Zellweger MINIGAS MK5 User manual

This publication is not intended to form the basis of a contract, and the company reserves
the right to amend the design and specification of the instruments without notice.
Distributor:
A company of the Zellweger Luwa Group
Manufactured By Zellweger Analytics Ltd, Poole Dorset, UK
MEMBERS OF THE ZELLWEGER ANALYTICS DIVISION
total environmental solutions
service manual
INTERNATIONAL ENQUIRIES
Zellweger Analytics Limited
Hatch Pond House
4 Stinsford Road
Nuffield Estate
Poole
Dorset
BH17 0RZ, UK
Tel: [phone]
Fax: [phone]
Email: postmaster@zellweger-
analytics.co.uk
Sales enquiries:
[phone]
Service enquiries:
[phone]
CORPORATE
HEADQUARTERS
Zellweger Luwa AG
Wilstrasse 11
CH-8610 Uster
Switzerland
Tel: [phone]
Fax: [phone]
ASIA PACIFIC
Zellweger Analytics Ltd
Asia Pacific Regional Office
84 Tuas Avenue 11
Singapore 639098
Tel: [phone]
Fax: [phone]
Email: [email protected]
FRANCE
Zellweger Analytics SA
33 rue du Ballon
ZI des Richardets
F-93165 Noisy le Grand Cedex
France
Tel: [phone]
Fax: [phone]
GERMANY
Zellweger Analytics GmbH
Bahnhofstrasse43
D-40764Langenfeld
Germany
Tel: [phone]
Fax: [phone]
ITALY
Zellweger Analytics srl
Via F. Primaticcio 168
I-20147 Milano
Italy
Tel: [phone]
Fax: [phone]
SPAIN
Zellweger Analytics SA
Avda Remolar 31
08820 El Prat de Llobregat
Barcelona
Spain
Tel: [phone]
Fax: [phone]
MIDDLE EAST
Zellweger Analytics
Middle East Regional Office
PO Box 52196
Dubai, UAE
Tel: [phone]
Fax: [phone]
Email:
[email protected]
BELGIUM
Zellweger Analytics nv/sa
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Chée de Louvain
B - 1932 Zaventem
Belgium
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Fax: [phone]
NETHERLANDS
Zellweger Analytics BV
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Tel: [phone]
Fax: [phone]
USA
Zellweger Analytics Inc
4331 Thurmond Tanner
Road
Flowery Branch
Atlanta
Georgia 30542
USA
Tel: [phone]
Toll Free: [phone]
Fax: [phone]
Email: [email protected]
MAN0588.P65 Issue 01 - 04/00
2987M5025
MULTIPLE DISPLAY 4-IN 1 GAS
MONITOR
MAN0588.P65 Issue 01 - 04/00
2987M5025
MINIGAS MK5

MAN0588.FM5 ISSUE 01 - 04/00
2987M5025
2
INSTRINSIC SAFETY APPROVAL
The Mini
g
as Mk5 has been approved as intrinsicall
y
safe b
y
authorities throu
g
hout
the world. It is important that, as a matter of safet
y
, the conditions of those
approvals are maintained.
The followin
g
warnin
g
was issued b
y
BASEEFA and appeared in the EECS
News
(
No. 14
)
dated April 1991:
"With re
g
ard to repairs, the owner of the e
q
uipment is responsible for ensurin
g
that the repairs are carried out in such a wa
y
that the explosion protection
measures incorporated in the e
q
uipment remain in compliance with the
appropriate standards. Not onl
y
should a competent repairer be used, but also
the specification for the repaired e
q
uipment should ideall
y
be obtained from the
manufacturer. Unless the repairs are carried out b
y
a manufacturer who holds
an appropriate EECS licence, we
(
BASEEFA
)
can offer no assurance with
re
g
ard to the
q
ualit
y
of the work done."
The followin
g
warnin
g
, applicable to intrinsicall
y
safe instruments, was issued
b
y
CoGDEM in October 1992:
CoGDEM
THE COUNCIL OF
GAS DETECTION EQUIPMENT MANUFACTURERS
WARNING
This is a certified product. Service, repair or calibration other than b
y
the
manufacturer or a manufacturer’s recommended a
g
ent ma
y
cause invalidation
of certification, warrant
y
, and result in a safet
y
hazard.
CONFORMITY WITH EUROPEAN
DIRECTIVES
Within the European Union, the CE mark is used in to show conformit
y
with
applicable New Approach Directives specif
y
in
g
the health and safet
y
re
q
uirements.
These re
q
uirements include EMC and ESD standards. Within the limits of EMC
Directive 891336/EEC, the desi
g
n of the Mini
g
as Mk5 has been tested to EN50
082-2: 1995 to ensure that it has ade
q
uate immunit
y
to RFI and ESD encountered
in recommended applications.
Similarl
y
, tests have been carried out to EN55 022: 1995 Class B to ensure that the
Mini
g
as Mk5 does not produce electrical interference.

