Micro Medical Micro CO Meter Service Manual

Contents page

Safety Precaution 2
Important safeguards 2
Looking after your Micro CO meter 2
Introduction 2

% C O H b

CO-

OFF

Before you begin 2
Micro CO system overview 3
CO sensor 5
Micro CO meter exploded view 6
Disassembling the Micro CO
meter for servicing 7
Replacing the internal battery 7
Replacing the fuel cell 7
Reassembling the Micro CO meter 8
Calibrating the Micro CO meter 9
Circuit description 11
Power supply 11

micro

Fuel cell amplifier circuit 12
Reset circuit 12
Display 12
Sounder 12
Indicator lights 13
RS232 interface 13
Specifications 14
Technical support 15
Parts List 16
Circuit Diagram 18

Micro CO Meter
Service Manual

073-12 Iss. 1.1 Sept 2005

Micro CO Meter

Service Manual

Information in this document is subject to change without notice and does not represent a commitment
on the part of Micro Medical Limited. Only the parts supplied by Micro Medical Limited should be used
to complete the service operation described in this manual. If in any way you feel unsure about the
successful completion of the service operation you should contact Micro Medical Limited or its
appointed agent in your country or region and arrange the despatch of the product to a Micro Medical
Limited Service Centre.

Copyright 2005 by Micro Medical Limited All rights reserved
Drawing no. 073-12
Version 1.1
September 2005
All other products are trademarks or registered trademarks of their respective owners.

1

Safety Precaution

The servicing of this device is intended to be carried out by a properly trained and
competent electronics engineer, or experienced in the maintenance and servicing
of medical devices. Read this manual thoroughly before proceeding with the
service. If in any doubt please contact the service centre at Micro Medical Limited
or their accredited agent in your country or region.

Important Safeguards

ο Read all of the instructions.
ο Keep the instructions in a safe place for later use.
ο Follow all warnings and instructions marked on the product.
ο When replacement parts are required, be sure to use replacement parts

specified by Micro Medical that have the same characteristics as the original
parts. Unauthorised substitutions may result in fire, electric or other hazards.

ο Do not place on an unstable table.
ο The product should be operated only from the type of power source indicated

on the label.

Looking after your Micro CO Meter

ο Avoid exposing the Micro CO Meter to solvents including alcohol and

disinfectants.

ο Avoid operating the Micro CO Meter in dusty conditions or near to heating

appliances or radiators.

ο Do not keep the Micro CO Meter in a damp place or expose it to extreme

temperatures.

Introduction

This service manual provides you with information to carry out the servicing of the
Micro CO Meter. It is a process, which is relatively straightforward but must be
carried out in a logical sequence. Our advice is to familiarise yourself with the
contents of this manual before attempting to carry out the procedure of replacing
the parts supplied in the sensor replacement kit for the Micro CO Meter.

Before You Begin

Before you begin the servicing operation, please read the section on Circuit
description very carefully:

2

Micro CO Meter system overview.

The Micro Medical Micro CO meter consists of a hand held microcomputer
unit (1) incorporating a CO sensor and is supplied with a mouthpiece
adapter with integral one way valve (2) together with disposable cardboard
mouthpieces (5).

The microcomputer unit is powered by a single alkaline PP3 battery (4)
and is supplied with a reducing connector for calibration (3).

1

2

%COHb

CO-PPM

OFF

Micro CO

3

4

5

3

The CO sensor is an electrochemical fuel cell, and works through the
reaction of carbon monoxide at one electrode and oxygen (from ambient
air) at the other. This reaction generates an electrical current proportional
to the concentration of CO exposed to the sensing surface of the fuel cell.

The current output signal from the sensor is conditioned using a current to
voltage converter and is applied to an analogue to digital (A/D) converter.

When the unit is first switched on the microprocessor records the baseline
reading on the A/D input and uses this value to auto zero the instrument.

The subject is requested to breathe in maximally, hold the breath for 10
seconds, and then to expire fully through the mouthpiece connected to the
microcomputer unit with the mouthpiece adapter. As an aid to timing the
breath holding period a countdown from 10 to 0 is displayed after the unit
is turned on.

The microprocessor then records the peak value obtained and displays
this on a 3½ digit LCD display.

The value can be displayed either as parts per million (ppm) concentration
in the expired air or as the equivalent percentage carboxyhaemoglobin
(%COHb) using the mathematical relationships described by Jarvis et al,
for concentrations below 90ppm and by Stewart et al for higher
concentrations.

