The present invention relates to systems providing an accurately variable known concentration of a vapour (that is, the gaseous form of a substance which is liquid at Standard Temperature and Pressure [STP]) in an airstream, and is particularly concerned with such systems for use in calibrating instruments such as gas detection devices.

Gas detection devices which provide a measure of tracer or pollutant gas (henceforth, for convenience, referred to as test gas) in an airstream are well known. Devices which operate on the principle of ionisation of the test gas by means of ultra-violet light are described in, for example, GB 1576474 and PCT/GB ^Q 2/01313. In the devices described in these documents an airstream containing test gas is subjected to ultra-violet light and is passed between a pair of electrodes. Ionisation of the test gas results in the flow of electrical current between the electrodes and measurement of this current can, with calibration of the devices, provide an indication of the concentration of the test gas in the airstream.

The conventional method of calibration relies on commercially provided supplies of bottled gases containing requisite ratios of calibrant gases in an undetectable carrier/buffer. This method has many disadvantages, amongst which are; economy, in that successful calibration requires a large number of different mixtures, and in that each separate mixture is seldom completely used: in fact each calibration usually only requires only very small quantities of each mixture, and at relatively infrequent intervals, practicality, in that detection devices of the type referred to require calibration on site, which requires a disproportionate effort to be devoted to the transport and manipulation of calibration equipment and mixtures, and technical validity, in that the calibration gases are supplied as nominal mixtures, which require further costly assay to confirm

their precise composition. Furthermore the carrier gas itself may be contaminated. It is also preferable that the actual test gases which will be used or met in the field should be used in the calibrations rather than commercial equivalents. A system providing accurate calibrations for gaseous systems is described in our pending Application PCT/GB93/02334. The present invention provides an extension to this system which allows the calibration of airstreams carrying vapour. For accurate calibration of the amount of a vapour in an airstream the substance providing the vapour must be completely vaporised with no globules of liquid and no condensation.

According to the present invention a vapour concentration calibration system includes means for supplying to a device to be calibrated a first metered supply of air (primary air) with which has been mixed a second metered supply of air (secondary air) which has been saturated with a vapour.

A preferred means for saturating the secondary air includes an air pump for pumping air through an air flowmeter and then- -, in the form of fine bubbles, through the liquid whose vapour cont< nt is to be calibrated.

A preferred method of forming the fine bubbles involves passing the air through a sintered gas diffuser, the diffuser being positioned, for example, towards the base of a sealed vessel holding a volume of the liquid. For liquids of low volatility it may be necessary to heat the liquid and to pass the saturated secondary air to a mixing point with the primary air via a heated tube.

The most convenient method of determining the amount of vapour in the saturated secondary air is, of course, to use physical tables and a knowledge of the temperature and pressure of the liquid being vaporised. The temperature can conveniently be measured by means of a thermometer in the liquid within the vessel. A pressure sensor within the secondary air supply line may be required on some occasions.

The vapour calibration system can conveniently be used with the system described in PCT/GB93/02334 in which a primary air supply includes an air filter, blower unit and a flow meter which might, for example, be a turbine flow meter. Control of this air flow is

effected, for example, by having a variable capacity blower or by the use of an air bleed valve.

Both primary and secondary air will usually be taken from the ambient air. The calibration system is preferably in the form of a portable pack.

According to another aspect of the present invention a method of preparing an accurately quantified sample of vaporised air includes the steps of saturating a metered supply of secondary air and of mixing the saturated air with a metered supply of primary air.