MAN0588.FM5 ISSUE 01 - 04/00
2987M5025
3
WARNINGS AND CAUTIONS
WARNING
The Mini
g
as Mk5 is to be serviced and the battery char
g
ed only in
a safe area.
Substitution of components may impair the intrinsic safety of the
Mini
g
as Mk5 - replacement parts must be approved by Zellwe
g
er
Analytics. This applies to all parts, includin
g
sensors, components
and accessories.
CAUTION
The Mini
g
as Mk5 must be serviced only by qualified personnel
trained by Zellwe
g
er Analytics, or by a Zellwe
g
er appointed a
g
ent.
Manual must be read and fully understood before servicin
g
the
Mini
g
as Mk5.
Note:Failure to observe and abide by the above warnings and
cautions may render the intrinsic safety approval void and may
remove any right of claim against Zellweger relating to product
liability or consequential damage to any third party.
CAUTION
The followin
g
g
ases or substances may contaminate the
Mini
g
as Mk5 explosive sensor:
Silicones
Halo
g
ens (>100 ppm)
Halo
g
enated hydrocarbons
Phosphorous-containin
g
compounds
Sulphur containin
g
compounds (>100 ppm)
Volatile or
g
anometallics
If the Mini
g
as Mk5 has been exposed to any of the above
contaminatin
g
g
ases or substances, carry out the calibration
check described in Chapter 6.

MAN0588.FM5 ISSUE 01 - 04/00
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4
WARNINGS AND CAUTIONS
Zellwe
g
er Analytics Limited reserve the ri
g
ht to chan
g
e or revise the
information supplied in this document without notice and without
obli
g
ation to notify any person or or
g
anisation of such revision or
chan
g
e.
HELP US TO HELP YOU
Every efforthas been made to ensure the accuracy in the contents of
ourdocuments,however,Zellwe
g
erAnalytics Limitedcanassume no
responsibility for any errors or omissions in our documents or their
consequences.
Zellwe
g
er Analytics Limited would
g
reatly appreciate bein
g
informed
of any errors or omissions that may be found in the contents of any of
our documents and to this end we include the followin
g
form for you
to photocopy, complete and return to us so that we may take the
appropriate action.

MAN0588.FM5 ISSUE 01 - 04/00
2987M5025
5
HELP US TO HELP YOU
To: Marketin
g
Communications,
Zellwe
g
er Analytics Limited,
Hatch Pond House,
4 Stinsford Road,
Nuffield Estate,
POOLE. Dorset.
BH17 0RZ.
United Kin
g
dom.
Tel : [phone]
Fax : [phone]
email : marcom@zellwe
g
er-
analytics.co.uk
From :
Address :
Tel :
Fax :
email :
I su
gg
est the followin
g
corrections/chan
g
es be made to Section ...........
Marked up copies attached (as appropriate): Yes / No
Please inform me of the outcome of this chan
g
e: Yes / No
For Marketin
g
Communications, Zellwe
g
er Analytics Limited:
Actioned By: Date:
Response: Date:

MAN0588.FM5 ISSUE 01 - 04/00
2987M5025
6

MAN0588.FM5 ISSUE 01 - 04/00
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7
CONTENTS
section
1. GENERAL DESCRIPTION 13
1.1 MINIGAS Mk5 MULTI-GAS MONITOR 13
1.1.1 Identification usin
g
the part number and serial
number 13
1.2 ASSOCIATED EQUIPMENT 15
1.2.1 Battery char
g
er 15
1.2.2 Oxy
g
en sensor 17
1.2.3 Explosive-
g
as sensor 17
1.2.4 Microprocessor and associated circuits 18
1.2.5 TXD/RXD port 18
1.2.6 Datalo
gg
in
g
circuit 19
1.2.7 Display module 19
1.2.8 Power supply 19
1.3 GENERAL DESCRIPTION - BATTERY PACKS 19
1.4 GENERAL DESCRIPTION - BATTERY CHARGERS 20
1.4.1 Fast char
g
er 21
1.4.2 Trickle char
g
er 22
1.4.3 Fuses 23
2. TOOLS AND TEST EQUIPMENT 25
2.1 TOOLS 25
2.2 TEST EQUIPMENT 25
2.2.1 Common test equipment 25
2.2.2 Special-to-type test equipment 26
2.2.3 Reconfi
g
uration equipment 27

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CONTENTS
8
3. FAULT DIAGNOSIS 29
3.1 INTRODUCTION 29
3.2 SELF TESTS 29
3.2.1 Initial checks 30
3.2.2 Fault codes 33
3.2.3 Low battery fault 33
3.2.4 Datalo
gg
in
g
warnin
g
s33
3.3 ELECTRICAL TESTS - MINIGAS Mk5 34
3.3.1 Battery power supply 35
3.3.2 Power consumption 35
3.3.3 Power maintain ' circuit power and reference
supplies 36
3.3.4 Switches 37
3.3.5 Toxic-
g
as channels 37
3.3.6 Oxy
g
en channel 37
3.3.7 Explosive-
g
as channel 39
3.3.8 ADC circuit 41
3.3.9 Speaker drive 42
3.4 ELECTRICAL TESTS - BATTERY PACK 42
3.5 ELECTRICAL TESTS - FAST CHARGER 43
3.6 ELECTRICAL TESTS - TRICKLE CHARGER 45
3.7 CONNECTIONS - MINIGAS Mk5 MONITOR 52
3.8 CONNECTIONS - BATTERY PACK AND CHARGERS 55
4. MAINTENANCE 57
4.1 INTRODUCTION 57
4.2 MINIGAS Mk5 MONITOR - DISASSEMBLY AND
REASSEMBLY 57
4.2.1 Sensor
g
rill 58
4.2.2 Disassembly of the monitor assembly (Fi
g
4.3) 60
4.2.3 Reassembly of the monitor assembly (Fi
g
4.3) 62

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CONTENTS
9
4.3 SENSOR REPLACEMENT 65
4.3.1 Oxy
g
en sensor 65
4.3.2 Toxic sensors 66
4.3.3 Explosive-
g
as sensor 67
4.4 CHANGES TO THE MONITORED GASES 68
4.4.1 Oxy
g
en channel 69
4.4.2 Explosive-
g
as channel 69
4.4.3 Toxic
g
as channels 69
4.4.4 Adjustin
g
the toxic
g
as null volta
g
e70
4.5 BATTERY PACKS - DISASSEMBLY AND
REASSEMBLY 71
4.5.1 Rechar
g
eable battery packs (Fi
g
4.6) 71
4.5.2 Dry-cell battery pack (Fi
g
4.7) 72
4.6 BATTERY CHARGERS - DISASSEMBLY AND
REASSEMBLY 76
4.6.1 Fast char
g
er (Fi
g
4.8) 76
4.6.2 Trickle char
g
er (Fi
g
4.9) 80
5. SOFTWARE OPTIONS 83
5.1 INTRODUCTION 83
5.2 SPECIFYING A CONFIGURATION CODE 84
5.3 CONFIGURING A MINIGAS Mk5 84
5.3.1 Installin
g
M4CONFIG 84
5.3.2 Usin
g
M4CONFIG 85
6. TESTING AND CALIBRATION 87
6.1 INTRODUCTION 87
6.1.1 General 87
6.1.2 EasyCal provides the followin
g
facilities: 87

