VQ41TSB
Combustible Gas Detector Elements
To be read in conjunction with "Introduction to Pellistor Gas
Sensors" and Pellistor Application Notes 1, 2, 3, 4, 6 and 7.
7217
COMPENSATOR
(BLACK NUMBER)
INTRODUCTION
The VQ41TSB is a flammable gas sensor optimised for the
detection of ammonia in air. It operates by detecting the heat
produced by the catalytic oxidation of ammonia on a pair of
matched elements. It may be used for the detection of
ammonia in air mixtures with concentrations of 600 ppm
upwards and is particularly suitable for monitoring over the
range zero to 1.5% by volume (zero to 10% LEL). This makes
50 k
O
ZERO SET
1
k
O
1 k
O
DETECTOR
R
(SEE
NOTE 4)
the sensor ideal for use in ammonia refrigerant detection
systems intended to comply with BS4434:1989.
Interference from water vapour or carbon dioxide is eliminated
by using a matched, inert compensating element. The use of an
improved poison resistant construction provides protection
against the common substances that can progressively
degrade the performance of a sensor and specifically provides
a high degree of protection against silicone poisoning agents. In
this latter case the protection is very much higher than that
provided by standard poison resistant catalytic flammable gas
detectors. Tests show no effect on zero or sensitivity to
ammonia when exposed to 100 ppm HMDS for greater than
one hour.
The VQ41TSB employs new catalyst chemistry and device
structure to optimise the ammonia response. There is negligible
sensitivity to xylene and very little sensitivity to methane under
normal operating conditions, but the device can be used to
detect hydrogen and butane.
GENERAL DATA
Electrical
This information relates to the sensor operating in the
recommended bridge circuit shown below.
Operation . . . . . . . . . . . . . . continuous
Bridge supply . . . . . . . . . . . 2.0
+
0.1 V
Sensor power consumption . . . . . . 250 mW max
Typical sensor current . . . . . . . . 100 mA
Sensitivity in SGX Sensortech test block (see note 1):
minimum . . . . . . . . . . . 15 mV/% NH
3
typical . . . . . . . . . . . . 20 mV/% NH
3
Typical sensitivity to hydrogen . . . . . 45 mV/% H
2
Mechanical
Outline . . . . . . . . . . . . . . . see page 2
Shock (see note 2) . . . . . 250 g, 5 blows in each plane
Vibration (see note 2) . . . . . . . . 20 g, 24 cycles
from 100 to 3200 Hz
MARKING
Each element carries a unique serial number written on the side
of the can. On the detector the number is red and on the
compensator it is black.
(RED NUMBER)
OUTPUT
Recommended Bridge Circuit (See note 3)
OPERATION
The output of the 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 devices must therefore be mounted in
a draught free environment. Ideally, the elements should be
mounted symmetrically side by side with the open ends
exposed to the gas being monitored. The elements are
supplied with ‘O’ rings so that gas tight seals may be made
between the elements and the mounting. As for all sensors
operating on the catalytic oxidation principle, the sensor should
be separated from the gas being monitored by a suitable flame
arrestor.
In use, the operating voltage should not be allowed to vary by
more than the specified
+
0.1 V, or the output in clean air may
change in sympathy. A typical plot of sensitivity against supply
voltage is shown in Figure 1.
NOTES
1. The sensitivity is partially influenced by the flow of gas to
the
elements. Consequently, it is affected by the geometry
of the
mounting arrangements. The quoted figures
were obtained
in SGX Sensortech’
s
standard test block,
details of which are
available on request.
2. Shock and vibration measurements are strongly dependent
on
the mounting arrangements of the sensor.
3 The values of the resistors in the bridge circuit are suitable
for circuits where negligible current is drawn from the
output terminals, such as when a high input impedance
output amplifier is used.
4. The elements are provided as a matched pair with a
trimming resistor R, which must be connected across the
compensator as shown. Occasionally a trimming resistor is
not
required and in such cases the slip packed with the
sensor
will be marked ‘N/R’ instead of quoting the value of
the
resistor supplied.
SGX Sensortech(IS) Ltd Registered in England No.08067077
A1A-VQ41TSB Issue 4, March 2007
SENSITIVITY
(mV)
4
Figure 1. Typical Response to 20% LEL
7216
NH
3
60
40
H
2
C
4
H
10
CH
20
AROMATIC C
8
H
10
0
0 1 2 3 4
BRIDGE VOLTAGE (V)
OUTLINE OF DETECTOR ELEMENT
7218
1
B
D
F
Ref Millimetres Inches
E
11.3 max 0.445 max
A
C
1
A
B
1
G
C
10.8 min 0.425 min
6.35 max 0.250 max
5.85 min 0.230 min
3.69 max 0.145 max
3.43 min 0.135 min
D
40.0 min 1.575 min
E
1.0 nom 0.039 nom
6.35 max 0.250 max
INSULATOR
METAL
H O-RING
F*
5.33 min 0.210 min
FLANGE
G
8.2 max 0.323 max
H
1.5 nom 0.059 nom
Inch dimensions have been derived from millimetres.
* For the compensator element, dimension F is 7.88 mm (0.310 inches) max, 6.86 mm (0.270 inches) min.
WARNING
To satisfy the requirements of the recognised approval authorities,
SGX Sensortech
recommends
that a suitable sintered metal flame arrestor be used with the device to ensure safe operation.
SGX Sensortech(IS) Ltd Registered in England No.08067077
A1A-VQ41TSB Issue 4, March 2007
