GB2165355A

GB2165355A - Apparatus for recording gas presence

Info

Publication number: GB2165355A
Application number: GB08524422A
Authority: GB
Inventors: Michael Keown
Original assignee: National Research Development Corp UK
Current assignee: National Research Development Corp UK
Priority date: 1984-10-05
Filing date: 1985-10-03
Publication date: 1986-04-09
Also published as: GB8425185D0; GB2165355B; GB8524422D0

Abstract

A hydrogen sulphide monitor for unattended use in sewers has a turntable 1 carrying a lead acetate indicator paper 2 covered by disc 3 with 48 holes 3a. The turntable is driven at one rotation per day by clockwork 10. The non-rotating casing of the clockwork 10 has a plate 4 with a hole 4a which registers with each hole 3a in turn, so that the indicator paper 2, by its darkening at each hole 3a, forms a record of the hydrogen sulphide concentration each half hour through the day. The apparatus may have a plurality of elements 2 each sensitive to a different gas. <IMAGE>

Description

SPECIFICATION Apparatus for recording gas presence This invention relates to apparatus for recording presence of a gas in an air. "Gas" is a specific gas or gases, and "air" is the atmosphere where the gas is suspected. Thus, the apparatus may for example record the presence of hydrogen sulphide in the air in sewers.

According to the present invention, apparatus for recording presence of a gas in an air at various times comprises an element sensitive to the gas, a window exposing one place on the element at a time to the air, a drive mechanism which moves the window relative to the element at a known rate, and a cover with apertures which are exposed in turn by the window, the cover being interposed between the element and the window and fixed relative to the element.

The apparatus could record a plurality of gases (or the same gas to a plurality of levels of detection) simultaneously, by having a plurality of elements each sensitive to a different gas (or each having a different level of sensitivity of the same gas), a window for each element and a common drive mechanism. The apertures in the cover are preferably equally spaced. The locus of the window on the element may be a circle or spiral or rasterwise scanning.

The element may be a substrate (such as filter paper) carrying a substance reactive with the gas; the reaction may suitably be one which is detectable to visible change, but reactions may be exploited which are detectable by subsequent treatment such as chemical or thermal development or chemical analysis.

Where the gas to be recorded is hydrogen sulphide, a suitable substance is a lead salt such as lead acetate; upon reaction, visible lead sulphide forms, in a quantity dependent on the duration of exposure and on the concentration of that gas.

Other gases detectable using appropriate elements would be for example ammonia (litmus) and chlorine (potassium iodide/starch).

The drive mechanism is preferably clockwork. This perhaps surprising preference arises from the risk of spark-ignited explosions; this risk can made electrochemical gas detection methods undesirable for sewers, and also can make it undesirable to use any electric clock mechanism to drive the apparatus set forth above. The drive mechanism may be arranged to rotate at any convenient (but known) rate, such as once per 24 hours or once per week.

The invention extends to a method of recording presence of gas in air at various times over a period, comprising placing apparatus as set forth above so that the air reaches the element, leaving the apparatus for the period, and then examining the element. The invention also includes the element resulting from the method.

The invention will now be described by way of example with reference to the accompanying drawing, which is an exploded view of apparatus according to the invention.

A turntable 1, which is 15 cm in diameter, is driven by a spring-driven clockwork mechanism 10 to rotate once per 24 hours. The turntable carries a thin resilient pad 1 a of polyether foam, a sheet 1 b of polyethylene and a sensitised recording element being a filter paper 2 of the same diameter impregnated with lead acetate solution and dried. The filter paper 2 is firmly covered by an aluminium disc 3 having forty-eight 5 mm diameter holes 3a spaced equally in a 13 cm diameter (62 cm radius) circle. The filter paper 2 is clamped between the turntable 1 and the disc 3 by a central axial splined collar 7a and nut 7b.Accordingly, the turntable 1, the pad la, the sheet 1 b, the filter paper 2 and the disc 3, which all have a central keyhole-shaped hole fitting the splined collar, rotate fixed together as an assembly. The pad 1a and sheet 1b ensure that no air can reach the filter paper 2 except via the holes 3a.

