Contents page
Safety Precaution 2
Important safeguards 2
Looking after your
Micro CO meter 2
Introduction 2
Before you begin 2
Micro CO meter
system Overview 3
CO sensor 5
Micro CO meter
exploded view 6
Disassembling the
Micro CO meter
for Servicing 7
Replacing the service
parts of the Micro CO 7
Reassembling the
Micro CO meter 8
Calibrating the
Micro CO meter 9
Circuit description 10
Specifications 13
Technical support 14
Parts List 15
Circuit Diagram 17
Micro CO Meter
Service Manual
037-16 Iss. 1.2 September 1998
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 1998 by Micro Medical Limited All rights reserved
Drawing no. 037-16
Version 1.2
September 1998
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
o Read all of the instructions.
o Keep the instructions in a safe place for later use.
o Follow all warnings and instructions marked on the product.
o 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.
o Do not place on an unstable table.
o The product should be operated only from the type of power source indicated
on the label.
Looking after your Micro CO Meter
o Avoid exposing the Micro CO Meter to direct sunlight.
o Avoid operating the Micro CO Meter in dusty conditions or near to heating
appliances or radiators.
o 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).
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.
3
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
inputs of the microprocessor.
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 20
seconds, and then to expire fully through the mouthpiece connected to the
microcomputer unit with the mouthpiece adapter.
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
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 as with other fuel cells.
The fuel cell uses a three electrode design which gives increased
selectivity to the measured gas compared with two electrode designs. The
three electrode cell consists of a sensing electrode, a counter electrode
and a reference 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 sensing electrode reacts at the surface of the
electrode by oxidation.
CO reacts at the sensing 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
The CO fuel cell requires a bias voltage of 220mV to be supplied
permanently to the reference electrode.
The low power circuitry supplying this voltage, is powered by an internal,
950mA –hr lithium cell with an operational life greater than the life of the
electrochemical fuel cell.
5
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