Jarvis MJ, Belcher M, Vesey C, Hutchison DCS

Low cost carbon monoxide monitors in smoking assessment.
Thorax 1986; 41:886-887

Stewart RD, Stewart RS, Stamm W, Seleen RP

Rapid estimation of carboxyhaemoglobin levels in fire fighters
JAMA 1976; 235:390-392
As a quick guide to the estimated smoking level, red, amber and green

indicator lights are provided. The following levels of CO activate these
lights:

CO(ppm) %COHb Cigarette consumption Indicator

0 - 5 0 - 0.8 Non smoker Green

6 - 10 1 - 1.6 Light smoker Amber

11 - 72 1.8 - 12 Heavy smoker Red

>72 >12 Suspected poisoning Red + alarm

4

CO Sensor

The sensor is an electrochemical micro fuel cell using gaseous diffusion
barrier technology resulting in a direct response to volume concentration
rather than partial pressure.

The cell consists of a working electrode and a counter electrode separated
by a thin layer of electrolyte. The gaseous diffusion barrier limits the flow
of gas to the sensing electrode and ensures the electrochemical activity of
the electrode is far in excess of the amount of gas with which it has to
deal.

Gas diffusing onto the working electrode reacts at the surface of the
electrode by oxidation.

CO reacts at the working electrode according to the equation:

CO + H2O → CO2 + 2H+ + 2e

The counter electrode acts to balance out the reaction at the sensing
electrode by reducing oxygen in air to water:

½O2 + 2H+ + 2e- → 2 H2O

-

5

Micro CO meter exploded view

Item 6

Item 5

Item 4 Item 3 Item 1Item 2

Item 10

Item 9

Item 7 Item 8

6

Disassembling the Micro CO meter for servicing.

The Micro CO microcontroller unit comprises of a solid state electronic circuit
in a robust ABS housing and does not require any preventative maintenance.

Routine maintenance consists of replacing the fuel cell and internal lithium
battery when they are exhausted. The 3.6 volt lithium battery is continuously
monitored by the microcontroller and the message bt2 will be displayed when
the voltage falls below 3 volts. The fuel cell exhibits a gradual loss of
sensitivity with time and has effectively expired when the unit can no longer
be calibrated.

When either the fuel cell or the lithium battery has expired, replace by
following the procedure below.

1. Turn the unit face down and slide back the battery compartment.

2. Remove the PP3 battery.

3. Remove the 2 self tapping screws (Item 1) and put to one side.

4. Lift the top moulding (Item 6) from the bottom moulding (Item 2).

5. Remove the PCB (Item 3) from the bottom moulding.

6. If the fuel cell (Item 4) is exhausted carefully remove by gently pulling
away from the PCB.

7. If the battery (Item 8) has expired, then note the orientation and remove
from the PCB-mounted clips by pulling on the battery strap (Item 7) directly
away from the PCB. Put the battery strap to one side and discard the
battery.

Replacing internal battery of the Micro CO.

1 Place the battery strap over the new battery (Cat No: BAT5900) and push

into the PCB-mounted clips ensuring the correct orientation. (Take great

care not to short circuit the terminations even momentarily as the
low internal impedance of lithium batteries will result in a high
current consumption and greatly reduced life).

Replacing fuel cell of the Micro CO.

1 Remove the CO sensor (Cat No: CEL7300) from the plastic container.
2 Carefully insert the CO cell into the PCB with the correct orientation.

7

Reassembling the Micro CO meter

1. Place the PCB into the bottom moulding.

2. Tuck the battery strap between the battery and the side wall of the bottom
moulding, away from the slide switch.

3. Ensure that the slide switch (Item 9) and the switch plate on the top
moulding (item10) are both positioned at the bottom of their travel.

4. Ensure that the rubber gasket (Item 5) is in place in the top moulding.

5. Place the top moulding on top of the bottom moulding ensuring the rubber
gasket fits over the cell correctly and secure using the two self-tapping
screws.

6. Reconnect the PP3 battery ensuring correct polarity.

8

n

Calibrating the CO meter

Calibration will remain stable to within 2% over one month and typically to
within 10% over 6 months. Micro Medical supplies calibration gas (20ppm CO
in air) and recommends that the unit is recalibrated on a 6 monthly basis. See
page 13 for calibration accessories.
To carry out the calibration locate the calibration switch on the right hand side
of the instrument as shown below.

Serial
port

Calibratio
Switch

Push the slide switch to the CO - PPM position and wait for the unit to display
zero.

