gas monitor will read lower than what is actually in the gas sample. For example, if a 1 to 1 dilution fitting is used, since it dilutes the sample by 50%, this means that the reading will be half of what is actually present in the test space. In order to understand what is the correct reading, it is necessary for the operator to multiply the meter reading by 2. If a dilution fitting is 2 parts dilution to 1 part sample, then it knocks the reading down to 1/3 of the actual value, and in this case it is necessary to multiply the meter reading by 3 to get the actual concentration. So, a reading of 50% LEL is actually 150% LEL.

Cautions
A dilution fitting ratio can be affected by changes in pressure of the incoming gas sample. The fitting is calibrated to provide the correct dilution if the sample is drawn from atmospheric pressure. If the pressure is different, it can change the ratio. For example, if the sample is drawn from a strong vacuum, the fitting may have a difficult time pumping enough gas through the sample hole, and therefore it would draw a larger proportion of the sample through the dilution hole. In this case, you would be getting more dilution of the sample, and so the readings would be lower than expected. If the sample is drawn from a pressurized vessel, it may force too much gas through the sample hole and the pump will not be able to draw the correct amount from the dilution hole. In this case the reading may be higher than expected. In the case where it is testing an inerted space with a catalytic sensor, if insufficient dilution occurs then the LEL reading may be low or near zero because the catalytic sensor is not responding properly due to a lack of oxygen.

Dilution Fittings Increase H2S Range
This special instrument has an internal dilution fitting. It is built inside the instrument to extend the range of the H2S sensor, and the user cannot remove or adjust it. The dilution ratio used is about 7 to 1, but it is not necessary for the operator to do any multiplying because the instrument is designed and calibrated to read correctly with the internal dilution present.

The range of this H2S EAGLE is 0 to 1,000 ppm. It is very important when calibrating this instrument to use a sample bag. Fill the sample bag, and then draw from the bag with the instrument. If a demand flow regulator is used, or if a fixed flow regulator is connected directly to the EAGLE, then the internal dilution will not work properly and the H2S calibration will not be correct.

Calibration frequency is one of the most commonly asked questions regarding the use of gas detection instruments. Regulatory agencies typically refer users to follow manufacturers recommended protocols for calibration.The calibration frequency for gas detection instruments really depends on the type of use a customer will give the instrument. For example, some users who require the readings to hold up in court as data for certain legal applications must calibrate both before and after each test or each series of tests, in order to remove all doubt of the proper functioning of the instrument. The other extreme is someone who only uses the instrument a couple times a year for non-critical applications. This type of user should calibrate their instrument before each use.

What we generally recommend is that users develop a frequency of calibration that is tailored to their application and usage. Initially, the user may begin by calibrating once per week, and note any changes or adjustments needed to the calibration. If, week after week, there is very little or no adjustment needed, then the calibration frequency can decrease to the point that there will be only a small adjustment needed when calibrating.

In general, for most users, this frequency ends up being somewhere between one and three months. For users who do not wish to develop their own frequency, we recommend that they calibrate once a month.
For users who “bump test” their instrument prior to each use, the calibration cycle can be extended to 3 to 6 months for instruments that successfully pass the bump gas test.

There is no universal standard for pass/fail tolerance on a bump test. The tolerance must be determined by the user based on frequency and usage. A typical tolerance could be +/- 20% or +/- 30%, or a simple triggering of the instrument’s alarm.
All of our newer instruments have auto-calibration. This feature makes calibration quick and painless. Using the 4 gas cylinder, a 4 gas portable monitor can calibrate all 4 channels together in just a minute or two. With this simplification of the calibration task, we encourage users to calibrate their instruments more frequently than they may have done in the past.

Calibration frequency of fixed systems depends upon the type of use you have and the sensor types. Typical calibration frequencies for most applications are between 3 and 6 months, but can be required more often or less often based on your usage.

