GB2099588A - Fluid component measuring and alarm device - Google Patents

Abstract

A device for sensing the presence of, or determining the concentration of, at least one component in a fluid comprises first and second matched measuring devices A, B each responsive to the same component, first and second matched modifying devices 12, 16, each for modifying a fluid sample so as to modify the concentration of the component to which the devices A, B are responsive by an amount dependent on the concentration of the component to be detected, a pump means 14 operable in two opposite directions to alternately draw unmodified fluid through one measuring device A and then its associated modifying device 12 and at the same time expel modified fluid from the other modifying device 16 to the other measuring device B and vice versa, the pump means 14 operating in each direction for equal periods of time and means 24 to continuously sum the outputs from each of the measuring devices A, B. <IMAGE>

Description

SPECIFICATION Improved alarm device It is often a requirement in an industrial environment, such as a coal mine or chemical production plant, that a dangerous substance, frequently a gas, is detected and an alarm given.

A detector for this purpose should operate continuously for long periods, often weeks or months, should be capable of accurate measurement of the concentration of the substance, i.e. zero drift should be minimised, and should be capable of reacting very quickly to the presence of gas, i.e. should have minimum "dead time" because a few minutes or even several seconds warning of danger may be useful in the circumstances.

A fluid monitoring device in which zero drift is minimised is described in the specification of GB Patent No. 1,559,683; the same sample of an atmosphere is passed through a sensor before and after a chemical modifying treatment, and the two exposures provide a measuring signal and a zero setting signal. Cyclic operation of such a device minimises zero drift so that the sensor gives a measurement of concentration which is accurate over long periods.However, a disadvantage of such an arrangement is that there is a considerable "dead time"; the device is not insensitive to a sudden change in concentration when the modified sample is passing through the sensor, and, since most sensors do not act instantaneously but build up a response over a period of seconds or even minutes, there is an additional period of low sensitivity just after an unmodified sample has started to pass through the sensor.

In the present invention, the "dead time" of the known type of monitor is minimised.

In the aforementioned patent specification, it is explained that the sensor is not necessarily sensitive to the gas which is to be sensed but is sensitive to an "active component" which, in addition to being the gas to be sensed may in some circumstances be a gas to which the gas to be detected is converted by the modifying step, or may be a gas whose concentration in the atmosphere under test is related to the gas to be detected. As an alternative to sensing a gaseous component of an atmosphere, the device may be used to sense the presence of a gas dissolved in a liquid, or a liquid component of a liquid mixture.

These possibilities are also applicable to the present invention.

According to the invention an improved alarm device for sensing the presence of at least one component in a fluid comprises first and second matched measuring means, each responsive to the presence of the same active component; first and second matched modifying means, each for modifying a sample of the fluid so as to modify the concentration of the active component by an amount which is dependent on the concentration in said fluid of the at least one component to be detected; pump means operable in two opposite directions, and arranged alternately to draw an unmodified sample of the fluid past one measuring means into the associated modifying means and simultaneously to expel a modified sample of the fluid from the other modifying means past the associated measuring means, and vice versa, the pump means operating in each direction for equal periods of time; and summing means to sum continuously the output signals from the first and the second measuring means.

In a preferred embodiment the apparatus is arranged in the order:-- first measuring means: first modifying means; pump means; second modifying means; second measuring means. The pump means may be two unidirectional pumps switched to operate alternately, or may be a single reversible pump.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of apparatus according to the invention; Figure 2 illustrates the form of the outputs from the two measuring means.

Figure 1 shows a first general filter 10, a first measuring means A of through-flow type, a first modifying means 12, a reversible vane pump 14, a second modifying means 16, a second measuring means B of through-flow type, and a second general filter 1 8. The items of apparatus are connected together by a pipe 20, the operation of the pump 14 is controlled by a timer and switch 22, and the first and second measuring means A and B are connected to an adding circuit 24 which supplies an output signal 26 to an alarm device or a display device.

Consider a system suitable for detecting the presence of a dangerous concentration of carbon monoxide in air. The first and second measuring means are electrochemical detectors sensitive to carbon monoxide, which in this case is both the component of the fluid to be sensed and the active component. The first and second modifying means are carbon monoxide scrubbers, such as palladium/alumina mixtures which catalyse the oxidation of CO gas by atmospheric oxygen to CO2 gas; the electrochemical detectors are insensitive to CO2.

