GB2320087A - Apparatus for monitoring ventilation of buildings - Google Patents

Abstract

A plurality of tracer gas sources are placed in a building and the tracer gas is monitored at one or more locations remote from the sources. A tracer gas source comprises a container (3) and a number of plugs (5, 5a, 5b) of different lengths through which a tracer gas has to pass, one of the plugs being used and being selected so that the source has a desired emission rate. The emission rate can thereby be varied by selection of a different plug. The plugs may be of silicone rubber.

Description

APPARATUS FOR MONITORING VENTILATION OF BUILDINGS This invention relates to apparatus for monitoring ventilation of buildings.

Adequate ventilation and air quality are necessary for occupant health, safety, comfort and productivity in a building. Too much or too little ventilation can both be disadvantageous.

A conventional technique for measuring ventilation in buildings involves tracer gas emitted and mixed with the air at one part of a building and which is then sampled at a chosen location, to indicate the rate of ventilation. However, in most tracer gas type systems, it is necessary to maintain an even mixture of the gas throughout the building and to take representative air samples. This can be difficult to achieve in large or multi-roomed buildings, particularly when the building is in use or is naturally ventilated, since rates of natural ventilation are variable throughout the building and over time.

More recently, a technique of 'passive' tracer gas monitoring has been developed. This technique involves simple, unobtrusive, passive devices which are distributed around a building and used to take measurements while the building is occupied. The devices are filled with a tracer liquid, of which very small amounts are released at a concentration of, perhaps, a few parts in one hundred billion. The gas may be a perfluorocarbon tracer (PFT) gas which is totally inert, odourless and non-toxic. Separate sampler devices collect air samples by diffusion onto an adsorbent material.

These samples are periodically analysed and a timeaveraged ventilation rate is determined from the mass of tracer absorb by the sampler and the known tracer emission rate in the building.

A problem with such techniques up to now has been in coping with different sized rooms and complicated environments where it is desirable to vary the rate at which emission of the tracer gas occurs. This may be done by putting a different number of tracer gas emitters in rooms but it is difficult to get accurate results this way.

The present invention arose in an attempt to provide an improved tracer element source.

According to the present invention there is provided a tracer element source comprising a container for receiving an amount of a tracer gas emitter and a plug permeable to the tracer gas and being mounted above the tracer gas emitter in the container so that the tracer gas can permeate through the plug, the size of the plug being chosen to provide a desired emission rate.

Thus, for one container a plurality of plugs of different sizes (ie different lengths for example) may be provided, each different size of which is associated with a different emission rate so that if a particular emission rate is required for a particular use, the appropriate plug is selected.

According to the present invention in a second aspect there is provided apparatus for providing a source of tracer gas, comprising at least one container for receiving an amount of a material which emits a tracer gas; and at least two plugs adapted to be received in the container and being permeable to the tracer gas so that the tracer gas can permeate through the plugs at a desired emission rate; wherein at least two of the plugs are of different sizes and thereby different emission rates.

According to the present invention in a third aspect there is provided a method of varying the emission rate of a tracer gas source, which source comprises a container into which is received a material which emits a tracer gas; wherein the method comprises inserting into the container a plug which is permeable to the tracer gas to enable the tracer gas to permeate through it and thereby be emitted, at a rate which is at least partially determined by the size of the plug, the size of the plug being chosen to vary the emission rate.

The plugs may be of different lengths.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows schematically a system for measuring the ventilation in a building; and Figure 2 shows a tracer gas source and plugs of various sizes.

Referring to Figure 1, a passive technique for measuring the ventilation in a building uses a plurality of sources 1 of a tracer gas which are distributed at various locations around the building. The sources contain perfluorocarbon tracer gas at concentrations of a few parts in one hundred billion. Presently preferred tracer gases are perfluromethylcyclohexane (PMCH) and perflurodimethylcyclohexane (PDCH). One or more detectors 2 are mounted at chosen measurement locations and these collect air samples by diffusion onto an absorbent bed.

The detectors are typically of the same size as the sources (eg roughly about the same size as a small pencil) and are of well known design. Usually, the samplers collect the air samples over a time period, which generally ranges from one day to several weeks and, at the end of the measurement period the samplers are removed for laboratory analysis. This determines the amount of tracer which has been adsorbed by the sampler over that time period and the time-averaged ventilation rate can be determined from the mass of tracer adsorbed over that period and the known tracer emission rate in the building.

