[aprssig] FEMA- Flood gauges (feedback)

Ross Whenmouth ross at topwire.co.nz
Thu Sep 15 15:42:54 CDT 2016

Hi Bob,

> Here is a web page on some of the Flood Gauge ideas… Bob
> http://aprs.org/flood-gauges.html

Rather than using float switches, would a level sensor with no moving 
parts be cheaper and more reliable?

For example, I am thinking that if you had a pair of insulated 
conductors (for example a loop of 300 or 450 ohm ribbon cable with both 
ends terminated at the top in the dry part of the tube) installed in the 
lower part of the tube (the part that gets filled with water), then when 
the tube is dry, the capacitance between the conductors is quite low 
(plastic + air dielectric capacitor). But as the tube fills with water, 
the capacitance between the conductors will increase (electrically only 
plastic dielectric between the conductors of the submerged ribbon = 
higher capacitance).

300 ohm ribbon in air has a capacitance between each conductor of about 
13 pF/m, so say you had 4m length in a 2m long loop, the dry capacitance 
would be about 52 pF. If one side of this capacitor was connected to 
0Vdc and the other side was connected to a digital output of the 
microcontroller by a say 1 Mohm resistor and to a high impedance voltage 
comparator connected to a digital input (some micro-controllers 
incorporate such a voltage comparator on-die), then it will take about 
52 usec to charge the capacitor from 0V to 2/3rds of Vcc. As the tube 
fills with water, and the capacitance increases, the time taken to 
charge (or discharge) the capacitor will increase accordingly. Power 
consumption would be super-low if the micro-controller slept between 
measurements (which might only be performed every few minutes? whilst 
the tube is dry).

Ross Whenmouth

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