GB2311604A

GB2311604A - Vapour generator for producing calibration samples

Info

Publication number: GB2311604A
Application number: GB9606545A
Authority: GB
Inventors: Robert Gittins
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-03-28
Filing date: 1996-03-28
Publication date: 1997-10-01
Also published as: GB9606545D0

Description

A DEVICE FOR PRODUCING CONTROLLED CONCENTRATIONS OF VAPOUR IN AIR This invention relates to a method of producing controlled concentrations in air of vapour from a volatile liquid.

Controlled concentrations of vapour in air are required for calibration and testing of instruments used for the measurement of the vapour concentration. In some cases the instruments are portable and the source of the controlled concentration of vapour is also required to be portable. The usual method of providing this controlled concentration is a pressurised gas cylinder which contains the air plus a known concentration of the vapour. There are limits within which this technique can be used. If the vapour pressure of the liquid is high then the pressure in the cylinder is limited to a value below that at which the partial pressure in the cylinder reaches the dew point of the liquid and condensation takes place. This limit of pressure limits the total volume of controlled concentration vapour in air which can be held in the cylinder. A further limit is at the other end of the scale with very low concentrations. Very low concentrations may be affected by the reactive or absorbtive property of the internal walls of the cylinder. Steel and aluminium, commonly used as materials from which cylinders are manufactured, both form oxide layers which can be reactive. In some cases special internal coatings are provided to reduce this effect which would otherwise cause the very low concentration of the vapour to fall. If such a cylinder has been made up as a standard for calibration and certified then it has a finite life and beyond this period there is uncertainty as to how much the concentration has been affected by reaction with the wall of the cylinder. A similar problem arises with chemical compounds which are reactive such as acids, aldehydes or alcohols but the problem is not limited to this group of compounds.

An alternate method of providing the controlled calibration is the continuous flow dilution method. In this method a known flow of air or gas is passed over or through the liquid providing the vapour at a known temperature so that it is saturated with the vapour at the vapour pressure of the liquid. The concentration of the vapour is known from published vapour pressure data or determined experimentally. The flow can be diluted a number of times with further flows of air to produce lower concentrations which are determined from knowlege of the air flows. In this method the flows are controlled by conventional flow controls such as needle valves and pressure regulators. The use of a reservoir of liquid makes it an unsuitable method for use as a portable device as it is impossible to generate only vapour in any orientation of a reservoir.

According to the present invention the continuous flow dilution method is used with a reservoir which acts as a source of vapour in any orientation and acts as a restrictor to control the flow from the reservoir.

specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings. Reference is made to preparation of air plus vapour but any gas may be used instead of air. Reference is also made to a liquid giving off vapour but this could also be any solid which gives off vapour.

The reservoir is a tube of inert material such as polytetrafluoroethylene. Any inert material may be used such as glass, stainless steel or any other inert plastic material. The tube is filled with an absorbent material such as any support material used as a column packing in the field of chromatography.

The material is of an evenly graded particle size such as 60-80 BS Mesh so that air may flow freely through the tube. The flowrate is controlled by the air pressure applied at the end and the flow resistance of the tube packed with the absorbent material. The absorbent may be brick dust, diatomaceous earth, or any absorbent material with a substantial surface area. The absorbent material is coated with the liquid for which the vapour is to be generated by injecting it until the whole tube is filled with the liquid. Air pressure is then applied at one end and the unabsorbed liquid is driven out of the tube. After the liquid has been driven off, the air emerging from the tube is saturated with the vapour of the absorbed liquid. The absorbent material wetted with the liquid usually has a different (darker) colour to the dry material. As the air flows through the tube a light coloured band of the dry material appears at the air inlet end. The edge of this light coloured band moves along the tube towards the outlet as the air plus saturated vapour is dispensed from the tube. The position of this band is a clear indication of the length of absorbent packing with the liquid absorbed still available to generate vapour.

The tube is connected in an arrangement as shown in Figure 1. Air pressure is applied at 1. The air flow passes through the packed tube 2 and through a bypass restrictor 3. The outlet from the two flows mixes at 4 and appears at 5. The pressure drop across the restriction of 3 controls the flow through the restriction and the tube and hence the concentration of the vapour at the outlet. The concentration is not dependent on the applied air pressure within wide limits. The flow through both the tube and the bypass restriction 3 are affected in a similar way by change of pressure.

A multiple dilution is shown in Figure 2. The air inlet 6, the packed tube 7 and the bypass restrictor 8 are in a similar arrangement to that of Figure 1. An additional restrictor 9 causes a low flow through another restrictor 10. This flow mixes with a further flow of air generated via the air flow through restrictor 11. The double diluted outlet emerges at 12.

Claims

1. A Controlled Concentration Vapour Generator using the flow dilution method in which the liquid giving off the vapour is absorbed on an inert material held in a tube through which air flows.

2. A device as in Claim 1 where the absorbent material is any packing material used in a chromatography column including brick dust and diatomaceous earth.

3. A device as in claims 1 or 2 where the tube containing the inert material is made of polytetrafluoroethylene, glass, stainless steel or any inert plastic.

4. A device as in claims 1,2,3,4 or 5 where the vapour is given off by a solid.

5. A device as in claims 1,2 or 3 where the amount of material available for desorbtion is detectable by the colour change of the absorbent when wetted or coated with solid material.

6. A device as in claims 1,2,3 or 4 where the vapour is generated in air or any gas.

7. A device as in claimsl,2,3,4,5 or 6 where the restrictions controlling the flows are fixed.

Amendments to the claims have been filed as follows 1. A Controlled Concentration Vapour Generator using the flow dilution method in which the liquid giving off the vapour is absorbed on an inert material held in a tube through which air flows.

4. A device as in claims 1,2,3,4 or 5 where the vapour is given off by a solid.

5. A device as in claims 1,2 or 3 where the amount of material available for desorption is detectable by the colour change of the absorbent when wetted or coated with solid material.

7. A device as in claims 1,2,3,4,5 or 6 where the restrictions controlling the flows are fixed.

8. A device as in claims 1,2,3,4,5,6 or 7 where the air or gas passing through the absorbent material becomes saturated with the vapour of the absorbed liquid or solid at the vapour pressure of the absorbed liquid or solid at the temperature of the absorbed liquid or solid.

9. A device as in claims 1,2,3,4,5,6,7 or 8 where the concentration of the vapour emerging from the absorbent material remains at the level of the vapour pressure of the liquid or solid as the band of saturation moves along the tube containing the absorbent material as the liquid or solid is removed from the tube as vapour in the air or gas flow.

10. A device as in claims 1,2,3,4,5,6,7,8 or 9 where the flow dilution method gives a continuous output.

11. A device as in claims 1,2,3,4,5,6,7,8,9 or 10 where the flow dilution is a single or a multiple dilution.