A precaution to note.
It is generally recommended that a bump test or calibration be performed if it is suspected that the instrument has been subjected to any condition that could have an adverse effect on the unit (sensor poisons, high gas concentrations, extreme temperature, mechanical shock or stress, etc).

In detecting combustible gases in oil and gas, petrochemical and other applications, choosing between the two most common gas sensing technologies used for this purpose will be critical in ensuring a safe, reliable and cost effective solution. These technologies are catalytic combustion and infrared. Both have advantages and disadvantages depending on an application specific needs.

RKI Instruments, a world leader in gas detection equipment, offers both technologies, providing the user with flexibility in selecting the best sensing technology for their situation. Of the many hydrocarbons that are found in industry today, most are detectable with a catalytic combustion sensor and many are detectable with an infrared sensor. It is important to consider the specific compounds to be monitored as there are some that do not readily lend themselves to detection with a general purpose infrared (IR) detector, such as hydrogen, acetylene, and aromatic compounds, like benzene and toluene, for example. We will look at some common compounds and discuss the basic principles of operation for the two technologies as well as their advantages and disadvantages.

Typical alkane gases monitored

• Methane
• Ethane
• Propane
• Butane
• Pentane
• Hexane

Other alkenes, alcohols, and amines monitored

• Butadiene
• Isopropylamine
• Propylene
• Ethylene Oxide
• Propylene Oxide
• Ethanol
• Methanol

Catalytic Detectors

Catalytic detectors are based upon the principle that when gas oxidizes it produces heat, and the sensor converts the temperature change via a standard Wheatstone Bridge-type circuit to a sensor signal that is proportional to the gas concentration. The sensor components consist of a pair of heating coils (reference and active). The active element is embedded in a catalyst. The reaction takes place on the surface of the catalyst, with combustible gases reacting exothermically with oxygen in the air to raise its temperature. This results in a change of resistance.

There is also a reference element providing an inert reference  signal by remaining non-responsive to gas, thereby acting as a stable baseline  signal to compensate for environmental changes which would otherwise affect the sensor s temperature.
Advantages
The major advantages of catalytic detectors:

• Robust.
• Simple to operate.
• Easy to install, calibrate and use.
• Long life with a low replacement cost.
• Proven technology with exceptional reliability and predictability.
• Easily calibrated individually to gases such as hydrogen which cannot be detected using infrared absorption.
• Can perform more reliably in dusty & dirty atmospheres as they are not as sensitive as optics to the build up of industrial contaminants.
• Can perform more reliably in high temperature applications.
• Are less sensitive to humidity and condensation.
• Not as significantly affected by changes in pressure.
• Can detect most combustible hydrocarbons.

Disadvantages
The limiting factors in catalytic detector technology:

• Catalysts can become poisoned or inactive due to contamination (chlorinated & silicone compounds, prolonged exposure to H2S and other sulfur &/or corrosive compounds).
• The only means of identifying detector sensitivity loss is by checking with the appropriate gas on a routine basis and recalibrating as required.
• Requires oxygen for detection.
• Prolonged exposure to high concentrations of combustible gas may degrade sensor performance.
• If flooded with a very high gas concentration, may show erroneously low or no response, and sensor may be damaged or rendered inoperable.

Infrared Detectors

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.

Conclusion

There is clear need for both IR and catalytic detectors in industry. When making a choice, be sure to consider the field environment and the variables in detector design. Life-cycle cost assumptions will not hold true in all environments. The same can be said for detector mean-time-to-repair or failure. Careful analysis of detectors, suppliers and field experience will help you to select the best catalytic or IR detectors for your application.

Calibration is a vital and necessary step to ensuring the proper performance of any gas detector. The calibration process requires use of a known concentration of test gas, also known as span gas or calibration gas. Use of incorrect or expired calibration gas can result in improper calibration. This can result in unsafe operation, as well as improper diagnosis of instrument malfunction. This article will focus on disposable (non-refillable) calibration gas cylinders for both reactive and non-reactive gases.