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CONTENTS
10
6.2 TOOLS AND TEST EQUIPMENT 88
6.3 PREPARATION FOR CALIBRATION 89
6.3.1 General 89
6.3.2 Adsorption of the Test Gas 89
6.3.3 Pur
g
in
g
the Gas Connections 90
6.4 CALIBRATION PROCEDURES 91
6.4.1 General 91
6.4.2 Enterin
g
Calibration Mode 91
6.4.3 Simultaneous Zeroin
g
92
6.4.4 Span Calibration 93
6.4.5 Calibration Failure 95
6.4.6 Stora
g
e of Calibration Results 96
6.5 POST CALIBRATION 96
6.6 SETTING GAS BOTTLE CONCENTRATIONS 96
6.7 MANUAL CALIBRATION PROCEDURE 97
6.7.1 Enterin
g
Manual Calibration mode 97
6.7.2 Oxy
g
en calibration (span) 98
6.7.3 Explosive calibration (null and span) 100
6.7.4 Explosive channel temperature compensation
and linearisation coefficients 102
6.7.5 Toxic calibration (null and span) 107
6.7.6 Manual Calibration failure 110
6.8 POST CALIBRATION 110
6.9 CALIBRATION FAILURE 110
6.10MINIGAS Mk5 - STANDARD SERVICEABILITY TEST 111
6.10.1 Gas tests 111
6.10.2 Final acceptance test 114
6.10.3 Displayin
g
the instrument serial number 116
6.10.4 Datalo
gg
in
g
warnin
g
s116
6.11FAST CHARGER - STANDARD SERVICEABILITY
TEST 117

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11
6.12TRICKLE CHARGER - STANDARD SERVICEABILITY
TEST 118
7. SPARES LISTS 119
7.1 MINIGAS Mk5 INSTRUMENT - SPARES LIST 121
7.2 RECHARGEABLE BATTERY PACK - SPARES LIST 124
7.3 DRY-CELL BATTERY PACK - SPARES LIST 126
7.4 FAST CHARGER - SPARES LIST 128
7.5 TRICKLE CHARGER - SPARES LIST 130
8. GLOSSARY OF TERMS AND ABBREVIATIONS 131
TECHNICAL BULLETINS AND SERVICE NOTES 135
MINIGAS CONFIGURATION REQUEST FORM 144
TABLES
Table Pa
g
e
Table 3.1 MiniGas Mk5 fault codes 33
Table 3.2 Explosive-
g
as pcb to analo
g
ue pcb connections 52
Table 3.3 Analo
g
ue pcb to di
g
ital pcb connections 55
Table 3.4 Di
g
ital pcb to display module connections 56
Table 3.5 Di
g
ital pcb to lo
gg
er pcb connections 56
Table 3.6 Battery pack connections 57
Table 3.7 Fast char
g
er internal connections 57
Table 3.8 Trickle char
g
er internal connections 58