In this system the general filters 10, 1 8 are not essential since no additional component of air is likely to interfere with the sensors, but the filters may be used to remove, for example, water vapour from the gas samples drawn into the apparatus.

In operation the pump 14 operates in the direction to draw a sample of the atmosphere through filter 10, through the first measuring means A, which provides an output signal proportional to the concentration of any carbon monoxide in the sample, and into the modifying means, in which any carbon monoxide is converted to carbon dioxide. When the pump operates in the reverse direction, the modified sample is expelled through the first measuring means, which provides a signal corresponding to absense of CO gas, through the filter 10 to the atmosphere; this second signal from the first measuring means can be regarded as a zero-reset signal.

Figure 2 illustrates the form of the output signal A' from measuring means A, and shows the relatively slow rise to a high signal level corresponding to CO at the concentration C' below a predetermined danger level, followed by a similar slow fall to the zero-reset level.

Simultaneously, the second measuring means B is working in the reverse direction, providing a decreasing signal B' when signal A' is increasing and vice versa, as can be seen in Figure 2.

The first and second measuring means are matched, i.e. they have very similar responses to carbon monoxide, and for each measuring means the response with time to the presence and absence of carbon monoxide is similar, therefore the sum of the two signals A' and B' is approximately equal to the maximum true output of one of the sensors; this sum can be compared with a danger level signal to provide an alarm. The repeated cycling gives repeated correction of zero drift by resetting the zero for each device alternately, but since one measuring device is responsive to gas concentration while the other measuring device is being reset, the "dead time" period during which an ineffective response is provided is minimised.

"Dead time" is not completely eliminated; referring again to Figure 2, in the period t device A has just begun its exposure to modified gas and is completely insensitive to a change in concentration of CO in the atmosphere, and device B is just beginning its response to CO in the atmosphere; if there is a small increase in concentration C', the sum of the signals would not reach a level at which an alarm is triggered, but the period t is very short compared with the time needed to zero-reset one of the devices.

Further, if the concentration of CO increased very suddenly to an amount well above the danger level, the response of the operative measuring means would follow a much steeper curve than that illustrated for concentration C', and the danger level signal would be triggered, giving a warning in a minimum time of a very high concentration. In some circumstances this may be useful.

A further advantage is that the output signal is approximately constant if the gas concentration is constant, in contrast with the alternately increasing and decreasing signal provided by a cyclically rezeroed single sensor.

An example of suitable measuring means A and B are polarographic cells using sensing electrodes of metallised membrane form, e.g. a 12 micron thick layer of polytetrafluoroethylene onto which is sputtered a layer of platinum, typically 10 millimetres in diameter, and which can be used to sense carbon monoxide at concentrations between 10 and 1,000 parts per million.

As an alternative to a reversible vane pump 14, two unidirectional pumps can be used, working in opposite directions and controlled by the time switch 22 to operate alternately for equal periods.

The invention has been described with reference to sensing a gas, carbon monoxide, which is itself the active species, is sensed by the measuring means A, B, and is acted on by the modifying means 12, 1 6. As described in the specification of GB Patent No. 1,559,683 other arrangements are possible. An inactive component to be sensed can be converted to an active component and the concentration of that component is then sensed.

If, for example, nitric oxide gas, NO, is to be detected, then a measuring means sensitive to nitrogen dioxide gas, NO2, and insensitive to NO can be used; a sample of atmosphere is drawn through the measuring means which gives a zeroreset signal; the modifying means comprises a layer of chromic acid which converts NO to NO2, and the second passage of the sample past the measuring means provides a signal proportional to NO2 concentration, which is directly proportional to the concentration of NO in the original sample.This arrangement can be used to sense NO even in the presence of NO2, because the signal as a sample is drawn in will given concentration of NO2 in the mixture, and the signal as the sample is expelled will give concentration of both original and converted NO2; subtraction of the signals gives the concentration of NO2 converted from NO.

In another arrangement, vinyl chloride monomer can be sensed by providing measuring means A, B, sensitive to chlorine and permanganate modifying means 12, 16. The indrawn samples then provide zero reset signals, the vinyl chloride contents of the samples are proportionately converted to chlorine, and the expelled samples give the concentrationmeasuring signals.