9

Screw the control valve firmly onto the cylinder and connect the gas supply as
shown below:

Plastic Tubing

Control
valve

20 ppm carbon
monoxide in air

Flow
indicator

Reducing
Connector for
calibration

The plastic tubing supplied with the gas should be pushed firmly over the
reducing connector.
Slowly turn the control knob anti-clockwise until the ball in the flow indicator is
between the two marks. This will then supply a gas flow of approximately

0.25 l/min. Apply this flow for 25 seconds and if the meter does not read

20ppm use the calibration tool to push the calibration switch.
The unit will then beep 3 times, store the new calibration value, and display
the following:

The gas supply should then be turned off.

10

Circuit description

(Refer to the parts list and circuit diagram)
The circuit is based on the Hitachi one time programmable (OTP)
microcontroller HD64F3687FP (U5) operating at a clock frequency of

14.745MHz.

This processor contains EPROM, RAM, and eight 10-bit analogue to digital
(A/D) converters. The current output signal from the sensor is conditioned
using a current to voltage converter and is applied to an A/D input of the
microcontroller. Voltages derived from the external PP3 battery and the
internal lithium battery are also connected to the A/D converter inputs.
When the unit is first switched on the microprocessor records the baseline
reading from the A/D and uses this value to auto-zero the instrument. The
signal from the CO sensor is continuously monitored and the peak of the
calculated carbon monoxide concentration is displayed and transmitted
through the RS232 driver (U7).

Power Supply

The unit has two separate supplies. The externally accessible alkaline 9 volt
PP3 battery (BAT 1) provides the main supply. The instrument may be
switched on and off with the slide switch and may also be switched off by a
signal from the processor. This is done if the unit is left on, without use, for a
period of 4 minutes in order to conserve battery power. The supply is
controlled by gates U2 and U3 arranged in a bi-stable configuration, and
powered continuously from BAT1. When the slide switch is moved to the
‘CO-PPM’ position one end of R7 is pulled low. This transition is differentiated
by the action of C15 and R6 so that a momentary pulse appears on pin 2 of
U3. This pulse will toggle the bi-stable circuit so that pin 4 of U2 will go low,
turning transistor TR2 on, and supplying 9 volts to the low drop-out regulator,
U4. When the slide switch is returned to the ‘off’ position pin 1 of U2 is pulled
low, the bi-stable action is reversed, and TR2 will be turned off. If the unit is
left on without use for approximately 4 minutes then pin 44 of U5 is driven
high, under software control, turning on TR1 which will also turn the unit off
via the bi-stable circuit. When this happens the slide switch must be pushed
to the ‘off’ position and then to the ‘CO-PPM’ position in order to initiate
another pulse through C15 to turn the unit back on again. The output of the 5
volt regulator supplies the processor and associated circuitry. C13 and C14
smooth the input and output of U4.
BAT1 is monitored by an A/D input of the microcontroller, pin 62 through the
potential divider, R13 and R14. When this battery falls below 6.6 volts a
battery low warning is temporarily indicated on the display upon switch on.
When the battery falls below 6.1 volts the message is displayed permanently
and the unit cannot be used.

11

The secondary power supply is provided by a single 3.6 volt lithium cell,
BAT2, used to permanently power the fuel cell amplifier circuit consisting of
the op-amp U1 and associated passive circuitry. The amplifier circuit draws
less than 11uA from the 1000mA-Hr lithium battery giving a nominal
operational life of greater than 10 years.
BAT2 is buffered by U8B and monitored by an A/D input of the
microcontroller, pin 63. R27, between BAT2 and the inverting amplifier,
prevents the battery from discharging through the op-amp when the supply is
switched off. The battery has an end point of 3 volts and when this level is
reached the message bt2 will be displayed. When this happens follow the
procedure outlined in Servicing.

Fuel cell amplifier circuit.

The fuel cell provides an output current, from the counter electrode,
proportional to the concentration of target gas at the sensing surface. This
current is converted to a voltage by the action of U1 and the associated
passive components at a nominal transfer factor of 387mV/μA. This signal is
further amplified by U8A with a gain of 2.08 and applied the A/D input of the
microcontroller, pin 44, after being filtered by the action of R8 and C4. The
sensitivity of the CO fuel cell is 0.045 +/- 0.015μA giving a nominal amplified
output of 36mV/ppm CO.

Reset Circuit

This microcontroller has internal reset circuitry that requires C12 and D5 for
proper operation.

Display

The display is a custom 3½ digit low power LCD. The segments and
backplane are driven directly by microcontroller. The backplane is driven by a
square wave of nominally 60Hz. The individual segments are driven by a
similar square wave that is in phase with the backplane when the segment is
off and 180 degrees out of phase when the segment is on.

Sounder

The sounder is operated by a 1kHz square wave generated by pins 24 and 25
of U5.

12

Indicator lights

The three indicator LED’s are driven from pins 51, 52 and 53 of the
microcontroller through transistors TR3, TR4 and TR5.