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12
FIGURES
Fi
g
ure Pa
g
e
Fi
g
ure 1.1 MiniGas Mk5: functional block dia
g
ram 16
Fi
g
ure 1.2 Fast char
g
er: functional block dia
g
ram 21
Fi
g
ure 1.3 Trickle char
g
er: functional block dia
g
ram 22
Fi
g
ure 3.1 Missin
g
sensor: typical oscilloscope waveforms 38
Fi
g
ure 3.2 Pellistor power circuit: typical oscilloscope
waveforms 40
Fi
g
ure 3.3 ADC circuit: typical oscilloscope waveforms 41
Fi
g
ure 3.4 Speaker drive circuit: typical oscilloscope
waveforms 42
Fi
g
ure 3.5 Explosive-
g
as pcb assembly 46
Fi
g
ure 3.6 Di
g
ital pcb assembly 47
Fi
g
ure 3.7 Analo
g
ue pcb assembly 48
Fi
g
ure 3.8 Battery pack fuse board 48
Fi
g
ure 3.9 Battery pack connectors 49
Fi
g
ure 3.10 Fast char
g
er pcb assembly 49
Fi
g
ure 3.11 Four-char
g
er bank: interconnection dia
g
ram 50
Fi
g
ure 3.12 Trickle char
g
er pcb assembly 51
Fi
g
ure 3.13 Trickle char
g
er mains transformer connections 51
Fi
g
ure 4.1 Sensor
g
rill assembly 58
Fi
g
ure 4.2 Sensor layout 59
Fi
g
ure 4.3 MiniGas Mk5: exploded view 64
Fi
g
ure 4.4 3-pcb assembly 68
Fi
g
ure 4.5 Rechar
g
eable battery pack: exploded view 75
Fi
g
ure 4.6 Dry-cell battery pack: exploded view 75
Fi
g
ure 4.7 Fast char
g
er: exploded view 78
Fi
g
ure 4.8 Trickle char
g
er: exploded view 81
Fi
g
ure 7.1 MiniGas Mk5 instrument 120
Fi
g
ure 7.2 Rechar
g
eable battery pack 124
Fi
g
ure 7.3 Dry-cell battery pack 125
Fi
g
ure 7.4 Fast char
g
er 127
Fi
g
ure 7.5 Trickle char
g
er 129

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13
1. GENERAL DESCRIPTION
1.1 MINIGAS MK5 MULTI-GAS MONITOR
The Mini
g
as Mk5 is a small, portable
g
as monitor desi
g
ned to be
carried or worn without hinderin
g
the user. Its purpose is to monitor
the atmosphere continuously for hazardous levels of four
g
ases
simultaneously.
The instrument is usually supplied with four sensors - for detectin
g
oxy
g
en, explosive
g
ases and two toxic
g
ases (selected from carbon
monoxide, hydro
g
en sulphide, chlorine, carbon dioxide or sulphur
dioxide). If required it can be confi
g
ured for monitorin
g
only one, two
or three of these
g
ases.
Power for the Mini
g
as Mk5 is provided by a removable battery pack,
three versions of which are available:
Rechar
g
eable nickel cadmium (NiCd) pack.
Rechar
g
eable nickel metal hydride (NIMH) pack.
Dry-cell (4 x AA) pack.
The NiCd and NIMH packs can be rechar
g
ed (either fitted to or
removed from the instrument) by means of a special-to-type battery
char
g
er (see Section 1.2.1).
1.1.1 Identification using the part number and serial number
The instrument part number isto be found on the label on the outside
ofthebase castin
g
andconsistsofupto 13 alpha-numeric characters
providin
g
the followin
g
'as supplied' information about the
Mini
g
as Mk5.
Ori
g
in of order
(1 character)
P = sales
S = sales
- = production
Instrument
(1 character)
U = Mini
g
as Mk5

MAN0588.FM5 ISSUE 01 - 04/00
2987M5025
1. GENERAL DESCRIPTION
14
Destination country
(1 character)
A = United States
(the ran
g
e of countries may be extended) C = Canada
E = United Kin
g
dom
F = France
G = Germany
L = Spain
S = Sweden
X = special instrument
Sensors fitted
(up to 4 characters)
0 = oxy
g
en
(the ran
g
e of sensors may be extended) F = explosive
(that is, flammable)
C = carbon monoxide
H = hydro
g
en sulphide
L = chlorine
D = carbon dioxide
S = sulphur dioxide
X = position not used
(dummy sensor fitted)
Software confi
g
uration code For example, A01
(3 alpha-numerics)
Display confi
g
uration code
(1 character)
M = multiple display
(Mini
g
as Mk5 is available only as a multiple display instrument)
Sub-division code
(1 digit)
1 = datalo
gg
in
g
, zinc
casin
g
Battery pack
(1 digit)
9 = monitor only, no
battery supplied
(the part number does not specify a particular battery pack - this is
supplied as a separate item). Mini
g
as Mk5 is only available as a data
lo
gg
in
g
instrument.
Example: Part number PUG-OFCH-A07-M19 indicates that the
instrument is a standard production Minigas Mk5 (minus
its battery pack) supplied to Germany, fitted with four
sensors (for monitoring oxygen, explosive gas, carbon
monoxide and hydrogen sulphide) with the alarm levels
configured to software code A07, and is a datalogging
instrument in a zinc case.

