Edinburgh Sensors GasCheck User manual

GasCheck
Operating Manual V2.4
GASCHECK CARBON
DIOXIDE SENSOR
Operating Manual
Edinburgh Instruments Limited
2 Bain Square, Kirkton Campus
Livingston EH54 7DQ
UK
44 (0)1506-425300
Fax [phone]

Edinburgh Instruments Limited, 2006
All rights reserved. No part of this document may be reproduced or
transmitted in any form or by any means without prior permission of the
copyright holder.
The product described in this manual is subject to continuous
development and, while every effort has been taken to ensure that the
information given is correct, Edinburgh Instruments Limited cannot
accept any liabilities for errors and omissions or their consequences.

GasCheck
Operating Manual V2.4
Contents
1. INTRODUCTION.........................................................................................................2
2. PRINCIPLE OF OPERATION......................................................................................2
3. INSTALLATION ..........................................................................................................3
3.1 A Mechanical installation - Basic GasCheck....................................................3
3.2 Aspiration .......................................................................................................4
3.2.1 Pumped aspiration ............................................................................4
3.3 Electrical connection for GasCheck..............................................................5
3.5 Power supply requirements for GasCheck ...................................................7
4. USING THE OUTPUT..................................................................................................9
4.1 A Linearisation of the GasCheck ......................................................................9
4.2 Temperature and Pressure correction ........................................................11
5. SERVICE AND MAINTENANCE................................................................................12
5.1 Calibration ....................................................................................................12
5.2 Trouble shooting..........................................................................................15
5.3 Source replacement .....................................................................................15
6. TECHNICAL DATA ...................................................................................................16
7. WARRANTY..............................................................................................................18
8. DOCUMENT HISTORY CHANGE ............................................................................19

GasCheck
Operating Manual V2.4
1.
INTRODUCTION
The GasCheck range of sensors employs high reliability infrared absorption
technology to detect carbon dioxide. By using a physical measurement
technique, the sensors offer high selectivity, fast response and are virtually
immune to poisoning. By careful design of the electronics and optics, the
GasCheck technology provides a stable sensor capable of accurate and reliable
measurement without the need for frequent recalibration. The result is a device
with both low initial cost and low cost of ownership.
The GasCheck range is designed to meet the needs of OEMs who require to
monitor CO2, but do not themselves have expertise in the specialist field of
infrared technology. The sensors comprise the heart of an infrared detection
system, offering the user a conditioned 0 - 1 V output for ease of installation.
Surface mount electronics technology enables the GasCheck range to provide
a complete infrared sensor sub-system in a compact format.
The main features of the GasCheck sensor are:
high accuracy
good long term stability
low temperature coefficient
conditioned voltage output
negligible cross sensitivity
low power
fast response
small size
low mass
The GasCheck range comprises of three models.
1. Basic GasCheck:- ranges 3000ppm, 3% or 10% with a 0-1V non-
linear output.
2. GasCheck with linearisation:- ranges 3000ppm, 3% or 10% with a
4 to 20 mA linear output.
3. GasCheck with self referencing:- ranges 3% or 10% with a 0 to 1V
non linear output. This model is available with either the standard
aspirated style or diffusion style sensor.

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Operating Manual V2.4
2
2.
PRINCIPLE OF OPERATION
Many gases, including CO2, absorb energy in the infrared band. This
absorption is selective and occurs at specific frequencies corresponding to the
resonant frequencies of bond vibrations within the molecule. Measuring a
characteristic absorption enables the gas to be detected and the strength of the
absorption gives a measure of the gas concentration.
The GasCheck sensor optics consists of an infrared source, a sampling cell, an
infrared filter and a detector. In addition, the sensor includes electronics to
drive the source and to process the signals from the detector.
source drive
electronics
sample cell
IR source
detector signal processing
electronics
Figure 2.1 Schematic of the GasCheck sensor
The infrared source is the hot filament of a tungsten lamp emitting a broad
band of radiation, which is then guided through the gas in the sample cell. The
infrared then passes through a filter before reaching the detector. The filter
selects a very narrow band of frequencies corresponding to a characteristic
absorption band for CO2. The amount of energy reaching the detector falls as
the CO2concentration increases. Because the absorption of infrared by the gas
is non-linear, the output signal is also non-linear.