RS232 interface

The transmit and receive ports for RS232 transmission are on pins 46 and 45
respectively of the microcontroller. The transmission is converted from logic
to RS232 levels by the U7.

13

Specifications

Type Electro - chemical fuel cell
Range 0 - 100ppm
Resolution 1ppm
Green indicator light 0 to 6ppm (0 to 1% COHb)
Amber indicator light 7 to 10ppm (1.1 to 1.6% COHb)
Red indicator light 11 to 72ppm (1.8 to 12% COHb)
Flashing red light + alarm >72ppm (>12% COHb)
Accuracy +/-5% of full scale of 1ppm whichever

is the greater

Sensitivity drift 0.5%/°C
Sensor Life >2 to 5 years
Response time <15 sec (to 90% of reading)
Hydrogen cross sensitivity <15%
Operating temperature 15 to 25°C
Operating pressure Atmospheric +/-10%
Pressure coefficient 0.02% signal per mBar
Relative humidity 15 – 90% continuous
(Non condensing) (0 – 99% intermittent)
Baseline drift 0ppm (auto zero)
Long term drift <2% signal loss per month
Power source Single Alkaline 9 volt PP3
Main battery life >30 hours of continuous use
Internal battery life 10 years
Weight 180g (Including battery)
Dimensions 170 x 60 x 26mm
Display 3½ digit LCD
Storage temperature -20 to +70 deg Celsius
Storage humidity 30% to 90%

14

Technical Support

Great Britain and World Headquarters

Micro Medical Ltd

PO Box 6
Rochester
Kent ME1 2AZ

Telephone + 44 (0)1634 893500
Fax [phone]
Web Site http://www.micromedical.co.uk
Email [e-mail]

Contact Micro Medical Ltd for the local agent in your region or
country for local service:

15

Parts List

Designation Description

U1 OP90GS. Precision micro power surface mount U2 BU4S11. Single NAND gate U3 BU4S11. Single NAND gate U4 LM2931M5.0. Low drop-out 5 volt regulator U5 HD64F3687FP. Hitachi microcontroller U6 24LC00. 16 Byte surface mount serial EEPROM U7 MAX3221CAE. Maxim RS232 transceiver U8 MCP602-I/SN. Microchip dual rail-to-rail OP-AMP D1 ZHCS750 Schottky diode D2 3mm green LED D3 3mm red LED D4 3mm yellow LED D5 BAS21. Small signal diode

DISPLAY (LCD 016-03) 3½ Digit custom display

TR1 DTC114EKA NPN transistor TR2 FMMT591 PNP transistor TR3 DTC114EKA NPN transistor TR4 DTC114EKA NPN transistor TR5 DTC114EKA NPN transistor

R1 330K Resistor
R2 12K Resistor
R3 1M Resistor
R4 10K Resistor
R5 10K Resistor
R6 1M Resistor
R7 1M Resistor
R8 100K Resistor

R9 10K Resistor
R10 560R Resistor
R11 560R Resistor
R12 820R Resistor
R13 100K Resistor
R14 100K Resistor
R15 100K Resistor
R16 10K Resistor
R17 1M Resistor
R18 10K Resistor
R19 10K Resistor
R20 10K Resistor
R21 27R Resistor
R22 1K Resistor
R23 10K Resistor
R24 180K Resistor
R25 360K Resistor
R26 390K Resistor
R27 1M Resistor

C1

C2 22pF Ceramic capacitor

C3

C4

10μF Tantalum capacitor

0.1μF Ceramic capacitor
1μF Ceramic capacitor

16

C5 22pF Ceramic capacitor

C6

C7

C8

C9
C10
C11

0.1μF Ceramic capacitor

0.1μF Ceramic capacitor

0.1μF Ceramic capacitor

0.1μF Ceramic capacitor
1μF Ceramic capacitor

0.1μF Ceramic capacitor
C12 10nF Ceramic capacitor
C13
C14
C15

0.1μF Ceramic capacitor

47μF Electrolytic capacitor

0.1μF Ceramic capacitor
C16 33pF Ceramic capacitor
C17 33pF Ceramic capacitor
C18
C19
C20
C21

1μF Ceramic capacitor
1μF Ceramic capacitor

0.1μF Ceramic capacitor

0.1μF Ceramic capacitor

TH1 NTC 47K thermistor
SW1 SLF2300. Double pole 3 position slide switch
SW2 Push switch

