Tank Level Sensors that Dont get Dirty

[BUR]

    I have a system made by Garnet.  It's a circuit board that mounts on the outside of the tank and it reads in 1% increments. Works very good, I would buy again. The remote panel has a readout for battery, fresh, grey, black water and LPG.        Good Busin'   Wilbur

[Tim Strommen]

How exactly do these external sensors work? I've Googled "Series tank systems monitor" but haven't found the product or manufacturer.  I've already bought a traditional monitoring system for may tanks that uses copper contacts inside the tanks - supposedly good for any type of liquid including sewage - but I have read a number of comments about such systems being unreliable. I'm afraid comments such as "my system is better - just use a flash light!" don't impress those of us who have tanks that cannot easily be accessed (in my case the tanks aren't inside the bus, or even in a bay).  Jeremy

Basically every material has a characteristic called capacitance.  ("Rough" explanation for the non-electrically aligned:) This is an effect that a material will resist a change in the voltage across it at low frequencies - which it does by absorbing charge if the attached circuit has more voltage than the material, and releasing charge if the attached circuit has less voltage than the material.  This effect if for a unit of measurement of the material - for instance say a piece of plastic tank 1/8"-cubed has a capacitance of 40pF (that's pico-Farads, "Farads" being the unit of measurement used for capacitance - pico meaning 0.000000001 Farads).  If you increased the volume of plastic to 1/8"x1/8"x1/4" you have doubled the volume and also the capacitance (so now you'd have 80pF).  If you recall I stated that capacitors resist change, well more capacitance = more resistance to change.  What you can do is alternate the voltage field across the material and watch how long it takes for the material to balance out with the circuit.

Again, all materials have a capacitance value - even people.  If you've ever used one of those cool lamps that you simply touch the plastic base and it turns on - you've seen this effect.  Your body adds so much capacitance to the circuit, that it takes a lot longer to balance out - a very simple circuit can set a switch point where you can say "pushed" or "not-pushed" based on the value - or in the case of a sender, you can build a circuit that reads how long it takes to balance out the circuit and refer to a calibrated Look-up-Table or equation to provide a human-usable value.  Even water and poo have capacitance values, and the more you add to a tank the more time it takes to balance out the circuit – so with a micro-processor you can tell when there is no water/poo or it is full of it, and show that on a display.  By the way, with a very high resolution grid on an LCD display, you can tell how far away from the screen a finger is or where on the screen a finger or multiple fingers are.  This is the technology that Apple uses in its "iPod Touch" and iPhone touch screens.

As to your inquiry about the "my system is better than yours" not being good enough - since capacitive sensors are external (mostly, there are some like the Centronics fuel tank sensor that are internal) they don't get in contact with the material being read, so they are less likely to rust.  Also, since there are no moving parts, they aren't as likely to break (plus being external they don't take up tank volume, which could get you an extra flush in a crisis ).  True, they can be affected by build-up in the tank - but that is a simple matter of maintenance - you either clean the tank to get back your original tank volume or you recallibrate (if you are truly lazy).  Since they don't require that you poke a hole in the tank to install it, they are also less likely to leak, or become a weak spot in cold weather.

A while back, we had a discussion about a Centroid type fuel sender and I had put u a link to an electronics article that showed how to build one of these yourself.  You can find that thread here

-Tim

[Joe Camper]

Thanks for taking the time to explaine that Tim.

I just cut and pasted the entire paragraph.


I hope you know what you are talking about LOL