Det-Tronics 505 Combustible Gas Detector Transmitter with Combustible Gas Sensor CGS - Manual (Page 4)

Section I —
Description and
Operating Characteristics
DESCRIPTION
The Model 505 Transmitter is used with one constant
voltage catalytic gas sensor to provide a linear 4 to 20
mA output signal corresponding to a 0 to 100% LFL
gas concentration (see Figure 1). Intrusive calibration
and sensor sensitivity checks are performed using a
standard digital voltmeter (not provided). An explosion-
proof junction box with removable cover is included with
all Model 505 Transmitters. Options include junction box
material (aluminum or stainless steel), number of conduit
entry ports (up to five), conduit entry threads (NPT or
Metric), orientation of entries (180 degree straight-through
or 90 degree “L”), and signal output load impedance
(125 or 500 ohms). Detector Electronics Corporation’s
(Det-tronics ® ) Model 505 with signal loop impedance
option “A” directly replaces the K-Series Transmitter (125
ohms), and the Model 505 with signal loop impedance
option “B” replaces the Model 500 Transmitter (500
ohms).
SENSOR
Det-Tronics constant voltage catalytic bead type
combustible gas sensors are used with the Model 505
transmitter family.
GENERAL APPLICATION
INFORMATION
A combustible vapor or gas is one that will burn when
mixed with air (or oxygen) and ignited. Every vapor has
a minimum and maximum concentration in air, which
together form the “flammable” or “explosive” range. The
lower flammable limit (LFL) is defined as the smallest
amount of the gas that will support a self-propagating
flame when mixed with air (or oxygen) and ignited.
The range of gas concentration measurement for most
catalytic sensor-based gas detection systems, including
the Model 505 transmitter, is 0 to 100% LFL, with 0% LFL
being a gas-free atmosphere and 100% LFL equaling
the gas concentration at its lower flammable limit. The
relationship between % LFL and % by volume differs from
gas to gas. ASTM E681 is the existing standard method
for determining flammable limits. Examples include:
Hydrogen (H2), 100% LFL = 4.0% by volume in air
Methane (CH4), 100% LFL = 5.0% by volume in air
Ethane (C2H6), 100% LFL = 3.0% by volume in air
Ethylene (C2H4), 100% LFL = 2.7% by volume in air
Pentane (C5H12), 100% LFL = 1.5% by volume in air
Propane (C3H8), 100% LFL = 2.2% by volume in air
For data on other gases, refer to NFPA Handbook 325M.
Typical alarm setpoints for combustible gas detection
systems are 20% LFL for the low alarm and 40% LFL for
the high alarm.
The LFL of a gas is affected by temperature and pressure.
As the temperature increases, the LFL concentration
decreases, and the explosion hazard increases. The
relationship between LFL and pressure is fairly complex,
however, a pressure increase usually lowers the LFL. The
LFL of a gas is not significantly affected by the humidity
fluctuations normally encountered in typical industrial
applications.
INSTRUCTIONS
Combustible Gas Detector
Transmitter Model 505 with
Combustible Gas Sensor Model CGS
3.1
© Detector Electronics Corporation 2012 Rev: 4/12 95-8472