GB2239952A - Sensor system eg for chlorofluorocarbons (CFCs) - Google Patents

Abstract

A sensor system compares a measurement of CFC concentration with a stored threshold value which varies with time according to predicted variations in the background concentration of CFCs, thus avoiding unwanted alarm output signals caused by high background values. A CFC detector 2a has a housing including a detection section 13 containing a sensor. A fan is also located within the housing, and forces air past the sensor at a known rate, ensuring that the quantity of CFCs present can be measured accurately. A sensor network includes a number of base stations, each connected to a central processor, to which sensors (2a-2c, 3a-3c, 4a-4c, Figure 1) are interchangeably connected. The sensors may be CFC detectors, or pressure or temperature sensors providing data relevant to the control of the heating or air conditioning system in a building. <IMAGE>

Description

kEN$PR This invention relates to a sensor, particularly a sensor for chlorofltorocarbons, and to a network of such sensor.

Chiorofluorocarbons (CFCs) are widely used as refrigerants. Thus, in buildings such as supermarkets, large quantities of CFCs are present. It has been suggested that CFCs which leak from refrigeration apparatus can be a cauee of the destruction of ozonein the upper atmosphere. In addition, CFCs are expensive substances. It is therefore common to locate CFC detectors near large scale refrigeration apparatus, in order to detect leakages, which can then be stopped.

It is known to provide a CFC detector comprising a high voltage source which is used to create a discharge through a sample of the ambient atmosphere. Since CFC molecules have an affinity for electrons, the presence of CFCs in the atmosphere will tend to reduce the current flow, and hence a measurement of the current can be used as an indication of the proportion of CFCs present in the atmosphere.

However, the known crc sensors have a number of disadvantages. One disadvantage is that the sensors are able only to detect the presence of CFCs in their immediate vicinity, and thus it would be necessary to provide a large number of such sensors in order to determine accurately the source of any leak. A second disadvantage of the known sensors is that it is inevitable that there will be some leakage of CFCe from a refrigeration system, and that the atmosphere will contain a certain proportion of CFCs. In order to deal with this, the known sensors are deliberately made to be relatively insensitive in order to reduce the possibility of false alarms. However, this has the effect that the sensors are able to detect only very substantial leakages of CFCs.

According to a first aspect of the present invention, there is provided a sensor system including at least one CFC detector and a processing unit, wherein an output alarm signal is provided on the basis of a comparison of a value for the concentration of CFCs measured by the detector and a time dependent threshold value stored in the processing unit.

The time dependent threshold value can be calculated to take into account the fact that, even in the absence of any leakage, the background concentration of CFCe will vary, and the sensor can therefore be made more sensitive to any leaks which do occur.

According to a second aspect of the present invention, there is provided a CFC detector comprising a sensor having a housing, the housing having an inlet for gas, and the detector further comprising a fan for forcing gas to flow into the housing.

In one preferred embodiment, the sensor comprises a high voltage source for creating a discharge within the housing.

Preferably, the housing has an inlet manifold, and a plurality of input pipes connected thereto and supplying gas to the housing from a plurality of respective locations, which may be separated from one another.

This arrangement has the advantage that the respective locations may, for example, be spaced apart along a pipe which contains the refrigerant, and a single sensor unit can therefore detect any leak along section of the pipe.

According to a third aspect of the present invention, there is provided a sensor network including a plurality of sensor base stations each connected to a central processor, and a plurality of sensors, each of which may be interchangeably connected to any of the eensor base stations, wherein each of the sensors supplies to the central processor measurement signals indicative of a value of at least one variable property of the ambient atmosphere in the vicinity of the sensor, and wherein each sensor base station has a respective address signal stored therein for supply to the central processor together with measurement signals which are supplied to the central processor from a sensor connected thereto.

