GB2219403A

GB2219403A - Self-testing combustion monitor

Info

Publication number: GB2219403A
Application number: GB8808204A
Authority: GB
Inventors: Terry M Egan
Original assignee: TELEGAN Ltd
Current assignee: TELEGAN Ltd
Priority date: 1988-04-08
Filing date: 1988-04-08
Publication date: 1989-12-06
Anticipated expiration: 2008-04-08
Also published as: GB2219403B; GB8808204D0

Abstract

A combustion monitor has a gas sensor and a temperature sensor, and automatic self-test means arranged to supply the sensor with a test gas sample, for example of atmospheric air, and to provide an indication if the resultant output lies outside a given range. Automatic temperature-correction means corrects the sensor output in dependence upon an output of the temperature sensor. Thus, the combustion monitor automatically self-tests and checks its own accuracy before any actual measurements are taken. The sensor may be electrochemical sensing oxygen or carbon monoxide. Temperature correction may be in dependance upon the difference between the sensor temperature and its temperature when it was last calibrated. If the output is outside the given range then further measurement is inhibited either until the monitor has been recalibrated or until it has successfully passed a subsequent self-test.

Description

COMBUSTION MONITOR The present invention relates to a combustion monitor, for example for monitoring the exhaust gases in a stack from a boiler.

Various types of combustion monitor are known, for example those described in UK patents 2,041,539 and 2,064,780. A common feature of all such monitors is a gas sensor (usually an oxygen meter), whereby the oxygen concentration in the exhaust gases from the boiler can be determined. The difficulty that arises is that such monitors do not necessarily produce an output which is representative of the oxygen concentration being measured unless rather careful precautions are taken. For example, the output of an oxygen meter will depend to some extent upon its age, upon its past history, upon the temperature at which the measurement is being taken and upon factors intrinsic to the meter- itself. Accordingly, oxygen meters always have to be calibrated prior to use.

One known method of calibration is the "negative offset" method, which is described in UK patent 2,064,780. In this method, prior to a measurement being taken with the instrument the oxygen cell is supplied with a standard sample, such as atmospheric air, and the oxygen cell output adjusted by applying a negative offset or calibration correction so that it reads the correct value; in the case of atmospheric air, of course, this will be 20.9%. The same calibration correction is then applied to the oxygen cell output when it is measuring the oxygen concentration of gases in the stack.

A difficulty with this method is that the measured oxygen concentration may not be that accurate, particularly because the temperature of the meter during use is not likely to be the same as the temperature at which it was calibrated.

It is an object of 'the present invention to provide a combustion monitor which at least alleviates these difficulties.

It is a further object of the invention to provide a combustion monitor in which the oxygen or other gas sensor output is of improved accuracy.

It is yet another object of the invention to provide a combustion monitor having provision for a self-test procedure, while avoiding the negative offset method described in GB2,064,780.

According to a first aspect of the present invention, a combustion monitor has a gas sensor and a temperature sensor, automatic self-test means arranged to supply the sensor with a test gas sample and to provide an indication if the resultant output lies outside a given range; and automatic temperaturecorrect ion means arranged to correct the sensor output when supplied with a gas to be monitored in dependence upon an output of the temperature sensor.

The sensor is desirably an oxygen sensor; but carbon monoxide or other sensors could also be used.

Conveniently, the sensor is electrochemical.

Desirably, the test gas sample may be atmospheric air, for which the oxygen concentration is known to be about 20.9%.

The sensor may include manual calibration means (for example a potentiometer), whereby its output can be adjusted. The temperature-correction means may be arranged to correct the sensor output in dependence upon the difference between the temperature measured by the temperature sensor, and the temperature at the time that the sensor was last manually calibrated.

Conveniently, this latter value may be stored in a microprocessor memory.

The said given range may comprise a fixed window or alternatively the size and/or position of the range may be dependent upon the output of the temperature sensor at the time that the monitor is undergoing automatic self-test. It is possible, although not essential, for the sensor output during the self-test phase to be corrected in dependence upon the output of the temperature sensor at the time.

If the resultant sensor output during the selftest lies outside the given range, then a visual or audible warning may be given. Alternatively, and preferably, the instrument is also arranged so that in this event the taking of measurements is inhibited (at least until the instrument has either been recalibrated manually or has successfully passed a subsequent selftest).

The automatic temperature correction means conveniently comprises a microprocessor, which may also be used to undertake any calculations that may be required, for example of.combustion efficiency. The monitor may provide visual indications (either all at once, or as selected by the user) of any or all of the following: temperature, oxygen concentration, carbon dioxide concentration, carbon monoxide concentration, ratio of carbon dioxide to carbon monoxide (or its inverse) and combustion efficiency. It will be evident, of course, that suitable sensors may be provided for these and for any other similar measurements that may be required.