Reactive Gas Mixtures
Reactive gas mixtures are calibration gas mixtures that include at least one component gas which is classified as reactive. This is a broadly used term for chemicals that have some instability under certain conditions, and may react with certain materials, moisture, oxygen, or other chemicals. Reactive gas mixtures include mixtures containing hydrogen sulfide, chlorine, sulfur dioxide, ammonia, hydrogen chloride, among others. Reactive gas mixtures are generally packaged in a special cylinder made of aluminum and treated (passivated) by a special process to minimize reactivity with the reactive gas. Reactive gas mixtures typically have a shelf life of one year or less. After shelf life has expired, it is likely that the concentration of the reactive gas will either decrease or eventually disappear all together.

Non-reactive Gas Mixtures
Non-reactive gas mixtures are calibration gas mixtures that do not include any reactive gases. This is a broadly used term for chemicals that are stable under most conditions, and are not affected by moisture, oxygen, or other chemicals. Non-reactive gas mixtures include mixtures containing alkane or alkene hydrocarbons (methane, ethane, propane, hexane, isobutylene, etc.), nitrogen, hydrogen, carbon monoxide, carbon dioxide, among others. Non-reactive gas mixtures are generally packaged in a cylinder made of steel. Non-reactive gas mixtures have a shelf life of three years.

Shelf Life For All Cylinders
The shelf life for a cylinder is RKI’s warranty. Below is a guide to the shelf life for RKI gas mixtures. As a general rule, all steel cylinders have a 3 year shelf life while aluminum cylinders range from 6-24 months.

How Do I Know When My Calibration Gas Cylinder Has Expired?
All RKI Instruments calibration gas cylinders include the statement “Best when used by” followed by month and year. Cylinders should not be used beyond this date.

The primary risk associated with combustible gases and vapors is the possibility of explosions. Explosion, like fire, requires three elements: fuel, Oxygen, and an ignition source. Each combustible gas or vapor will ignite only within a specific range of fuel/Oxygen mixtures. Too little or too much gas will not ignite. These conditions are defined as the Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL). Any amount of gas between the two limits is explosive. It is important to note that each gas has its own LEL and UEL, as shown in the chart below. The gas concentrations are shown by percent of total volume, with the balance as normal air.

Between these two limits explosions can occur under some conditions, with the maximum explosive energy available at approximately the midpoint. Note that these limits are sometimes referred to as LFL (Lower Flammable Limit) and UFL (Upper Flammable Limit). These limits are empirically determined, and various authorities sometimes quote slightly different figures, based on slightly different experimental procedures.

as Tracer: Has a high sensitivity ppm sensor for detection of natural gas leaks down to 10 ppm in leak check mode. When equipped with the proper sensors, the GasTracer has both barhole and leak check modes available. The Gas Tracer has a version where the charcoal filter is removed from the CO sensor to allow for detection of H2S. The readout is displayed as ppm CO (not H2S), and there is no way to differentiate CO from H2S readings.

GX-2012: Has the capability of H2S detection with a dedicated H2S sensor. With the Gas Tracer, H2S sensor is not available.

There are no standards for placement of O2 sensors, or for gas sensors in general for that matter. Think of the sensors like a smoke detector. The sensor responds to what is immediately around the sensor. In the case of using an oxygen sensor to detect the level of oxygen as it is displaced by another gas, the inert gas has to migrate to the sensor from the source. So, when placing sensors, the following variables must be taken into account.

• Source location of gas
• Ventilation pattern
• Size of area
• Layout of area
• Desired speed of response
• Density of the gas. Some inert gases (argon or helium) are lighter than air, nitrogen is virtually the same density as air, so for nitrogen you would want to locate the sensors in the breathing zone (4 – 6 ft. above the floor), but again ventilation must also be taken into consideration