I am working on a project that will measure the fluid level of a corrosive/nasty substance in a 55 gal drum. I have read many posts that suggest using a capacitance type probe, but since this is nasty stuff, I don't want to be inserting and removing a probe every time a barrel runs dry and needs replaced. The same problem prevents any type of float type measure. The best thing that I can come up with is some type of pressure sensor to measure the weight of the barrel (~40lbs - 575lbs).
Does anyone have any good ideas for the sensor? Or perhaps some other way to measure the level?
Perhaps look into hitting the drum with a hammer and measuring the frequency (audio recording) at which the drum resonates. It will be a function of how much fluid is in the drum.
Assuming barrel is "sealed" (apart from a vent) and you want a completely non-contact system then your simplest option is to weigh it (once you know the tare weight of an empty barrel.
Alternative non contact systems are nuclear density measurement to detect the interface. Not really available to the home builder !!
Radar detection of the interface surface but these are expensive and need to be within the barrel
Ultrasonic detection as per radar
If you can tolerate something going into the barrel then you could use a pneumatic dip leg. Basically insert a tube (6mm 316 grade stainless steel) into the barrel right down to the bottom, with a small space to allow air out. Then you blow a slow flow of air down the tube to produce a stream of small bubbles. The pressure required to do this (which is your measured variable) is equal to the hydraullic head (depth) of the liquid. Fit a wiper to the tank opening to wipe or wash the leg when you need to remove it.
Alternatively if you want to experiment, build a hammer system to tap barrel then listen to its ring natural frequency. Empty barrels ring low, full barrels ring high and part full is a linear scale between the two.
if the liquid is not pumped out of the drum, but is gravity-fed, then you can use a pressure sensor before the valve.
If you want to mesure the weight of the barells, then use strain gauge (ripped off a digital bathroom-scale) and a wheatstone bridge.
If you want to make your own sensor from scratch, then you can still use Capacitance type sensor. the barrel would then stand on a plastic/wodden plate with aluminium foil glued on the bottom (or ironed into the plastic). This plate rests on a thin but not too thin rubber pads on top of an identical plate as the top, but with the aluminium side up. Wires connected to each side to the arduino.
As the weight of the barrell presses on the rubber, then the distance of the aluminium foil will increase/decrease , then the capacitance of the plates will too. then calibrate the sensor.....
Instead of aluminium you can of course use two unetched circuit-boards or whatever you like, just as long it is thin, as the Arduino cannot handle thick metals.
It is also best to spraypaint or tape over the metal layer to prevent short-circuit the whole thing in case of overloading.
XXXXXXXXXX non conductive plate
---------------- metal film with wire to arduino
I I rubber spacer
---------------- metal film with wire to arduino
XXXXXXXXXX non conductive plate.
Thanks for the input everybody. I think I am going to go with weight of the barrel, but I am intrigued by the ultrasonic rangefinder. I could build a custom vent cap that easily be switched from barrel to barrel, but how might I protect the sensors? Is there any chemical resistant material that ultrasonic can shoot through?
Just to clarify: I am trying to avoid any type of sensor that will have to be in contact with the fluid. These barrels are loaded into a machine, their charge is expended and then they are tossed out. The device cant be permanently mounted to the barrel nor do I want a worker to have to pull a dripping probe out of one barrel to insert it into another.
You may want to look at loadcell technology to see how they are constructed. In industry ultrasonic detectors are used to measure liquid levels in partially filled pipes and such from the outside. You might make a unit of this type to put on the bottom of a barrel.