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3
3. INSTALLATION
3.1 A Mechanical installation - Basic GasCheck
The sensor is designed to be mounted directly onto a printed circuit board
(PCB). The sensor should be secured with two M4 screws (M4 x 8 mm pan
head screws with a plain and spring coil washer recommended). Pads should
be provided on the PCB for electrical connection (1.9 mm pads with 1.0
mm holes recommended) and the pins on the sensor should be soldered
directly to these pads.
In designing the PCB to accept the sensor, consideration should be given to
the inlet and outlet pipes to ensure that these do not collide with tall
components.
140
4
113
7.5 7.5
130
31.5
38.0
8
2.54 between pins
51
15.7
Figure 3.1 Dimensions (mm) of the GasCheck 3000ppm
4
7.5 7.5
31.5
38.0
8
2.54 between pins
51
15.7
30
41
48
Figure 3.2 Dimensions (mm) for the GasCheck 3% and 10%

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Operating Manual V2.4
4
3.1 B Mechanical Installation - GasCheck with linearisation Board
Four mounting holes suitable for M4 screws are provided on a 45 x 140mm
grid. If spacers are used, these should be a least 6mm high to provide
adequate clearance for the surface-mount components on the underside of the
PCB.
3.2 Aspiration
The sensors are available with two options for the aspiration of the cell:
pumped aspiration
diffusion aspiration through a permeable Vion strip (only available for
the GasCheck with self-referencing option)
3.2.1 Pumped aspiration
With pump-aspirated sensors, a separate pump is required to flow gas into the
sensor. The flow rate should be between 0.2 and 1.0 litres per minute. Lower
flow rate will lead to slow response times; higher flow rates will cause the
output to become unstable. In calculating the response time, consideration
should be given to the dead volume within the complete system comprising
both the sensor head and the pipework. The dead volumes for the various
sensor heads are:
Range
Dead volume
3000 ppm
9 cm3
3 %
4 cm3
10 %
4 cm3
Connection to the sensor is via the barbed gas ports on the side of the head.
These are suitable for N3 (3 mm bore) soft flexible tubing. Neoprene and PVC
tubing is suitable, but other materials may be used provided that they are
compatible with the gas stream. Natural rubber is not recommended due to its
tendency to perish. Silicone rubber tubing is not suitable because it readily
absorbs and releases CO2.
Since the gas flows through the instrument, it is essential that particulate
contamination be removed from the gas stream. A filter which is capable of
removing particles above 1 m in size should be fitted upstream of the sensor.
It is good practice to use a hydrophobic filter (which prevents the passage of
liquid water) such as a porous PTFE membrane. Suitable filters are available
from Edinburgh Instruments Ltd.

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Operating Manual V2.4
5
NOTE
Never operate the sensor without a filter. If the sensor is
used without a filter, it will function initially but the
optics will gradually become contaminated leading to
drift in the readings and premature failure of the sensor.
The sensor is capable of operating with up to 100 %RH without ill effect
provided that the conditions are non-condensing. If there is any possibility that
condensing conditions might occur, the sensor should be protected by a water
trap.
If an outlet pipe is connected to the sensor, care should be taken to ensure this
does not create a back-pressure in the cell which would lead to erroneous
readings (see Section 4.2). The outlet should be free venting via tubing, which
is neither too narrow nor too long. It is not advisable to connect the outlet to
other sensors or components, which might restrict the flow.
3.3 Electrical connection for GasCheck
The sensor head requires only a +15 V supply (connected to both the source
board and the detector board) and provides a 0 to 1 V output, offset from the 0
V rail by 5 V (the +5 V rail is also provided as a reference output voltage)
PL1 on
the
detector
board
PL1 on
the
source
board
5
1
1
2
Figure 3.4 Location of the PL1 connectors
Source PCB, PL1
Detector PCB, PL1
pin 1: +15 V
pin 1: +15 V
pin 2: ground
pin 2: ground
pin 3: 0 to 1 V output
pin 4: +5 V offset

