Gas sensor

1. A gas sensor instrument, comprising:

first and second housing parts which mate together to form an instrument housing; and

first and second diffusion chamber parts which mate together to form a diffusion chamber;

a detector located within the diffusion chamber; and

a first printed circuit board housed within the instrument housing;

where the printed circuit board is arranged to process a signal from said detector;

wherein at least one of the first and second diffusion chamber parts is integrally formed with at least one of the first and second housing parts.

2. A gas sensor instrument as claimed in claim 1 , wherein the first diffusion chamber part is integrally formed with the first housing part and the second diffusion chamber part is integrally formed with the second housing part.

3. A gas sensor instrument as claimed in claim 1 , wherein one of the first and second diffusion chamber parts is an inner diffusion chamber part and the other is an outer diffusion chamber part, and wherein the first and second diffusion chamber parts overlap with each other such that a perimeter wall of the inner part is disposed inside a perimeter wall of the outer part.

4. A gas sensor instrument as claimed in claim 3 , wherein the overlap is at least 5 mm.

5. A gas sensor instrument as claimed in claim 3 , wherein the overlap is in the range 5 to 15 mm.

6. A gas sensor instrument as claimed in claim 3 , wherein the first and second diffusion chamber parts are each formed as a tube, open at one end and closed at the other, wherein a second printed circuit board is disposed within the diffusion chamber and wherein the second printed circuit board is clamped between the open end of one diffusion chamber part and the closed end of the other diffusion chamber part.

7. A gas sensor instrument as claimed in claim 3 , wherein the first and second diffusion chamber parts are each formed as a tube, open at one end and closed at the other, and wherein the inner diffusion chamber part comprises one or more spacer projections on a rim thereof to ensure a minimum air gap adjacent the rim.

8. A gas sensor instrument as claimed in claim 3 , wherein one of the first and second diffusion chamber parts is provided with one or more spacers in the overlap region to ensure a minimum separation between the two chamber parts.

9. A gas sensor instrument as claimed in claim 3 , wherein one of the first and second diffusion chamber parts is provided with a channel for accommodating an electrical conductor passing through the overlap region from the inside to the outside of the chamber.

10. A gas sensor instrument as claimed in claim 9 , wherein the channel is formed in the outer diffusion chamber part and wherein the diffusion chamber wall has increased thickness in the region of the channel.

11. A gas sensor instrument as claimed in claim 1 , wherein the inner surfaces of the first and second housing parts are electrically conductive.

12. A gas sensor as claimed in claim 1 , wherein the sensor is a radon gas sensor.

13. A method of making a gas sensor instrument comprising first and second housing parts first and second diffusion chamber parts, a detector and a first printed circuit board for processing a signal from said detector, wherein the first diffusion chamber part is integrally formed with the first housing part and the second diffusion chamber part is integrally formed with the second housing part and wherein the method comprises simultaneously mating the first housing part with the second housing part and the first diffusion chamber part with the second diffusion chamber part such that the detector is located between the first and second diffusion chamber parts and the first printed circuit board is housed between the first and second housing parts.

14. A method as claimed in claim 13 , wherein one of the first and second diffusion chamber parts is an inner diffusion chamber part and the other is an outer diffusion chamber part, and wherein the mating step causes the first and second diffusion chamber parts to overlap with each other such that a perimeter wall of the inner part is disposed inside a perimeter wall of the outer part.

15. A method as claimed in claim 14 , wherein the overlap is at least 5 mm.

16. A method as claimed in claim 14 , wherein the overlap is in the range 5 to 15 mm.

17. A method as claimed in claim 14 , wherein the first and second diffusion chamber parts are each formed as a tube, open at one end and closed at the other, wherein the method comprises inserting a second printed circuit board into the outer diffusion chamber part and wherein the mating step clamps the second printed circuit board between the open end of one diffusion chamber part and the closed end of the other diffusion chamber part.

18. A method as claimed in claim 14 , wherein the first and second diffusion chamber parts are each formed as a tube, open at one end and closed at the other, and wherein the inner diffusion chamber part comprises one or more spacer projections on a rim thereof to ensure a minimum air gap adjacent the rim.

19. A method as claimed in claim 14 , wherein one of the first and second diffusion chamber parts is provided with one or more spacers in the overlap region to ensure a minimum separation between the two chamber parts.

20. A method as claimed in claim 14 , wherein one of the first and second diffusion chamber parts is provided with a channel for accommodating an electrical conductor passing through the overlap region from the inside to the outside of the chamber.

21. A method as claimed in claim 20 , wherein the channel is formed in the outer diffusion chamber part and wherein the diffusion chamber wall has increased thickness in the region of the channel.

22. A method as claimed in claim 13 , wherein the inner surfaces of the first and second housing parts are electrically conductive and wherein the mating step electrically connects the first and second housing parts.

23. A method as claimed in claim 13 , wherein the sensor is a radon gas sensor.