MAN0588.FM5 ISSUE 01 - 04/00
2987M5025
1. GENERAL DESCRIPTION
15
The instrument serial number is shown on the label on the outside of
the base casin
g
and can also be displayed as part of the switch-on
sequence(seeSection6.7.3).Theserialnumbercomprisesan8-di
g
it
code:
First three di
g
its The date of manufacture - for example,
126 indicates December 1996 and 027
indicates February 1997.
Remainin
g
five di
g
its A number unique to each instrument.
Note:If the serial number contains the letter 'Z', then the instrument
has been made to a special customer requirement.
1.2 ASSOCIATED EQUIPMENT
Associated equipment is defined as special-to-type equipment
essential to the operation of the Mini
g
as Mk5. There is only one such
item, that is the char
g
er unit for use with the NiCd and NIMH battery
pack versions. The dry-cell version has no associated equipment.
1.2.1 Battery charger
The battery char
g
er is available as a fast or trickle char
g
e unit with a
number of power supply options, ac and dc. The ac fast char
g
e units
can be supplied as sin
g
le-way or multi-way char
g
ers, the multi-way
version consistin
g
simply of four units fitted to
g
ether with a sin
g
le
power supply cable and internal wirin
g
.
No operator controls are fitted and the power supply lead is fixed
permanently, the type of lead dependin
g
on the input volta
g
e and
destination country. A
g
reen li
g
ht indicates 'power on' when no
battery is inserted.
When a battery pack is inserted in the fast char
g
er, the char
g
er
displays a red (fast) li
g
ht to show that the battery is bein
g
char
g
ed. A
dischar
g
ed battery takes approximately 21/2 to 3 hours to be fully
char
g
ed, after which time it displays both the red and
g
reen li
g
hts to
show that it is on trickle char
g
e.

MAN0588.FM5 ISSUE 01 - 04/00
2987M5025
1. GENERAL DESCRIPTION
16
Note:The NIMH battery pack will switch into trickle charge when it has
reached approximately 85% full charge and, therefore, requires
a further 2 hours on trickle charge to reach full capacity. The
NiCd battery pack, however, will not switch to trickle charge until
it is 100% fully charged.
When a battery pack is inserted in the trickle char
g
er, the char
g
er
displays a second
g
reen li
g
ht to show that the battery is bein
g
char
g
ed. A dischar
g
ed battery is fully char
g
ed after a maximum of 19
hours, althou
g
h it is safe to leave it on char
g
e for lon
g
er periods.
Figure 1.1 Minigas Mk5: functional block diagram
Toxic
sensor 1
Battery
Pellistor
assembly
Toxic
sensor 2
Oxygen
sensor
Power fail
detect
Voltage
regulator
and
power
maintain
Micro-
processor
and config.
data store
Switch
input
circuit
LCD
Alarm
(red)
LEDs
(4)
Confirm.
(green)
LEDs
(2)
Backlight
LEDs
(2)
Watchdog
circuit
Calendar
clock Logger
memory
LED
drives
Speaker
connections
TOX1/TOX2
amplifier
and
missing
sensor
circuit
Oxygen
amplifier
A-to-D
converter
Pellistor
circuit
TOX 1
TOX 2
JP1
JP3
JP2
JP2
1.2V
JP4
JP1
JP3
B2 B3
B1
-
+
Charging current
To battery charger
Ground
B5
F3F2
F1
B3
Micro/alarm
power
Circuit
power Ground Data
B4
B2
B4 B1
JP1
JP1
JP5
TXD/RXD
JP1
JP2
JP4
Calibrate
Select
(green) Backlight
(red)
Explosive pcb
Analogue pcb
Battery pack
Display
module
Digital
pcb
Logger pcb

