GB2041539A

GB2041539A - Oxygen level meter

Info

Publication number: GB2041539A
Application number: GB8004315A
Authority: GB
Original assignee: TELEGAN Ltd
Current assignee: TELEGAN Ltd
Priority date: 1979-02-09
Filing date: 1980-02-08
Publication date: 1980-09-10
Also published as: GB2041539B

Abstract

This invention is a portable oxygen level meter which incorporates an oxygen level cell (11) having a sensitive area exposed to an opening (16) protected by a filter (15) and opening into a duct (22) through which exhaust gases can be pumped from the stack at (23) by a hand pump (24). A digital volt meter (19) is calibrated in terms of oxygen level. It is also possible to incorporate in the meter a switch enabling a reference supply to be supplied between terminals (31), or to give a measure of a voltage applied between terminals (31), or to give a measure of stack gas temperature, or to give a measure of the efficiency of the boiler. <IMAGE>

Description

SPECIFICATION Oxygen level meter This invention relates to an oxygen level meter for example for measuring the amount of oxygen in the atmosphere, possibly in a mine, or a factory, but particularly in the exhaust gases in a stack from a boiler. The amount of free oxygen present in the exhaust gases is a measure of the efficiency of combustion, and if it can be maintained at at least one fairly low level to be sure that there is sufficient oxygen for complete combustion, and below another slightly higher level to be sure that there is not too much excess oxygen to be heated, the fuel can be burnt very efficiently.

A portable meter enabling the oxygen level in a gas sample to be measured can therefore be a most useful tool.

According to the present invention a portable oxygen level meter comprises a duct and means for passing a sample of gas through the duct, the duct having an opening exposing gas passing through the duct to the sensitive area of an oxygen level cell.

Certain oxygen level cells are known, but a particularly suitable cell consists of a metal/air battery with a metal anode separated from a cathode by an electrolyte, the surface of the cathode being exposed to gas through a gaseous diffusion barrier in the form of a capillary device. At the cathode, oxygen is reduced to hydroxyl ions, which oxidise the metal anode and the current is proportional to the rate of consumption of oxygen, so that if the cell is connected across a fixed resistor, then a volt meter connected across that resistor gives a direct measure of the rate of use of oxygen.

It has been found that such a cell is very sensitive to water in the gas sample, and accordingly in a preferred form of the invention, the gas in the duct is exposed to the sensitive area of the cell only through a filter capable of passing gas but not water, for example, a disc with very fine pores.

The means for passing the sample of gas through the duct conveniently comprise a hand pump, and an inlet tube which can be inserted in the stack or in the other source of the gas whose oxygen level is to be measured. The passage to the duct is preferably arranged to cool the gas, possibly merely by being a long passage with a large heat loss, and then there may be a lambswool or other filter for filtering out much of the water obtained from condensed steam as the gas is cooled. Such a filter can hardly remove all the water, and the micro-porous disc is also advisable at the inlet to the cell.

In order to prevent the gas present in the duct from being diluted by an ingress of fresh air through the exhaust port of the instrument when the pump is not being operated, the duct may lead to exhaust through a long tortuous path.

The cell may be connected to a digital volt meter directly calibrated in percentage oxygen level. That may be by way of an amplifier, or by way of an adjustable calibrating resistor or both.

An amplifier would require a battery to power it, and such a battery could of course be used as an external source of reference voltage, for example, for testing meters. The volt meter for measuring the oxygen level could also be used for measuring signals obtained at various points in control apparatus, and according to a preferred form of the invention, a portable meter is capable of being set to measure oxygen level, or to provide a reference potential and/or to measure a signal voltage.

Where the meter is to be used for measuring the oxygen level in the exhaust gases in a boiler stack, it would also be convenient if a measure of the stack gas temperature could be easily obtained, and in a modified form of the meter there are means for connecting the output of a thermocouple in the stack between terminals for a voltage to be measured or for providing a reference potential, with the meter being adapted to be set to give a direct indication of gas temperature. Once that temperature is known, the boiler efficiency can be fairly simply calculated if a computer can be supplied with inputs representing the stack gas specific heat the rate of fuel supply, and the calorific value of the fuel. And accordingly the meter may also be arranged to give a measure of the boiler efficiency which varies with stack gas temperature and fuel setting.

