The VQ500 Series is a complete range of miniature, fully
certified, flameproof sensor heads containing a high
quality, low power pellistor pair optimised for the detection
of combustible gases or % volume gases in thermal
conductivity mode. They have an outline identical to
modern miniature electrochemical cells and are ideally
suited for use in portable instruments.
Within the VQ500 Series there are specific sensors that
are able to detect most combustible gases and vapours
and ammonia at LEL levels. The thermal conductivity
version will detect most gases at % volume concentrations
which have thermal conductivities different from that of air.
The VQ500ZD/W variants have had a temperature test
performed during its manufacture to confirm the
temperature performance of the sensor meets the
datasheet ensuring compliance to the various mining
performance standards (Group 1) with regard to the
temperature performance.
The VQ548ZD-S variant has extra burn-in for enhanced
stability.
If a flammable gas is present when the bead is heated to
about 400 to 500 °C, the gas will oxidise and the resultant
release of energy will heat the bead still further. This increase
in temperature is detected as an increase in resistance of the
coil.
The temperature of the coil is also affected by ambient
temperature and by variations in thermal conductivity of the
air caused by the possible presence of inert gases such as
carbon dioxide. To compensate for temperature changes not
caused by the oxidation of the flammable gas a second, inert
bead is used. This compensator is made in the same way as
a detector bead except that instead of incorporating a catalyst
in the ceramic bead, the bead is treated so that oxidation
cannot take place. The two beads are then used in a circuit
that detects the difference in their resistances. Since the two
beads are generally of a different colour, they have different
emissivity and hence different slope resistances. Therefore, to
obtain the best temperature performance, it is necessary to
connect a fixed resistor in parallel with the compensator to
correct for its higher slope resistance.
FEATURES
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Optimised for combustible gases and vapours
Thermal conductivity versions available
Certified explosion proof component
Integral pellistors
Low power
Standard miniature body size
Poison resistant – silicones and hydrogen sulfide
Shock resistant
ATEX certified
IECEx certified
CSA certified
UL recognised
Temperature Rated between -40°C and +55°C
TECHNICAL SPECIFICATION
Mechanical
Dimensions
Body material
Weight
see outline, page 3
Stainless Steel
22g
Environmental
OPERATING PRINCIPLES
The detector consists of a fine coil of platinum wire
embedded in a ceramic bead incorporating a noble metal
catalyst. The coil acts both as an electrical heater and as a
resistance thermometer. The bead is mounted on a
header with connecting leads and is surrounded by a
metal can with the end open to the atmosphere.
Temperature range for
operation and storage
Humidity range for operation
and storage
−40 to +55 °C
0 to 80%
(non-condensing)
Electrical
Supply Voltage
Supply Power
3.00 V to 4.25 V;
(Depending upon type)
135 mW to 230 mW
(Depending upon type)
Whilst SGX has taken care to ensure the accuracy of the information contained herein it accepts no responsibility for the consequences of any use thereof and also
reserves the right to change the specification of goods without notice. SGX accepts no liability beyond the set out in its standard conditions of sale in respect of
infringement of third party patents arising from the use of SGX products in accordance with information contained herein.
In case of modification of the product, SGX disclaims all liability.
SGX Europe sp .z o.o. Poland REGON: 362332227
DS-0223, Issue 2, 10-May-2016, Page 1
SGX Europe Sp. z o.o.
Building 11
Ligocka St. 103,
40-568 Katowice,
Poland
T: +48 (0) 32 438 4778
E: sales.is@sgxsensortech.com
www.sgxsensortech.com
PERFORMANCE
Linearity:
Catalytic
Thermal Conductivity
Response Time (T
90
)
Long Term Zero Drift
-
ZD-S Variants
-
All other Types
Linear up 60%LEL
Refer to Pellistor Application
Note 5
T
90
< 20 seconds
<5%LEL / month
(based upon a 1 month period)
OPERATION
The output of a sensor is dependent on a complex
combustion process established around the sensing
beads. It is imperative that the layers of gas around the
beads are not disturbed, otherwise the bead temperature
and hence device output will change. The design of the
VQ500 housing achieves this requirement, provided the
user takes care not to allow gas flows directly onto the
sinter.
It is recommended that the detector and compensator be
run in a Wheatstone Bridge circuit so that a difference
signal may be obtained. A suitable circuit is shown in this
document. In use, the bridge supply voltage should be
stable to within + 0.1 V, or the output in clean air may
change in sympathy. Although it is generally
recommended that pellistors should be run with a constant
voltage supply to the bridge, it is possible to use a
constant current supply provided that the voltage across
the bridge in clean air remains within the recommended
limits.
<5%LEL / month
(based upon a 12 month period)
Long Term Sensitivity Drift
-
ZD-S Variants
-
All other Types
<5%Full-Scale / month
(based upon a 1 month period)
<5% Full-Scale / month
(based upon a 12 month period)
Zero Offset with Temperature
(ZD/W Variant Only)
Warranty Period
≤ 0.2% methane variation
between -10°C to +40°C
when compare to the 20°C
reading
12 months
MAXIMUM GAS CONCENTRATIONS
(See Note d)
VQ546M, VQ546MR
VQ547TS, VQ548, VQ549
100%v/v
100%LEL
NOTES
a)
b)
c)
d)
The sensitivity is measured in the standard
manufacturer’s test jig with no filter or other material
between the gas and the sinter.
A temperature test is performed on all ZD/W sensors
to confirm the temperature performance of the
sensor is within specification.
A burn-in for several weeks is performed followed by
a week drift test.
The calibration of the sensor should be checked if it
has been exposed, whilst operating, to gas
concentrations greater than the Lower Explosive
Limit. Exceeding this limit may permanently damage
the sensor.
It is not recommended to direct the gas flow onto the
sinter.
The response to flammable vapours, especially
heavy aromatic compounds, may be different to the
theoretical figures (k factors) listed in Pellistor
Application Note 3. These k factors are for guidance
only and ideally the sensors should be calibrated to
the actual vapour concentration required.
Under no circumstances should the sensor pins be
soldered directly to a PCB or wires. Excessive heat
could cause irreparable damage to sensor elements.
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