In another modification, if the sum of the concentrations of an active component and an inactive component of an atmosphere are required, the inactive component may be converted to the active component by the filters 10, 18, and the sum of the components will then be sensed. For example, the sum of the concentrations of NO gas and NO2 gas may be measured by drawing samples through chromic acid layers in filters 10, 1 8 to convert NO to NO2, past NO2 measuring means A, B to give concentration-dependent signals, into NO2 scrubbing layers in the modifying means 12, 16, and then expelling the scrubbed samples to give zero-reset signals.

In another modification, if an atmosphere contains two active components and the concentration of only one component is required, the addition of selective scrubbing layers in the filters 10, 18, may be required. For example, if the concentration of CO gas is to be measured in the presence of NO2 gas, and the measuring means A, B, are sensitive to both gases, the provision of charcoal layers in the filters allows the removal of NO2 gas before the sample reaches the sensor.

In another modification, the concentration of one gas in a gas mixture may be measured by measuring the concentration of another gas in the mixture, modifying the gas to be measured so as to change the partial pressure of said another gas, and again measuring the concentration of said another gas in the modified sample.

In one arrangement, samples are caused to pass over absorption layers which completely remove the gas to be measured. For example to measure concentration of carbon dioxide gas in the atmosphere it is possible to measure the oxygen concentration in samples of the atmosphere, pass the samples through a CO2 absorbent in the modifying means, and measure the oxygen concentration in the modified samples. The difference in the measured partial pressures of oxygen can be used to calculate the CO2 concentration.

In another arrangement samples are caused to pass over catalytic or reactive means to convert the gas to be measured into another gas to which the measuring means are not sensitive; to alter the partial pressure in this case it is essential that each molecule of the gas to be measured is converted to a higher or lower number of molecules of said another gas.

In yet another arrangement, the gas to be measured is caused to react with the active component itself, which must be present in excess.

While the invention has been described with reference to the measurement of the gases CO, CO2, NO, NO2, and vinyl chloride in air, it is not CO2 NO NO limited to such gases but can be used to determine the concentrations of other gases in air, of a component of a gas mixture, and of a component of a liquid mixture or solution.

Claims (10)

Claims

1. An alarm device for sensing the presence of at least one component in a fluid comprises first and second matched measuring means each responsive to the presence of the same active component; first and second matched modifying means each for modifying a sample of the fluid so as to modify the concentration of the active component by an amount which is dependent on the concentration in said fluid of the at least one component to be detected; pump means operable in two opposite directions, and arranged alternately to draw an unmodified sample of the fluid past one measuring means into the associated modifying means and simultaneously expel a modified sample of the fluid from the other modifying means past the associated measuring means, and vice versa, the pump means operating in each direction for equal periods of time; and summing means to sum continuously the output signals from the first and second measuring means.

2. An alarm device according to Claim 1 in which the apparatus is arranged in the sequence, first measuring means; first modifying means; pump means; second modifying means; second measuring means.

3. An alarm device according to Claim 1 or Claim 2 in which the pump means is a reversible pump.

4. An alarm device according to Claim 1 or Claim 2 in which the pump means comprises two unidirectional pumps arranged to operate alternately.

5. An alarm device according to any preceding claim further comprising means to compare said continuous sum of the output signals with a predetermined value and to provide an alarm signal when the predetermined value is reached.

6. An alarm device according to any preceding claim for sensing a component of the fluid which is an active component, in which the first and second modifying means remove the active component from each sample.

7. An alarm device according to any one of Claims 1 to 5 in which the first and second modifying means convert the at least one component in each sample into an active component to which the first and second measuring means are responsive.

8. An alarm device according to any one of Claims 1 to 5 for sensing at least one component of a fluid, the concentration of the component being dependent on the concentration of an active component in which the first and second modifying means remove the active component from each sample.

9. A method of sensing the presence of at least one component of a fluid comprises exposing first and second matched measuring means each responsive to the presence of the same active component to alternate successive samples of the fluid; modifying each sample of the fluid so as to modify the concentration of the active component by an amount which is dependent on the concentration in said fluid of the at least one component to be detected; and expelling each modified sample past its respective measuring means, the arrangement being such that one measuring means is exposed to an unmodified sample while the other measuring means is exposed to a modified sample; and continuously summing the output signals from the first and second measuring means.

10. An alarm device substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.