The invention may be carried into practice in various ways, and one embodiment will now be described by way of example, with reference to the accompanying drawings; of which Figure 1 is a view of the casing of a portable multi-purpose meter embodying the invention; Figure 2 is a diagram showing the construction and arrangement of the meter; and Figure 3 is a diagram showing a modification.

The meter involves the use of an oxygen level cell 11, consisting of a metallic anode 12, an electrolyte 13, and a cathode 1 4 having a sensitive surface protected by a diffusion barrier 1 5 and a disc 1 6 having very fine pores capable of passing gas but not water. The anode and cathode are connected across a potentiometer 17, whose tapping is connected through an amplifier 18, and a digital volt meter 1 9 in series to earth.

The exposed face of the disc 1 6 is mounted against an opening 21 in the wall of a duct 22, through which gas to be tested is pumped from an inlet 23 by way of a hand pump 24, and a lamb's wool filter 25. The exhaust from the duct consists of a long tortuous passage 26 leading to atmosphere at 27.

The inlet may for example be inserted through a small hole in the side of a boiler chimney stack, so that exhaust gases are pumped into the duct 22 when the hand pump is operated. The inlet path 23 is arranged to be long enough for the gases to be cool, so that much of the moisture is condensed and caught in the filter 25. The porous disc 1 6 prevents any remaining moisture from reaching the exposed surface of the cathode 14, and preventing a correct reading being given.

The amplifier is powered from a battery 28 and by operation of a switch 29 the cell 11 and amplifier can be disconnected from the meter 1 9 and the output from the battery connected to a pair of output terminals 31 to be available as a source of a signal, whose value is indicated on the meter at 19, for test purposes through plug-in connections.

In a third position of the switch 29, a voltage received through plugs at the sockets 31 can be arranged to be directly read at the volt meter 19, so that the value of that signal can be easily measured.

The invention enables a small hand-held unit to be used for measuring oxygen level, or alterDatively for providing a reference voltage, or for measuring a voltage signal.

A 'battery low' lamp 33 can be arranged to give an indication when the battery needs replacing.

It is also possible to obtain a measure of a temperature, for example, the temperature of the gases in a boiler stack. A thermocouple 35 is positioned in the stack and electrically connected between the terminals 31. In a further position of the switch 29, the circuit is as shown in Fig. 3, and the variable resistors 36, 37, 38, 39 can be adjusted so that stack temperature is directly indicated on the meter 19.

The efficiency of a boiler can be estimated knowing the stack gas temperature, the stack gas specific heat, and the rate of fuel supply and its calorific value. The meter can embody a calculator circuit arranged to give a direct measure of that efficiency, using the measured temperature and set values for the other parameters.

Claims

1. A portable oxygen level meter comprising a duct and means for passing a sample of gas through the duct, the duct having an opening exposing gas passing through the duct to the sensitive area of an oxygen level cell.

2. A meter as claimed in Claim 1 in which the cell consists of a metal/air battery with a metal anode separated from a cathode by an electrolyte, the surface of the cathode constituting the sensitive area.

3. A meter as claimed in Claim 1 or Claim 2 including a gaseous diffusion barrier in the form of a capilliary device between the sensitive area of the cell and the opening in the duct.

4. A meter as claimed in Claim 3 in which the diffusion barrier is capable of passing gas but not water.

5. A meter as claimed in any of the preceding claims in which the sample passing means comprises a hand pump leading from an inlet tube.

6. A meter as claimed in any of the preceding claims including a filter between the sample passing means and the duct for filtering out water from condensed steam.

7. A meter as claimed in any of the preceding claims in which the duct leads to exhaust through a long tortuous path.

8. A meter as claimed in any of the preceding claims including a digital volt meter calibrated in percentage oxygen level.

9. A meter as claimed in any of the preceding claims including a source of power constituting a battery.

10. A meter as claimed in Claim 9 including a switch enabling the meter to be set to read either percentage oxygen level, or a voltage applied to input terminals.

11. A meter as claimed in Claim 10 in which in one position of the switch a thermo- couple for measuring a temperature is connected between the terminals.

12. A meter as claimed in Claim 10 or Claim 11 in which in one position of the switch, the battery is arranged to provide a reference potential between the terminals.

1 3. A meter as claimed in Claim 11 including a computing device arranged to give a direct measure of efficiency using as one input a temperature signal supplied between the terminals.

14. An oxygen level meter constructed and arranged substantially as herein specifi cally described with reference to Figs. 1 and 2, or Figs. 1 and 2 as modified with reference to Fig. 3 of the accompanying drawings.