Preferably, at leapt some of the sensor units are crc detectors, while others may, for example, be temperature and pressure sensors. This allows a single sensor system, including a single processing unit, to be used for detecting CFC leakages and for supplying data which are necessary for the control of, for example, the heating and air-conditioning systems in a supermarket.

For a better understanding of the present invention, and to show how it may be brought into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a schematic diagram illustrating a sensor network in accordance with the first aspect of the present invention; Figure 2 shows a sensor in accordance with the second aspect of the present invention; and Figure 3 shows the sensor of Figure 2 and its connection to the sensor network.

Figure 1 shows a network including a central system executive 1, to which are connected a plurality of sensors 2a, 2b, ..., 2c, 3a, 3b, ..., 3c, 4a, 4b, ..., 4c, etc., which are located in respective zones 2, 3, 4, etc. The number of zones in the system, and the number of sensors in each zone, may be varied as required. In the preferred embodiment, the maximum number of sensors per zone is 16, as is the maximum number of zones in the system. However, if the number of sensors connected to a particular zone is reduced, the minimum interval between successive operations of a particular sensor is also reduced, and so the speed of response of the system may be improved.

The central system executive 1 includes a microcomputer programmed, in a manner known per gq, to carry out the functions described below. In addition, the central system executive includes power sources to drive the sensors and communications facilities for transmitting and receiving data to and from the sensors. The central system executive 1 includes a keypad, by means of which an operator can input data and access the functions of the system, and a display, by means of which information can be supplied to the operator.

Figure 2 shows a representative sensor 2a in the form of a CrC detector, which includes an intake section 11, an intermediate section 12, a detection section 13, and a connection 14. The intake section 11 includes a manifold with inlets 15a, lSb, 15c, Included within the intermediate section 12 are a filter, for removing solid impurities from the intake gas, and a fan. The detection section 13 includes a corona sensor, including a high voltage source and a current detector for measuring the discharge current created thereby. Power to the high voltage source, and measurement signals from the detection section, are passed between the connection 14 and the detection section 13 along the wire 16.

figure 3 shows the same sensor 2a connected to a remote unit 17, which has wires 18, 19 connecting it to the other sensors in the eame zone and to the central system executive 1. The wires 18, 19 are different sections of a twisted wire pair, which supplies power to the sensors, and also allows for data communication between the sensors and the central system executive 1.

Zn addition, each sensor includes a microcontroller, which allows a large amount of routine computing to be conducted at the point of measursment.

Such routines include health checks on the sensor and associated circuitry, multiple readings at variable time intervals, averaging data, exception reporting, and data storage for example. Updates in software and alterations in sensor routines may be loaded automatically from the central system executive, making it easy to tailor the system to a particular environment or customer.

When a particular sensor is activated, the fan within the intermediate section 12 is operated for a fixed period of time, for example 20 seconds, to establish a sample of the ambient air, Duxing the next 10 seconds, measurementg are taken from the characteristics of the corona discharge current. Prom this information, the quantity of CFC in the particular sample is computed, and is stored in the micro controller within the sensor in the form of a number within the range 0 to 127, with higher numbers representing higher concentrations. The measured value is then compared with a tolerance level, which is generally set centrally by the system executive 1, although the centrally set tolerance level may be manually changed at the sensor.In normal operation, each sensor within a particular zone is operated once every 10 minutes or so, but if a measured value exceeds the relevant tolerance level, the unit then operates continuously for a longer period, If the presence of CFCs in the atmosphere is then confirmed, the event is reported to the central system executive. An alarm may also be sounded. A reset button is provided, allowing the alarm to be silenced in the event that the CFCs are coming from a known source which is being dealt with, but if the alarm is silenced and the tolerance level is still exceeded after a period of one hour, the alarm will again be activated.

The sensor unit 2a has an intake manifold with more than one inlet. Tubes can be connected to the inlet manifold, to allow air to flow into the sensor from a number of different locations which may be located remotely from one another and from the sensor.

