I may or may not start a project, and thought I would ask for you all's input.
The project would be to see how full a tank is. I might be automating processes and I would need to see when the tank was empty. The steel of the tank is too thick to detect temperature changes.
With that being said, would it be sensible to drill a hole in the bottom of the tank, put in a pressure tap, and make the pressure tap go higher than the tank (that way the fluid wouldn't touch the sensor, as i doubt its waterproof). Then at the end of the pressure tap put a:
BMP085 pressure sensor. Then when the pressure of the air in the tube drops to ambient pressure I would know the tank is empty. Does anyone have any better ideas on how to do this? And if this doesn't make sense I can draw a schematic.
The main problem I see is attaching the pressure tap to the sensor, but perhaps I could just hot glue it?
This would measure the differential between the bottom of the tank and top. Water can touch the element and is used commercially to measure the liquid level in tanks. This principle is use to measure liquids such as liquid Nitrogen and Argon in dewers.
Well... since we're coming into the industrial side of things...
You can also try a Vega transducer (http://www.vegacontrols.co.uk/). Their sensor communicates through I2C to their calibrator thingy, so if you get access to the procotol, it might be possible to hook up the I2C on the Arduino to it. If not, there's a 4-20mA or 0-10V output.
If you're measuring liquids that won't have foam, using a ultrasonic device is ok, if not, then you can use the rod version or cable version and hook it up to the tank. These are quite versatile with very little error.
Otherwise, a standard pressure sensor will let you know the pressure and if you know the specific weight of whatever is in the tank, you can calculate the actual level. It won't be too accurate though...
If you want or need to calculate density, you can always put two pressure sensors a meter apart in the vertical and calculate it from there.
Any sort of pressure sensor is going to be sensitive to temperature fluctuations (not necessarily the sensor itself, but the pressure variations as temperature increases/decreases). If the tank is in a controlled environment with a constant temperature, this should be manageable. If it's outside year round where temperatures can vary considerably, it'll require some sort of temperature sensor and code to compensate for temperature variations to be remotely workable.
Wow guys thanks for all the good answers. I'm new to this so its good to get some insight from people that have worked with electronics before.
A quick review of the project is in order I suppose. We are processing vegetable oil and converting it into biodiesel. The process takes a long time and needs to be reduced so it will be economically viable. I plan on automating a lot of the processes to try to cheapen the cost of making it. I have a lot of questions about the project already but I'm waiting until I review the process in person (still in school for two more weeks) until I focus on the project a lot.
Anywho. Right now I think the ultrasound sensor would work the best. The surface is not foamy. If I'm not mistaken the ultrasound sensor will detect the distance between the sensor and whatever it hits, meaning that it will be able to tell when the tank is empty very easily, without having to mess with the tank at all. Thanks for all the advice, learned a lot from this thread
The inside of that tank is going to be a very "echo-y" place... maybe not ideal for ultrasonic sensor.
But your circumstances are very sensor, etc, friendly.
I'd look hard at the pressure sensor answer.
Also- if you only need a few "has it dropped this far yet" points, there is a neat trick used by at least some Canon printers...
They have a little prism inside their ink tanks. When it is "wet" with ink, a light beam bounced through it does one thing. When the ink has gone, the light does something else. Changes to "internal reflection", I would guess...
ff you have a small steady air supply, you can make a "bubbler" level indicator. Put a vertical tube into the tank with a small metered air supply. Measure the backpressure in the air line and compute the level.
A potentiometer and a float could also possibly work
you can also compute level from weight if its practical in your application
Destroyyoutoo:
Anywho. Right now I think the ultrasound sensor would work the best. The surface is not foamy. If I'm not mistaken the ultrasound sensor will detect the distance between the sensor and whatever it hits, meaning that it will be able to tell when the tank is empty very easily, without having to mess with the tank at all. Thanks for all the advice, learned a lot from this thread
If you need to monitor, I'd go with the cable version of those sensors I just pointed out. I haven't seen the tanks, but it is quite normal that there are agitators inside them or pipes into the bottom to avoid free fall of liquid that will turn into bubbles. And this will affect the reading taken from the ultrasound sensor. The VEGA ones allow you to filter out some of that, but it's not something you'll enjoy doing and if for some reason, something changes, you might have to do it again.
Again, it depends on the design of the installation, but I'd have a look at these details before settling with ultrasound.
There are also some industrial level tranducers that are based on pressure and compensate for temperature.
I'm currently looking for something like this as well and I haven't had much luck. I have been aiming for ultrasonic but dang the price is just out to way far left field.
Another option that I have seen used in the Oil industry is a capacitive level probe (also called RF admittance). You would use a rod/wire dropped into the tank and a sensing circuit to read the difference in signal getting through. You would be comparing the difference between the air as the insulator and the vegetable oil as the insulator. My only real experience is with industrial units (http://www.drexelbrook.com/), but it is another option to look into. You could do it with one probe if your tank was metal, but you could also measure between two probes if neccessary.
Another option is to use a thermistor. If you pass large current through it, it will heat up. But if it is placed inside of liquid such as oil, the oil will take away the heat. When the thermistor is cooled down, its resistance increases. You can tell if the liquid has dropped below the level where the thermistor or not by looking at its resistance.
As others have said, a cheap ultrasonic transducer will do the job, providing that you can keep it dry.
The splash proof versions are about 10x the price.
I have 2 Water Tanks on my sailboat and a black water tank(bathroom waste)
I have been looking for a way to measure how much water is still in the tanks without having to drillholes on the tank to install a float or other means of reporting back how much liquid is in it. I found a ultrasonic sensor that sits ontop of the tank, BUT it's $95 EACH.