MAN0588.FM5 ISSUE 01 - 04/00
2987M5025
1. GENERAL DESCRIPTION
17
On instruments not confi
g
ured for monitorin
g
a toxic
g
as, the
analo
g
ue pcb is provided with the circuits required for carbon
monoxide and hydro
g
en sulphide monitorin
g
. Therefore, in the event
of such a Mini
g
as Mk5 bein
g
modified to monitor either or both of
these
g
ases, there will be no need to chan
g
e the3-pcb assembly, but
a different assembly will be required if it is desired to monitor any of
the other toxic
g
ases.
As part of the instrument's self-test routine, a 'missin
g
sensor' circuit
checks for an opencircuit fault in the sensor resultin
g
from either a
defective sensor or its deliberate or accidental disconnection. This
sensor makes use of the sensor self-capacitance to indicate the
presence of the sensor.
1.2.2 Oxygen sensor
The oxy
g
en sensor is a 2-terminal device located in the top cover
assembly, the connections from the sensor receptacle plu
gg
in
g
into
JP2 onthe analo
g
ue pcb. The sensor produces a current proportional
to the percenta
g
e volume of oxy
g
en in the atmosphere which is
amplified and passed as an analo
g
ue si
g
nal to the ADC circuit.
1.2.3 Explosive-gas sensor
The explosive-
g
as sensor, known as a pellistor, is a 4-terminal device
whichplu
g
sintoJP1onthe analo
g
uepcb andthenconnects, viaJP4,
to the explosive-
g
as pcb.
The pellistor bead forms part of a Wheatstone brid
g
e circuit and is
maintained at a constant temperature by applyin
g
electrical power to
it from a switch-mode power supply circuit. The principle of operation
is that if a flammable
g
as is present it bums on the pellistor with the
result that less electrical power is needed to maintain the bead
temperature (that is, the volta
g
e across the pellistor is reduced).
Thus, the presence and concentration of explosive
g
as in the
atmosphere is measured by passin
g
the pellistor volta
g
e via JP1/JP4
to the ADC circuit.
To compensate for the instrument's wide ambient temperature
operatin
g
ran
g
e (si
g
nificant in comparison with the pellistor bead
temperature), a thermistor circuit provides temperature
compensation data via the ADC circuit.

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1. GENERAL DESCRIPTION
18
1.2.4 Microprocessor and associated circuits
The microprocessor on the di
g
ital pcb is an 8-bit device runnin
g
at
3.6864MHz. Normally the device is a mask-pro
g
rammed ROM type
83Cl54D, buttheMini
g
ascircuitisdesi
g
nedsothatitcanbemodified
to employ an EPROM type 87L58 or 87L51 FC instead.
Analo
g
ue si
g
nals from the sensors are applied to the ADC circuit and
addressed sequentially by the microprocessor. After analysis, these
si
g
nals are passed as serial data to the display module via JP1.
Instrument confi
g
uration data (alarm levels, calibration data and
display modes) are stored in an EEPROM, and communication with
the microprocessor is via a serial data bus.
The microprocessor also controls the audible warnin
g
speaker (fitted
in the sensor tray) and the LCD and the red warnin
g
,
g
reen
confirmationandbackli
g
ht LEDs on the display module. Drive circuits
for the speaker and LEDs are provided on the di
g
ital board.
The operation of the microprocessor is monitored by a watchdo
g
circuit. A pulsed si
g
nal from the microprocessor keeps the watchdo
g
circuit dormant but, in the absence of this si
g
nal, the watchdo
g
circuit
resets the microprocessor. In this event, the instrument
g
oes throu
g
h
its self-test routine a
g
ain and, if the reset was due to a fault, the
appropriate fault code is displayed; otherwise, normal operation is
resumed.
The microprocessor has three normally-open circuit-control lines,
each activated byapushbuttonswitch andmonitoredbythesoftware.
The three switches are the calibrateswitch on the explosive-
g
as pcb,
and the on/select (left-hand
g
reen button) and backli
g
ht (ri
g
ht-hand
red button) switches on the analo
g
ue pcb. When any of the switches
are pressed, the associated control lines are
g
rounded and the
microprocessor pauses its current process to determine the action to
take. The on/select switch also initiates the supply of power to the
circuit.
1.2.5 TXD/RXD port
TheTXD/RXDportonthe microprocessorisaserial datalink allowin
g
the Mini
g
as Mk5 to communicate with an external computer via the
battery pack contact B1 on the di
g
ital pcb. To allow downloadin
g
of
lo
gg
ed data, the battery pack is replaced by a data transfer unit.