The secondary air is preferably saturated by passing it, as a stream of bubbles, through a liquid, which may be heated. Pipes carrying the saturated secondary air to a mixing point with the primary air may be heated to prevent condensation. One embodiment of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, of which;

Figure 1 shows the mechanical layout of a test system according to the GB 9224303, Figure 2 shows part of a system according to the present invention, and

Figure 3 shows the results of calibrations conducted with the system according to the present invention over a range of temperatures and pressures. A calibration system as used for gases, according to

PCT/GB93/02334 (Figure 1) has a blower 10 connected to an ambient air supply by a filter 11. The blower 10 is connected by tubing 12 to a T junction 13 from which a first leg leads to an air bleed valve 14 and a second leg leads via a flow straightener 15 to a turbine flow meter 16. The turbine meter 16 is connected to a mixing manifold 17 which has a inputs 21 from a series of syringes l4l, 143- In a typical arrangement, as illustrated, six syringes are employed, these being combined in two units. Four small capacity syringes 141 are ganged together by a ganging member 142 in a first unit 122, and are driveable by a stepping motor (not shown) . Typical capacities for these might be 10 mL for two, 250uL for a third and 2.5π-L for a fourth. Two large capacity syringes 1 3 are ganged together by a

ganging member 144 in a second unit 123 and are driveable by a stepping motor 14 . A typical capacity for each syringe might be lOOmL.

Each syringe 141, 143 can be connected to a test gas supply line

5 connection 28 or to the mixing manifold 17 by a solenoid valve l4θ. A test gas supply line 23 connectable to a test gas supply 24 has an inlet valve 25 and an expansion chamber 26, and has a connection 27 to the mass flow controller 19 and connection 28 to the solenoid valves 140.

10 The mixing manifold 17 is connected by a mixing tube 30 to a diffuser 31 to which can be attached a device to be calibrated 32.

A calibration system according to the present invention (Figure 2) has a primary air line 50, which might in practice be the mixing manifold 17 illustrated in Figure 1. Secondary air is supplied from a

15 supply 1. which will usually be ambient air, to an air pump 52 connected to a flowmeter 53- The flowmeter 53 is connected by a tube 4 to a sintered glass diffuser 55 situated towards the base of a vessel 57 which has a digital thermometer 58 taking a reading from a thermocouple 59 and a heating element 60 controlled by a thermocouple

2061 and temperature controller 62. The vessel 57 is connected by a supply line 63, which may have heating means (not shown), to the primary air line 50.

In use liquid 56 is introduced into the vessel 57 and (if the volatility of the liquid so requires) is heated to an appropriate

25 temperature by the heater 60 under control of the controller 62. The pump 5 is operated to drive a flow of secondary air through the flowmeter 53 and line 54 to the diffuser 55- In the diffuser 55 the airflow is dissipated into a plurality of small bubbles which pass through the liquid 56, becoming saturated during passage therethrough.

30 The saturated air then passes through the supply line 63, which is heated, if necessary, to prevent condensation of vapour from the secondary air, to a metered supply of primary air in the primary air line 50. The resultant calibration gas is then passed to an instrument 64 to be calibrated.

35 It will be realised that the concentration of the vapour in the calibration gas will be sufficiently low for there to be little danger of condensation at this stage. It will also be realised that with

this arrangement, with primary and secondary air taken from ambient there will be substantially no pressure rise despite the action of the various pumps, so a pressure sensor will usually not be required. The concentration of vaporised liquid in the metered secondary air, and hence in the mixture of this with the primary air, will be directly obtainable from the temperature measurement from the thermometer 58 and from known physical characteristics.

All the various parameters relating to the characteristics of the final calibration gas and to calibration of the instrument 64 may, of course, be fed directly to a computer (39. Figure 1) for processing.

It will be realised, of course, that many variations of the above described system are possible within the scope of the invention.

For example, many types of diffuser may be used as alternatives to the glass diffuser 55. The diffuser must, of course, produce air bubbles small enough to ensure saturation. This may be determined by calibration or from known physical characteristics of various liquids. A series of eight separate test calibrations was carried out over a range of temperature from 10 to 30° C and a range of pressures from 640 to 780 mmHg. The results are shown in Figure 3. from which it will be seen that there was a high level of consistency for the results.