Product Description; Product Function; Description Of Operation - GasTech D-Guard2 Operator's Manual

8. PRODUCT DESCRIPTION

8.1.

Product Function

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The D-Guard smart Gas detector's function is to detect the presence of gas (or reduction in oxygen
concentration) in the environment immediately surrounding it. Such gases may be toxic, flammable or
asphyxiant, where concentrations may exceed safe exposure limits for occupants or combust in the presence
of ignition sources. The D-Guard
responsible for activating ventilation and/or sounding alarms to mitigate the risks associated with the presence
of increased concentrations of gases or low levels of breathable oxygen.
8.2.

Description Of Operation

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The D-Guard smart gas detectors are designed to sense gases that enter them via the process of natural
diffusion through the gas entry port.
The sensor type used within the D-Guard
gas needing to be monitored. If the gases to be monitored pose a risk of being toxic, then a D-Guard
electro-chemical type cells must be used. With the exception of hydrogen, if the reason for monitoring gas is
due to the risk of ignition, then a D-Guard
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hydrogen, a D-Guard with an electro-chemical cell should be used. For monitoring oxygen levels for depletion,
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a D-Guard with an oxygen sensor must be used.
In any environment, gases released into an open space will spread-out (diffuse) to equally occupy the volume
of space available. The diffusion of gases occurs due to mechanisms such as natural or forced air currents as
well as molecular collisions.
Whether the space into which gas is released is totally empty of any gas molecules, or already contains gas
molecules (e.g. air), the molar concentration (a measure of the actual number of gas molecules) or the
volumetric concentration (a measure of relative amounts of each gas component in a mixture) of a gas at any
given location and time will relate to its concentration at the point of release, the volume of space into which it
expands and influences such as ventilation that affect the rate at which diffusion occurs. In most cases, over a
long enough time, the concentration of gas anywhere within the space will have equal concentration. However,
in areas with enclosed spaces with poor ventilation, gas may 'pool' in higher concentrations if the rate of gas
leakage into the space is greater than the rate of diffusion out of it. Pooling can occur in roof spaces for gases
that are less dense than air or in pits and tunnels for gases that are more dense than air.
The various diffusion processes are the mechanism by which gas enters the D-Guard
Gas molecules will diffuse through the gas-permeable waterproof membrane and into the sensor body, where
they react with specific sensor mechanisms to produce signals proportional to the gas concentration.
For toxic gases entering electro-chemical sensors, an electrical current proportional to concentration will be
produced. For flammable gas molecules entering an infra-red sensor, infra-red energy will be absorbed
proportionate to gas concentration. This absorption is processed to produce a concentration related signal.
Signals from the various sensor types feed into the detectors electronics and signal processing software where
they are converted to scaled values and used for display indication, concentration and alarm signalling.
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D-Guard signals gas concentration to remotely connected equipment via its two power supply connections. It
does this by varying the amount of current it draws from the power supply in direct proportion to the gas
concentration measured. This is generally referred to as 'current loop signalling' and commonly uses 4mA to
represent zero gas conditions and 20mA to indicate full scale. D-guard
with other data using a digital communications system called 'HART' (Highway Addressable Remote
Transducer/Transmitter). This digital signal is super-imposed onto the current loop 4-20mA signal. Additionally,
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D-Guard can (optionally) provide volt-free relay contacts that can be programmed to respond at user-set gas
concentrations. Gas concentration values and other information is also presented on the display for localised
reading. Sensor operating errors are also signalled via a current loop level above or below the normal 4-20mA
range.
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detector range is designed to connect to other equipment or systems that are
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to detect the gas is dependent upon the specific characteristics of the
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fitted with Infra-Red sensors should be used. For detection of
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D-Guard
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in order to be detected.
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can also signal gas concentrations, along
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with