GB2044462A - Introducing gaseous samples into test apparatus - Google Patents

Abstract

A gas detecting or measuring device has a cell (2) containing a gas-sensitive electrical element (3) connected to an indicator device (4). At one end, the cell is connected to a supply line (6) for the gas to be tested, and, at the other end, the cell is connected to a pump chamber (9) containing a heater filament (10) and glass wool (11). When an electrical current is passed through the heater filament (10), the gas in the pump chamber (9) is heated and expands, so that a portion of the gas is expelled from the chamber. Upon cooling of the chamber (9) a sample of the gas to be tested is drawn from the supply line (6) into the cell (2). <IMAGE>

Description

SPECIFICATION Gas detecting or measuring device This invention relates to a gas detecting or measuring device.

Gas detecting or measuring devices are used for detecting or measuring gaseous constituents in gas mixtures, for example harmful substances in the atmosphere or substances in human breath. In many cases, they must be easily transportable.

The manner in which the sample is taken is of particular importance as a representative sample of the gas mixture to be tested has to be taken.

The known portable gas indicator described in German Patent Specification No. 2,324,404 has a measuring cell containing a gas-sensitive element.

A ventilator is connected to the measuring cell.

The ventilator is driven by an electromotor which in turn is driven by an electric battery mounted in the housing of the indicator. The ventilator draws the gas to be tested via an inlet in the housing and conveys it to the measuring cell. The measured value of the measuring cell is indicated by an indicator. The drawback of this indicator is that the ventilator driven by the electromotor is expensive, is prone to faults and requires attention.

Furthermore, its weight is a disadvantage for a portable device.

In the known gas measuring device described in British Patent Specification No. 1,448,557, a measuring cell containing a gas-sensitive element is connected on the one hand via a suction line to a supply pipe for the gas to be tested. On the other hand, a hand-operated piston pump is connected to the measuring cell via a connecting line, the cylinder of the pump being open only to the connecting line. If the pump piston is depressed, against the force of a compression spring, the gas is forced out of the cylinder via the connecting line, the measuring chamber and the suction line into the supply pipe. When the piston pump is released, movement in the opposite direction occurs as the compression spring expands, so that gas is drawn out of the supply pipe into the measuring cell and measured there. The measured value is indicated by an indicator.The drawback of this is that the movable parts are prone to faults and wear, require attention and may become unsound.

According to the present invention, there is provided a gas detecting or measuring device comprising (a) a detecting or measuring cell for detecting or measuring a constituent of the gas, and (b) an expansion pump driven by heat, connected to the detecting or measuring cell for drawing a sample of the gas to be detected or measured into the cell.

The gas measuring device of the invention does not need to have any movable parts, and therefore can be light and easy to seal. The advantages achieved by this invention arise from the absence of moving parts. The gas is conveyed only by the heat-produced expansion of the gas present in the expansion pump. The volume of gas which is thereby displaced returns as a gas sample during cooling after the heating has been switched off.

The result is an efficient operation requiring no attention. The sample taker can be light in weight and easy to seal. A portable gas detecting or measuring device, in particular can be constructed in accordance with the invention.

Preferably, the expansion pump comprises a pump chamber which is connected to the detecting or measuring chamber and which contains a heater filament. This form of construction is particularly simple and reliable, and may be light in weight. The pump chamber may additionally contain a material, such as mineral absorbent wool, which prevents or reduces mixing of heated and unheated gases. This material reduces the effect of changes in position and jolts of the device, such as can occur with portable devices, upon the gas content of the chamber. The gas flow during sample taking is not hindered by the presence of the wool.

Preferably, the detecting or measuring cell and the pump chamber have a common housing, whereby the device has a compact construction which is particularly suitable for portable devices.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a schematic representation of a device of the invention; and Figure 2 is a schematic representation of part of another device of the invention.

Figure 1 shows a measuring cell 2 containing a gas-sensitive element 3 connected to an indicator device 4, the cell 2 being arranged in a suction line 1. At the end 5, the suction line 1 opens into an attachable supply pipe 6 for the gas mixture to be tested. At the other end, an expansion pump 7 is connected to the suction line 1. The expansion pump 7 consists of a vessel 8, the interior of which forms a pump chamber 9, and a heater filament 10. The pump chamber 9 is filled with glass wool 11. The heater filament 10 is connected via line 1 2 to a control device 1 3 which determines the current supplied to the heater filament 10.

The measuring process is initiated, by the control device 13, by brief heating of the heater filament 10. In this way, that gas in the pump chamber 9 is heated and expands so that some of this gas is expelled via the section line 1.

Accordingly, a uniform sample of the gas mixture to be tested is drawn into the cell when the heater filament 10 and the pump chamber 9 are cooled.

As a result of the low thermal inertia, the sample is quickly taken. If the pump chamber 9 has a volume of 7 cm3 and the measuring cell 2 a volume of 0.5 cm3, and if the heater filament 10 is heated for 5 seconds to 9800C, 1 cm3 gas is displaced, and, during a cooling time of similar duration, 1 cm3 of the sample is drawn into the cell. When the sample comes into contact with the gas-sensitive element 3, there is produced a voltage whose maximum value is the measured value indicated by the indicator device.

In the partial representation of Figure 2, the design of the gas measuring device differs only in that the measuring cell and the pump chamber are combined in a common housing 14 which contains both the gas-sensitive element 3 and the heater filament 10. A compact construction is thereby attained. The measurement is conducted in a similar manner.

Claims (6)

1. A gas detecting or measuring device comprising (a) a detecting or measuring cell for detecting or measuring a constituent of the gas, and (b) an expansion pump driven by heat, connected to the detecting or measuring cell for drawing a sample of the gas to be detected or measured into the cell.

2. A device as claimed in claim 1, wherein the pump comprises a pump chamber which is connected to the detecting or measuring chamber and which contains a heater filament.

3. A device as claimed in claim 2, wherein the pump chamber additionally contains a material which prevents or reduces mixing of heated and unheated gases.

4. A device as claimed in claim 3, wherein the material is a mineral wool.

5. A device as claimed in any of claims 2 to 4, wherein the detecting or measuring cell and the pump chamber have a common housing.

6. A gas detecting or measuring device, substantially as hereinbefore described with reference to, and as shown in Figure 1 or Figure 2 of the accompanying drawings.