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1. GENERAL DESCRIPTION
19
By this means it is also possible to pro
g
ram instrument confi
g
uration
data into the EEPROM.
Downloadin
g
of data is accomplished by usin
g
the Lo
g
View software
packa
g
e, and confi
g
uration of the instrument usin
g
the M4CONFIG
pro
g
ram.
1.2.6 Datalogging circuit
Circuits associated with the datalo
gg
in
g
option are contained on the
lo
gg
er pcb, which is fitted to the di
g
ital pcb and contains a 1.2V
back-up battery, clock/calendar and additional memory.
1.2.7 Display module
Data and messa
g
es from the microprocessor are displayed on a
96-se
g
ment LCD usin
g
3:1 multiplexin
g
and provided with a
user-activated backli
g
ht facility. The LCD driver communicates with
the microprocessor via a serial data bus operatin
g
at a frequency of
approximately 125kHz.
1.2.8 Power supply
Dependin
g
on the state of the battery, the battery volta
g
e can be
between 3.5V and 6V, and it is applied to the monitor circuits at
contacts B2 (circuit power, unswitched), B3 (micro/alarm power,
unswitched) and B4 (
g
round) on the di
g
ital pcb.
'Circuit power' is used by the explosive channel pellistor circuit, and
alsoby thepowerfailcircuit tomonitorthebattery volta
g
eandprovide
the 'battery low' indication as the battery becomes dischar
g
ed. The
current is typically between 35 and 45mA.
'Micro/alarm' power is used to
g
enerate the 3.3V re
g
ulated supply
(Vcc) to the analo
g
ue and di
g
ital circuits, and also to power the
speaker and LED drive circuits. Without alarms, the current is
typically between 5 and 7mA. Under alarm conditions, the maximum
additional load is 23mA.
1.3 GENERAL DESCRIPTION - BATTERY PACKS
Three versions of the battery pack are available:

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1. GENERAL DESCRIPTION
20
• NiCd version containin
g
four encapsulated 600mAh nickel
cadmium cells, series-connected to provide a nominal 5V
supply.
• NIMH version containin
g
four encapsulated 900mAh nickel
metal hydride cells, series-connected to provide a nominal 5V
supply.
• Dry-cell version containin
g
four AA cells, series-connected to
provide a nominal 6V supply.
In allversions, the battery packinterfaces electrically with the monitor
via three connectors B2, B3 and B4 on the fuse board. These mate
with three similarly coded contacts on the monitor di
g
ital pcb.
In the case of the NiCd and NIMH versions, connection to a battery
char
g
er (Section 1.6) is via two studs B1 and B5 at the opposite end
of the battery char
g
er from the monitor. These are connected to the
battery pack fuse board by a pair of nickel strips.
The followin
g
fuses are located on the battery pack fuse board:
Input char
g
in
g
current (NiCd and NiMH version only) F1 1A
Circuit power supply F2 62mA
Micro/alarm power F3 62mA
1.4 GENERAL DESCRIPTION - BATTERY CHARGERS
Exploded views of the fast and trickle char
g
e units are shown in
Chapter 4, Fi
g
4.8 and 4.9, and functional block dia
g
rams in Fi
g
1.2
and 1.3. Both types of char
g
er are available as ac and dc power
supply options - the essential difference bein
g
that the dc variant in
each case has the transformer and rectifier circuit replaced by a fuse.
In the ac versions, the supply volta
g
e is internally link-selectable for
European and North American mains volta
g
es.