This allows a single sensor to sample the atmosphere at a number of locations, which may otherwise be relatively inaccessible, such as in ducts or in ceilings.

5n normal use of a refrigeration system, there will always be some leakages of crc.. The extent of these leakages will depend upon the amount of cooling which is being provided by the refrigeration system, and thus will tend to change throughout the day and throughout the year, depending upon the external temperature amongst other factors. In addition, the quantity of CFCs present in the atmosphere within a supermarket, for example, will be increased when aerosol sprays, using CFCs as a propellant, are used.

It may be that such sprays are used during cleaning operations, in which case they will tend to occur at regular times. In view of these factors, the background level. of CFCe vary, although on a basis which is relatively predictable. Thus, the central system executive iets tolerance levels which vary throughout the day and throughout the year on the basis of previously measured background levele, Thus, these tolerance levels can accurately represent the background level, and alarm signals can be provided when the CFC level is only slightly above an existing tolerance level, while being avoided if the only reason for a high level is that the background level has increased.

Moxeover, the provision of a large number of sensors spaced throughout a region means that it is possible to derive an accurate picture of the CFC levels throughout the atmosphere which is of interest.

This means that increases in the background level will tend to affect the readings at a large number of sensors, while increases resulting from leakages will tend to increase the readings at only a small number of sensors, at least initially. Moreover, it is possible to determine the source of a leak by considering which sensor or sensors first detect excess CFCs.

Signals sent from each sensor to the central system executive 1 include a data component derived from the sensor and an address component, indicating the location of the particular sensor. The address component is stored in a printed circuit board contained within the unit 17, thus allowing the sensors to be interchanged without reprogramming.

Although the invention has been described above with the sensors being CrC sensors, the invention also relates to a sensor network in which the sensors are of a variety of types, and may include CFC sensors, sensors for detecting other gases or airborne contaminants, temperature sensors and pressure sensors.

Using euch a network, a single system can provide data about a wide range of functions, and may, for example, be connected to a central computer controlling the heating or air-conditioning systems within a building.

Claims (12)

1. A sensor system including at least one CFC detector and a processing unit, wherein an output alarm signal is provided on the basis of a comparison of a value for the concentration of CFCs measured by the detector and a time dependent threshold value stored in the processing unit.

2. A system as claimed in claim 1, wherein the threshold value is calculated to take into account a variable background concentration of CFCs.

3. A CFC detector comprising a sensor having a housing, the housing having an inlet for gas, and the detector further comprising a fan for forcing gas to flow into the housing.

4. A detector as claimed in claim 3, wherein the sensor comprises a high voltage source for creating a discharge within the housing.

5. A detector as claimed in claim 3 or 4, wherein the housing has an inlet manifold, and a plurality of input pipes connected thereto and supplying gas to the housing from a plurality of respective locations, which may be separated from one another.

6. A sensor network including a plurality of sensor base stations each connected to a central processor, and a plurality of sensors, each of which may be interchangeably connected to any of the sensor base stations, wherein each of the sensors supplies to the central processor measurement signals indicative of a value of at least one variable property of the ambient atmosphere in the vicinity of the sensor, and wherein each sensor base station has a respective address signal stored therein for supply to the central processor together with measurement signals which are supplied to the central processor from a sensor connected thereto.

7. A sensor network as claimed in claim 6, wherein at least some of the sensor units are CFC detectors.

8. A sensor network as claimed in claim 7, comprising CFC detectors as claimed in any one of claims 3 to 5.

9. A sensor system as claimed in claim 1 or 2, comprising a CFC detector as claimed in any one of claims 3 to 5.

10. A sensor system substantially as herein described with reference to the accompanying drawings.

11. A CFC detector substantially as herein described with reference to, and as shown in, Figures 2 and 3 of the accompanying drawings.

12. A sensor network substantially as herein described with reference to, and as shown in, Figure 1 of the accompanying drawings.