GasCheck
Operating Manual V2.4
6
+
1
5
V
s
o
u
r
c
e
d
r
i
v
e
s
i
g
n
a
l
p
r
o
c
e
s
s
o
r
P
3
P
4
o
u
t
p
u
t
P
2
P
1
+
1
5
V
7
8
L
0
5
1
0
0
n
F
(
+
5
V
)
0
-
1
V
1
k
R
Figure 3.5 Equivalent circuit for GasCheck sensor
There is a 1kresistor in series with the output. This is included to protect the
output from damage in the event of it being shorted by the supply rails. The
output may be connected to external high impedance circuits (such as non-
inverting, op-amp based amplifiers) or low impedance circuits (such as
inverting, op-amp based amplifiers). If connected to a low impedance circuit,
the output resistance should be included in any gain calculations.
Output resistance = 1 k+ 1% (50 ppm /C temperature coefficient)
3.4 Electrical Connection - for GasCheck with linearisation board
Electrical connection is normally made at terminal block TB1 on the sensor.
The nominal 24V DC supply should be connected between the terminals
marked 0V and +24V. The analogue output (current or voltage) is available at
the terminals marked +and -.
The - terminal is connected to the 0V terminal thereby referencing the
analogue output to the negative power rail.

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Operating Manual V2.4
7
Alternatively, connections may be made via the 9 way D-type connector PL1
that carries additional signals used during factory testing. The connections for
PL1 are:
Pin Number
Function
1
+24V Power Input
2
+Analogue Output
3
AC signal from sensor head
4
+5V rail
5
+15V rail
6
0V Power Input
7
-Analogue Output
8
DC signal from sensor head
9
0 to 2V linear output
3.5 Power supply requirements for GasCheck
The sensor should be connected to a regulated, clean 15 V power supply. The
sensor is insensitive to the actual power supply volts and will achieve
specification for steady supply volts in the range 14.2 V to 15.8 V.
The load current taken by the sensor is not constant and contains significant
components at 8 Hz, 1 kHz and their harmonics. The peak current required
during normal operation is 300 mA although the average current (to be used
for power dissipation calculations) is only 60 mA. The power supply should
be capable of supplying this load whilst maintaining a clean regulated output.
The inrush current at power-up is 1.2A and the power supply should be
capable of supplying this although the supply voltage need not be maintained
at 15 V during this time.
A standard 7815 series regulator (not 78L15 version) with a 100 nF output de-
coupling capacitor and a 470 nF input de-coupling capacitor is recommended
for use with the sensor.

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Operating Manual V2.4
8
3.6 Power Supply Requirements - for GasCheck with linearisation board
The sensor is designed to operate from a nominal 24V DC supply. The power
supply should maintain the voltage between 18V and 30V DC for all load
conditions (see below) and all mains voltages.
The power input contains a high frequency filter, but is recommended that a
regulated clean power supply be used.
The load current taken by the sensor is not constant and contains significant
components at 8Hz, 1kHz and their harmonics. The peak current required
during normal operation is 330mA although the average current (to be used for
power dissipation calculations is only 100mA). The inrush current at power-up
is 1.2A, the power supply should be capable of supplying this although the
supply voltage need not be maintained during this time.
3.7 Electrical Connections for the GasCheck with self-referencing
The GasCheck with self-referencing requires a regulated 15V DC supply
which must be connected to the terminals labeled 15V (+ve) and 0V (-ve) on
PL1.(PL1-1 and PL1-2)
Two outputs are provided:-
1. A 0 to 1V output between the terminals +sig and -sig on PL1. (PL1-3
and PL1-4) This output is non-linearly proportional to gas
concentration (refer to appendices at the back of this manual).
2. A calibration error warning output between the terminals marked K
(+ve) and A (-ve) on PL1, (PL1-5 and P1-6) suitable for driving a small
LED.

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Operating Manual V2.4
9
4. USING THE OUTPUT
4.1 A Linearisation of the GasCheck
The absorption of infrared energy by a gas is non-linear and therefore the
output from the sensor is also non-linear.
The output volts as a function of gas concentration for the various ranges are
given in Appendices 1- 3. This may be used to set a trigger or alarm level
corresponding to a particular gas concentration.
The output may be linearised using a segmented linearisation technique in
which the linearisation curve is approximated to a series of straight lines. This
may be achieved, with negligible errors, using the data given in Appendices 1
- 3. Note that the transmitter must always be separately calibrated at zero and
span despite this linearisation.
4.1 B Output options for the GasCheck with linearisation
The sensor is supplied configured for a 4-20mA current output with a fault
state being indicated by 0mA. Alternative current/voltage outputs are
available by changing links on the board and fitting a resistor (voltage outputs
only).
To change the output from 4-20mA to 0-20mA, the link on LK1 should be
removed and fitted to LK2.
To disable the function, which forces the output to 0mA in the event of the
failure of the sensor head, remove LK3.
To provide a voltage output, the current output should be configured as a 0-
20mA output (see above) and appropriate resistors should be fitted to
positions R36/R37 on the PCB. The output is generated by passing the output
current through the parallel combination of R36 and R37. Table 1 gives
typical resistance combinations for voltage outputs.
Required Voltage Output
Resistor Values (R36 and R37)
0 - 1V
fit 2 x 100
0 - 3V
fit 2 x 300
0 - 5V
fit 2 x 500
0 - 10V
fit 2 x 1k
TABLE 1

