[aprssig] FEMA- Flood gauges (feedback)

Ross Whenmouth ross at topwire.co.nz
Thu Sep 15 16:25:08 CDT 2016

Hi Kirby,

> What happens when gunk grows on the wire? Granted gunk will interfere
> with the float switch, but I would think it would be more likely to fail
> on. I would think failing on would be preferable to failing off.
Gunk on the insulated ribbon conductor will either have a higher 
dielectric constant than air (if dry) or be electrically conductive (if 
wet) and both cases will tend to "fail on" (indicate a higher water 
level than is actually present) as the capacitance between the 
conductors will be increased by the gunk.

A pair of insulated rods spaced apart from each other (no plastic 
webbing between them unlike with 300 ohm ribbon) or a single insulated 
rod located co-axially inside a metallic (or otherwise electrically 
conductive) tube would probably be less prone to mis-operation due to 
fouling, but would probably cost more to manufacture.

Ross Whenmouth

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