BAT2 SL235 CVHZ Sonnenschein 1A-Hr ½ AA Lithium cell

X1 14.745 MHz crystal

CO CELL 1000 ppm CO fuel cell

17

1

TR3

DTC114KE

6

32

11

D.G.Brown

R12
820R

D4
RED

D

C

B

A

54321

C11

0.1uF

R1

R2
12K

R22
1K

A1

B1

F1

C1

G1

D1

B2

E1

A2

DP1

F2

C2

G2

D2

NC

E2

B3

DP2

A3

C3

F3

D3

G3

E3

NC

1

NC

NC

FVC

NC

FEV1

PEF

NC

NC

BLOW

FER

BLOW

NC

BP

BP

DISPLAY_MICRO

DIS1

C17
33pF

330K

C21

0.1uF

3

2

21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40

5V

R18

10K

R19

10K

R20

10K

MIDI 40

(2112B3000 Eco-Sure)

TR1

1

DTC114EKA

18
17
16
15
55
56
57
58
40
39
38
37
32
33
34
31
30
29
28
50
49
48
22
21
20
19
14
13
47
41
42
43

P85
P86
P87

R9
10K

1

W

C

CELL

2

6

7

8

20
19
18
17
16
15
14
13
12
11
10

9
8
7
6
5
4
3
2
1

5

SW1B
SWITCH_SLF2300

D

D1

1 3

ZHCS750

12

C

B

A

1 2 3 4 56

23

BAT1
PP3

5V

R21
27R

C10
1uF

TR2

FMMT591

R13

100K

R14

100K

5V

R4
10K

C8

0.1uF

C6

0.1uF

C7

0.1uF

C9

0.1uF

1

8

C13

0.1uF

JP1

1

10K

R16

U4
LM2931M-5.0

VIN

VOUT

236

2

U7

1
2
3
4
5
6
7
8

MAX3221CAE

2

3

R5
10K

CALIB_SWITCH

EN
C1+
V+
C1 ­C2+
C2 ­V­RIN

1

7

47uF

MODE

FOFF

5V
TO

FON

TIN

INV

RO

J1

JACK

53

U2

R17

1

4

BU4S11

53

U3

4

BU4S11

5V

1

C14

+

5V

2SW2

1

5V

16
15
14
13
12
11
10
9

1M

2

R61MR7

1M

1

2

+

C1
10uF

C15

0.1uF
C2
22pF

X1

14.745

22pF

C5

RST

5V

C12
10nF

C3

0.1uF

LED3

BATT2

LED2
LED1

2SPK1
1

C18

1uF

2

3

4

D5

1 3

BAS21

U5
H83687

11

OSC1

10

OSC2

7

RST

8

TEST

4

X2

5

X1

12

VCC

9

GND

6

VCL

3

AVCC

54

P17/IRQ3

53

P16/IRQ2

52

P15/IRQ1

51

P14/IRQ0

25

P12

24

P11/PWM

35

NMI

46

P22/TxD

45

P21/RxD

23

P10

44

P20/SCK3

62

PB0/AN0

61

PB1/AN1

60

PB2/AN2

59

PB3/AN3

63

PB4/AN4

64

PB5/AN5

1

PB6/AN6

2

PB7/AN7

27

P57/SCL

26

P56/SDA

36

P60/FTIOA0

R23
10K

U6

SDA3Vcc

SCL1Vss

24LC00

1

SW1A
SLF2300

P67/FTIOD1

P66FTIOC1
P65/FTIOB1
P64/FTIOA1
P63/FTIOD0
P62/FTIOC0
P61/FTIOB0

P76/TMOV
P75/TMCIV
P74/TMRIV
P72/TXD_2
P71/RXD_2
P70SCK3_2

P55/WKP5
P54//WKP4
P53//WKP3
P52//WKP2
P51//WKP1
P50//WKP0

5

2

32

P37
P36
P35
P34
P33
P32
P31
P30

P24

P23
P85
P86
P87

5V

3.6V

+

-

OP90

4 7

TH1
47K

R3
1M

1

5

U1

8

5V

C19
1uF
for MCU

R24
180K

6

R15
100K

R25
360K

C16
33pF

3.6V

12

R8
100K

C4
1uF

LED1 LED2 LED3

1

DTC114KE

Title

B

Date: 21-Jan-2004 Sheet of
File: G:\Development\ARCHIVE\073-01.s ch Drawn By:

R26
390K

5V

U8A

2

­1

+

MCP602

4 8

BATT2

6

-

5

+

5V 5V 5V

R10
560R

D2
GREEN

32

1

DTC114KE

BAT2
BAT3.6V

TR5

3

R27
1M

C20

0.1uF

U8B

7

MCP602

R11
560R

D3
AMBER

32

TR4

Micro CO - Circuit diagram

Number RevisionSize

073-01 1.1

18