GasCheck
Operating Manual V2.4
10
If you require a voltage output that is different to the ones listed in TABLE 1, then
you can calculate the required resistor values using Ohm’s Law.
RV
I

i.e. for 0-10 V output
select 0-20 mA
RV
I
  

10
20 10 500
3

500
3736
3736 RR RR
 kRR 1
3736
500resistance is obtained by fitting 1K resistors to R36 and R37. The maximum
available output voltage is limited to 12 volts approximately.

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Operating Manual V2.4
11
4.2 Temperature and Pressure correction
Because gases are compressible, there are various ways of expressing the gas
concentration: volume percent, partial pressure, density, mole density. Infrared
instruments operate by measuring the amount of infrared energy absorbed by a
sample of the gas and therefore fundamentally measure mole density (the
number of molecules in the path of the beam). Most users calibrate their
equipment against gases supplied in cylinders and therefore consider the
output to be an indication of volume concentration.
The mole density of the gas released from the cylinder depends not only on the
gas mixture in the cylinder but also on the ambient pressure and temperature.
This means that the output volts from a sensor connected to the same cylinder
will vary from day to day depending on the ambient conditions.
The sensor has built in compensation for temperature variation. Pressure
correction can be calculated from the ideal gas law, but there is also an
increase in absorption per molecule with pressure due to line broadening
effects. The sum of theses two effects is that the True %V/V is related to the
displayed value from the GasCheck by the following relationship.
(mBar)25.1013P
(mBar)pressureambientPwhere
5.05.1
/%
0
0
0









 PP
P
ValueIndicatedVVTrue
For example: if a GasCheck 3000ppm sensor output is 0.978 V, then the
linearisation data (Appendix1) states that the reading is 2852ppm. If the
ambient pressure is 980 mbar then the true reading can be derived:
corrected %V/V = 2852ppm x
)25.10135.09805.1( 25.1013 
= 3000ppm
NOTE
All units are factory set to the corrected gas
concentration on the day of the manufacture. For
maximum accuracy all on-site calibrations should also
use pressure and temperature correction.

GasCheck
Operating Manual V2.4
12
5. SERVICE AND MAINTENANCE
A full after sales calibration and service facility is provided by Edinburgh
Instruments. Service parts and equipment are also available.
5.1 Calibration
Edinburgh Instruments products are inherently stable and will maintain their
calibration over extended periods with minimal maintenance. We recommend
that the calibration of the GasCheck is checked at least once a year. Any
adjustments should be small.
ZERO (nitrogen) and SPAN calibration gases must be applied to the sensor in
order to check its calibration. To supply gas from a pressurized bottle a
pressure regulator, a needle valve, a gas flow meter and an inlet pipe are
needed. When supplying gas the following conditions should be maintained:

the CO2concentration of the SPAN gas should be between 80 and
105 % of the full scale

the tolerance of the CO2volume concentration of the gas bottle
should be 2% or better

the output pressure from the regulator should only slightly exceed the
ambient pressure
CAUTION
Never connect cylinders or other pressure sources
directly to the sensor as the high pressure in the
cylinder may damage the sensor unit.
Gas may be safely connected to the instrument by providing excess flow and
venting off the excess gas through an open T-piece with a 1 m pipe connected
to it. The procedure for checking/adjusting the calibration is as follows:
1. Any adjustments are made using the ZERO and SPAN adjust
potentiometers on the detector printed board (the printed board on the
sensor head, see Figure 4.1).
Note: on the linearised version the offset and gain potentiometers
VR1 and VR2 on the main PCB are for adjusting out electrical
tolerances and should not be used.