The invention also extends to a method of using a combustion monitor, and to a method of monitoring combustion. In particular, according to a second aspect of the invention a method of monitoring combustion comprises providing a gas sensor with a test gas sample, automatically testing the sensor by determining whether the resultant sensor output lies inside a given range, and if not, providing an indication of test-failure; and if the sensor passes the test, automatically correcting the output of the sensor, when subsequently supplied with a gas to be monitored, in dependence upon the measured temperature of the gas.

The invention may be carried into practice in a number of ways, and one exemplary embodiment will now be described.

The exemplary combustion monitor is a portable device, operating as a hand-held unit, and having a probe which can be inserted into the stack. Gases from the stack are drawn through the probe, by means of an electric pump, and into the body of the unit where they are analysed. The end of the probe incorporates a thermocouple for determining the temperature of the exhaust gases.

The unit includes an oxygen sensor, a carbon dioxide sensor, and a carbon monoxide sensor. Digital displays are provided so that the iiser may selectively view the temperature, the oxygen concentration, the carbon monoxide concentration, the carbon dioxide concentration, or the ratio of oxygen to carbon monoxide. The latter ratio is calculated by a microprocessor within the unit. In addition, although not necessarily, the microprocessor may also be programmed to provide a read out of combustion efficiency in a conventional manner (for example by the use of the standard Seigert's formula).

The way in which the oxygen sensor output is calibrated and checked will now be described.

Initially, the unit is factory calibrated by supplying the oxygen sensor with atmospheric air and manually adjusting a potentiometer until the sensor gives the expected reading of 20.9%. The purpose of this is to allow for variation in output between different sensors. Let the temperature at this time be Tf.

On site, normal operation of the instrument consists of a period of automatic self-test followed by a measurement period.

During the self-test procedure, the sensor is supplied with atmospheric air and the output corrected to allow for the difference between the test temperature Tt and Tf. This correction may take the form of multiplying the sensor output by a correction factor, the relevant correction factor being chosen from a table of values stored in the microprocessor memory, with addressing of the table being by the value of Tf alone. However, it is possible that the correction could also take some other form, such as an application of some predetermined formula to the values of Tf and to the sensor output.

The temperature-corrected output is then compared with a window, the position and size of which may vary with Tt. For example, the window size could be a constant value plus a certain proportion of Tt; or alternatively it could be chosen from a look-up table in dependence upon the value of Tt. If the corrected output falls within the window, the instrument proceeds to the next stage; otherwise, it indicates self-test failure, and must be returned to the factory for manual recalibration.

Once the instrument has passed the self-test procedure, it proceeds to the measurement stage. The probe is now inserted into the stack, so that the oxygen sensor is supplied with the gas to be measured.

The output of the sensor is corrected to allow for the difference between the measurement temperature Tm and Tf. The correction could, for example, take the form of a multiplicative value comprising a constant plus a fixed proportion of the temperature difference. The resultant corrected value is displayed, and may then be used in the calculation of combustion efficiency, for example using Seigert's formula. It is to be noted that the sensor output during the self-test phase is not used in any way during the measurement phase. The only information which is passed to the measurement phase is the fact that the instrument has passed the window test.

Claims

1. A combustion monitor having a gas sensor and a temperature sensor, automatic self-test means arranged to supply the sensor with a test gas sample and to provide an indication if the resultant output lies outside a given range; and automatic temperature correction means arranged to correct the sensor output when supplied with a gas to be monitored in dependence upon an output of the temperature sensor.

2. A combustion monitor as claimed in Claim 1 in which the sensor is electrochemical.

3. A combustion monitor as claimed in Claim 1 or Claim 2 in which the sensor is an oxygen sensor.

4. A combustion monitor as claimed in Claim 1 or Claim 2 in which the sensor is a carbon monoxide sensor.

5. A combustion monitor as claimed in any one of the preceding claims in which the automatic self-test means are arranged to supply the sensor with atmospheric air.

6. A combustion monitor as claimed in any one of the preceding claims in which the temperaturecorrection means is arranged to correct the sensor output in dependence upon the difference between the temperature measured by the temperature sensor, and the temperature at the time that the sensor was last manually calibrated.

7. A combustion monitor as claimed in any one of the preceding claims in which the sensor output during the self-test phase is corrected in dependence upon the output of the temperature sensor at the time.

8. A combustion monitor as claimed in any one of the preceding claims in which if the resultant sensor output during the self-test lies outside the given range, then a visual or audible warning is given.

9. A combustion monitor as claimed in any one of Claims 1 to 7 in which, if the resultant sensor output during the the self-test lies outside the given range, then the taking of measurements is inhibited at least until the monitor has either been recalibrated manually or has successfully passed a subsequent self-test.

10. A combustion monitor as claimed in any one of the preceding claims in which the automatic temperature correction means comprises a microprocessor.

11. A combustion monitor as claimed in Claim 10 in which the microprocessor is further used to calculate combustion efficiency.

12. A combustion monitor substantially as specifically herein described.