Printed capacitive liquid level sensor for fire suppression

1. A fire suppressant storage device ( 20 ) comprising:

a tank ( 22 ) having a first port and an interior for storing a fire suppressant; and

a discharge assembly mounted to the first port and comprising:

a discharge valve ( 48 ); and

a discharge conduit ( 50 ) at least partially within the interior and having:

an interior surface ( 60 ); and

an exterior surface ( 58 ),

wherein the discharge assembly further comprises:

a capacitive gauge ( 80 ) on the discharge conduit comprising:

a first conductor ( 82 A);

a second conductor ( 82 B) spaced apart from the first conductor;

a plurality ( 180 A) of first branches ( 182 A) of the first conductor extending away from the second conductor on the discharge conduit; and

a plurality ( 180 B) of second branches ( 182 B) of the second conductor extending away from the first conductor on the discharge conduit opposite respective said first branches.

2. The fire suppressant storage device of claim 1 wherein the gauge is along the exterior surface.

3. The fire suppressant storage device of claim 1 wherein:

the first branches are a plurality of groups at intervals along the first conductor; and

the second branches are a plurality of groups at intervals along the second conductor.

4. The fire suppressant storage device of claim 1 wherein the gauge further comprises:

a first insulative layer ( 90 ) between:

the first conductor; and

the discharge conduit; and

a second insulative layer ( 94 ) over the first conductor and the second conductor.

5. The fire suppressant storage device of claim 4 wherein the first and second insulative layers are polymeric layers.

6. The fire suppressant storage device of claim 4 wherein the first and second insulative layers comprise a polyimide.

7. The fire suppressant storage device of claim 1 wherein the gauge further comprises:

the plurality ( 180 A) of first branches ( 182 A) of the first conductor extend circumferentially away from the second conductor; and

the plurality ( 180 B) of second branches ( 182 B) of the second conductor extend circumferentially away from the first conductor opposite respective said first branches.

8. The fire suppressant storage device of claim 1 further comprising:

said fire suppressant within the tank interior, the discharge conduit at least partially immersed in the fire suppressant.

9. The fire suppressant storage device of claim 8 wherein:

said fire suppressant comprises a clean agent.

10. The fire suppressant storage device of claim 8 further comprising:

a compressed gas propellant in a headspace of the tank.

11. The fire suppressant storage device of claim 8 wherein:

the tank has a second port;

a closure ( 130 ) seals the second port; and

first ( 92 A) and second ( 92 B) leads from the gauge extend to the closure.

12. The fire suppressant storage device of claim 8 further comprising:

an electronics module ( 132 ) electrically coupled to the gauge and comprising a function generator ( 324 ) for supplying an AC signal to the gauge.

13. The fire suppressant storage device of claim 12 wherein:

the electronics module is configured to split an output of the function generator between said signal to the gauge and a signal to a reference capacitor ( 320 ).

14. A method for using the fire suppressant storage device of claim 1 , the method comprising:

applying a voltage across the gauge; and

measuring a response of the gauge.

15. The method of claim 14 wherein:

the applying a voltage comprises applying an AC signal; and

the measuring a response comprises taking a difference between said AC signal and an AC signal applied to a reference capacitor ( 320 ).

16. The method of claim 14 further comprising:

from the response, determining a level of said fire suppressant in the tank.

17. A method for manufacturing a fire suppressant discharge assembly, the discharge assembly comprising:

a discharge valve ( 48 ); and

a discharge conduit ( 50 ) having:

an interior surface ( 60 ); and

an exterior surface ( 58 ),

the method comprising:

forming, in situ on the discharge conduit, a gauge ( 80 ) having a first conductor ( 82 A) and a second conductor ( 82 B), the forming including providing a plurality of branches of the first conductor on the discharge conduit extending away from the second conductor and a plurality of branches of the second conductor on the discharge conduit extending away from the first conductor.

18. The method of claim 17 further comprising:

prior to the forming in situ of the first conductor and the second conductor, forming in situ a first polymeric layer on the discharge conduit; and

after the forming in situ of the first conductor and the second conductor, forming in situ a second polymeric layer over the first conductor, the second conductor, and the first polymeric layer.

19. The method of claim 17 wherein:

the forming in situ of the first conductor and the second conductor comprises spraying.

20. The method of claim 17 further comprising:

connecting respective first and second leads ( 92 A, 92 B) to the first and second conductors by conductive paste.