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Operating Manual V2.4
13
Fig. 4.1 Location of the ZERO and SPAN adjustment potentiometers
2. The sensor should be powered for at least 30 minutes before
checking its calibration.
3. Detach both pipes from the sensor head and attach first the ZERO gas
source to one of the ports. Adjust the needle valve to provide approx.
0.5 l/min to the instrument and allow the instrument to settle (for at
least 1 minute) before checking its reading (the instrument reading
may kick when opening the gas line). If the reading is outside the
required specification, adjust the ZERO potentiometer on the detector
printed circuit board.
4. Repeat step 3 above with SPAN gas. Adjustment is achieved using
the SPAN potentiometer on the detector printed circuit board. For
maximum accuracy, the bottle concentration should be corrected for
pressure (see next page).
NOTE
For the diffusion version of the GasCheck with self-
referencing, in step 4 remove the rubber cup from the
calibration port and then attach zero gas. Remember to
refit the rubber cap to the calibration port at the end of
the procedure.
Always make adjustments with the ZERO gas first and
then with the SPAN gas.
Gas may also be safely connected to an instrument by transferring it from a
cylinder or a gas mixer using gas balloons. Separate balloon assemblies must
be used for zero and span gas and different balloons should be used for
different span gas mixtures.
NOTE
Balloons should be used only for calibration checking
but not for accurate calibration.

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Operating Manual V2.4
14
When using balloons, take the gas balloon, release the clip valve and expel
any air by squeezing it, then close the clip valve. To purge the balloon attach
the balloon assembly to the gas cylinder, open the clip valve and allow the gas
to flow into the balloon so that the diameter of the balloon increases
somewhat. Stop the gas flow and empty the balloon by squeezing it to expel
the gas. Close the clip valve.
Again, attach the balloon assembly to the gas cylinder, open the clip valve and
fill to a diameter of approximately 20 cm. Then stop the gas flow and close the
clip valve. Connect the balloon to the calibration port and after allowing a
settling time check the calibration. Check that the balloon is still inflated at
the end of the procedure.
Because the sensor fundamentally measures mole density and not volume
percent (see Chapter 3.2), it is advisable to calculate the targeted instrument
reading corresponding to the span gas concentration with the ambient pressure
taken into account in order to achieve maximum accuracy from the GasCheck
sensor.
The targeted concentration reading is given by equation
Po PoPBC
TCR )5.05.1(  
(2)
where: TCR = targeted concentration reading (ppm or %),
BC = bottle concentration (ppm or %),
P = ambient pressure (hPa or mbar)
P0= 1013.25 (mbar)
EXAMPLE:
If the GasCheck 3000 is to be calibrated with 3000 ppm gas on a day when the
ambient pressure is 980 hPa, using equation (2) the output reading should be
adjusted to read an equivalent to
ppm2852
25.1013 )25.10135.09805.1(3000
TCR 

ppm
.
From the linearisation curve, this corresponds to 0.978V.
NOTE
All units are factory set to the corrected gas
concentration on the day of manufacture. For maximum
accuracy all field calibrations should also use pressure
and temperature correction.

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15
5.2 Trouble shooting
If your GasCheck sensor fails to function, we suggest you return it to
Edinburgh Instruments for repair.
However, if you wish to attempt a repair yourself, in the following list you
will find a few symptoms and likely causes to assist your efforts. Before
proceeding, please familiarize yourself with the operating principle (Section
2).
NOTE
Service should only be carried out by personnel
experienced in the repair of electronic equipment
including surface mount technology; suitable
professional tools and equipment will be required.
Symptom
what to do
output stuck at overange
check that reading could be valid; apply
zero gas and re-check output

check that source is flashing; flashing
light should be visible through source PCB,
if not see below

check that S.O.T.(Select On Test) resistor
R5 on detector PCB is fitted properly

check PL1-5 on detector PCB for the
presence of a rectified sinewave
source not flashing
check power to source PCB

depower sensor and measure source
resistance which should be 7 ; if reads
open circuit, the source has failed: see
Section 5.3

check oscillator running on source PCB;
IC22 pin 9 should have CMOS level at
crystal frequency
sensor unresponsive to gas

check that gas is reaching sensor

check that S.O.T. resistor R13 on detector
PCB is properly fitted
no output

check power to detector PCB

check 5 V and 12 V power rails on the
detector PCB
5.3 Source replacement
The infrared source comprises a tungsten filament lamp. The lamp has been
specially designed and treated to give stable output in the infrared over an
extended period. In case of a source failure, the GasCheck must be returned to
Edinburgh Instruments for repair.