In a variation of the procedure, 'spot' measurements are made over periods of, say, 30 to 60 minutes using automated portable air sampling pumps.

Figure 2 shows a tracer source in cross section.

The source comprises a container 3, typically in the form of an open topped cylindrical container of any suitable material. It may be of metal, for example aluminium.

Typically, its height will be around 5 cm. A small amount (typically about 0.4 ml) of a PFT in liquid form 4 is injected into the container. A plug 5 of a material (eg a plastics material such as a silicone rubber) which is permeable by the PFT gas which is continually released from the liquid 4 is then applied into the opening 6 in the container. The plug is preferably resiliently compressible and may have a diameter which is equal to the internal diameter of the container 3, so that the plug needs to be slightly laterally compressed to fit into the container. It thereby forms a good seal with the container so that all the gas released from liquid 4 has to permeate through the plug and cannot escape peripherally around it. In one embodiment, the diameter of the plug is 6 mm.

The rate of emission of a source of this type is determined by, inter alia, the temperature and the size (for example the length) of the plug. If the temperature varies then the rate of emission of gas from liquid 4 will vary. Also, if the size of the plug alters, then the amount of material through which the gas has to permeate alters and so the emission rate will also alter, in a controlled manner. In environments where temperature remains fairly constant (eg typically when temperature does not vary by more than, say, + 5"C cyclically above and below an average or by more than about 50C if not cyclically varying), then the emission rate can be varied by varying the length d of a plug.

Figure 2 shows two further plugs 5a, 5b of the same diameter but different depths. Plug 5a is of half the depth of plug 5 and plug 5b is of three quarters of the depth of plug 5. By substituting one of these for plug 5 then a tracer source of different emission rate per unit volume of space in which they are deployed is achieved. Other relative and absolute plug lengths may of course be used.

A kit of parts may therefore be provided comprising at least one container 3 and a plurality of two or more plugs 5 of different sizes. In one example, the plugs are, respectively, 12 mm, 9 mm and 6 mm long.

In use, it is found to be desirable to establish in a building a first room or area of nominal unit size.

The precise size does not matter. A single source of 'unit' emission rate per unit area may be placed in this room. Sources of proportion emission rate per unit area may then be placed in rooms of different sizes. For instance, a source of half the emission rate per unit volume may be placed in a room having half the volume and a unit having three quarters emission rate per unit area may be placed in a room having three quarters the volume of the nominal single unit room. In practice, a plurality of sources may be placed in each room but the rooms can be normalised to a unit room. Provided the levels of tracer gas are such that they can be detected at a sampler 2, it is the relative values of emission rate which are of importance, rather than absolute values.

Instead of the depth of a plug, the width of a plug (and thereby also the width of a container) may also be altered to vary the emission rate. Other functions of its size and shape may also be altered accordingly.

Claims (9)

1. Apparatus for providing a source of tracer gas, comprising at least one container for receiving an amount of a material which emits a tracer gas; and at least two plugs adapted to be received one at a time in the container and being permeable to the tracer gas so that the tracer gas can permeate through the plugs at a desired emission rate; wherein at least two of the plugs are of different sizes and thereby different emission rates.

2. Apparatus as claimed in Claim 1, wherein the at least two plugs are of different lengths.

3. Apparatus as claimed in Claim 1 or Claim 2, wherein the plugs are of plastics material.

4. Apparatus as claimed in any preceding claim, wherein the plugs are resiliently compressible and are of a diameter greater than the internal diameter of the or each container.

5. A method of varying the emission rate of a tracer gas source, which source comprises a container into which is received a material which emits a tracer gas; wherein the method comprises inserting into the container a plug which is permeable to the tracer gas to enable the tracer gas to permeate through it and thereby be emitted, at a rate which is at least partially determined by the size of the plug, the size of the plug being chosen to vary the emission rate.

6. A method as claimed in Claim 5, wherein plugs of different lengths are chosen to vary the emission rate.

7. Apparatus for providing a source of tracer gas substantially as hereinbefore described with reference to, and as illustrated by, the accompanying drawings.

8. A method of varying the emission rate of a tracer source substantially as hereinbefore described with reference to the accompanying drawings.

9. A method of monitoring the ventilation of buildings substantially as hereinbefore described with reference to the accompanying drawings.