The non-rotating casing of the clockwork mechanism 10 is fastened to a (non-rotating) PTFE-faced aluminium plate 4 of diameter 18 cm. The plate 4 has, at radius 6- cm, a single window 4a which registers in turn with each hole 3a one at a time as the assembly 1, 2 and 3 rotates. The disc 3 rubs against the PTFE face of the plate 4 in gas-tight fashion. The PTFE facing is thick enough to give a gas-tight resilience and lowers the friction of the rubbing. The rotating assembly 1-3 is held just sufficiently firmly against the nonrotating plate 4 by a screw 6 inserted from the turntable 1 side of the assembly 1-3 into the driving spindle 10a of the clockwork mechanism 10.

In use, the apparatus is suspended (via a manhole) by a non-rotting rope 5 in a sewer where hydrogen sulphide is present or suspected. The plate 4, being larger than the parts 1-3, takes any knocks if the apparatus swings and hits the sides of the manhole, thereby protecting the apparatus.

Each half hour, a new hole 3a is exposed through the window 4a to the air in the sewer. Any hydrogen sulphide present will blacken the lead acetate filter paper 2, according to its concentration. At the end of one day, the apparatus is removed and taken apart. The filter paper 2 is removed and studied to learn of any presence of hydrogen sulphide (and if so, its relative concentration) for each half hour of that day; this will show up as 48 patches of varying degrees of darkening around the filter paper, the darker the patch, the higher the hydrogen sulphide concentration. A fresh piece of identical filter paper is loaded into the apparatus if continued monitoring is required.

The window 4a may be the same size as the holes 3a, or may be slightly larger (subtending up to one forty-eighth of a circumference).

One or more of the holes 3a may have a distinctive shape to mark (say) the noon-to1230 interval of the filter paper.

Claims

1. Apparatus for recording presence of a gas in an air at various times, comprising an element sensitive to the gas, a window exposing one place on the element at a time to the air, a drive mechanism which moves the window relative to the element at a known rate, and a cover with apertures which are exposed in turn by the window, the cover being interposed between the element and the window and fixed relative to the element.

2. Apparatus according to Claim 1, wherein the apertures are equally spaced.

3. Apparatus according to Claim 1 or 2, where the locus of the window on the element is a circle.

4. Apparatus according to Claim 1 or 2 wherein the locus of the place on the element is a spiral or boustrephedon.

5. Apparatus according to any preceding claim, wherein the element is a substrate carrying a substance reactive with the gas.

6. Apparatus according to Claim 5, wherein the substrate is a filter paper.

7. Apparatus according to Claim 5 or 6, wherein the substance is a lead salt.

8. Apparatus according to any preceding claim wherein the drive mechanism is clockwork.

9. Apparatus according to any preceding claim, for recording a plurality of gases simultaneously, comprising a plurality of elements each sensitive to a different gas, a window exposing one place at a time on each element to the air and a common drive mechanism.

10. Apparatus according to any of Claims 1 to 8, for recording the same gas to a plurality of levels of detection simultaneously, comprising a plurality of elements each having a different level of sensitivity to the same gas, a window exposing one place at a time on each element to the air and a common drive mechanism.

11. Apparatus according to Claim 1, substantially as hereinbefore described with reference to and as shown in the accompanying drawing.

12. A method of recording presence of a gas in an air at various times over a period, comprising placing apparatus according to any preceding claim so that the air reaches the element, leaving the apparatus for the period, and then examining the element.

13. A method according to Claim 11, when dependent on Claim 7, wherein the gas is hydrogen sulphide.

14. A method according to Claim 12, substantially as hereinbefore described.

15. The element which has been exposed to the air in a method according to Claim 12, 13 or 14.