Water level detection, where water is somewhat polluted and has significant dissolved salts

Question

Found this excellent QnA here which provides several options for measuring or detection water-level. For my case, it is enough to differentiate 4 levels of water (very low, low, middle, full), of the tank.

I need to do the same, but in my case, I am having to deal with a concern, that this is not exactly clean potable water. The water, which is pumped out from a deep tube-well, appears to be somewhat polluted with silt (the rusty dark brown/blackish kinds) and lot of dissolved salts. While the silt is filtered by a 5 micron felt/microfiber based sediment pre-filter, in the post-pump section, it is present in the ground-level sump. The salt is a big concern too, because it stains (salt deposits) metal, plastic and pretty-much any surface. Any metal / plastic part left submerged for long-enough, gets a rather significant deposit of salt, which needs to be scrapped off (with difficulty).

Given these operating environment characteristics, I was wondering which ones of the following methods (all from the previous QnA cited above) of detecting water level, can be expected to work most reliably, and with minimum maintenance required over time.

1. Differential Pressure Transducer at the bottom of tank. What's the chance that the sensing surface or it's edges might get choked by the salt/silt deposit, and stop functioning in a few months. The salt/silt deposit itself may introduce errors in pressure reading over time.

2. Float switch - As this has moving parts, I wonder if they can be rendered immobile (lose mobility of the float) by the salt/silt deposit ?

3. Capacitive sensing - This is not explained in great detail in the cited QnA, but I've read elsewhere that it depends on an average dielectric coefficient of water, where-in 2 insulated probes are placed along the depth of tank / sump, in fairly close proximity. The capacitance of this, varies with water level, which is measured. The exact mechanism is not clear. Also, apparently, low voltage AC current is apparently used for this, but again, how part is not clear. Also, what roles the salt/silt deposits may play in change in capacitance over time, isn't clear to me.

4. Conductivity testing at multiple levels - Electrodes are placed at various levels, which are supposed to operate as closed circuits when water reaches the particular circuit placed at certain levels. Electrical current (say something like 24VDC) is passed through the circuits, periodically, for a short period to determine which all circuits are closed, and infer water level from such information. Now again, does salt/silt change the conductivity ? And can they corrode the electrodes ? Can I use aluminum or copper electrode ?

5. Ultrasonic, proximity sensing - Placing such a sensor face-down i.e. looking straight at the bottom of tank, and use the standard proximity sensing mechanism to detect water levels. To me, this seems most promising, as the sensor is rarely (if at all ever) in contact with water. Also, it's installation should be easiest. Of all of these, this however, is probably the most expensive approach, and possibly less robust (i.e. might not do well under the extreme humidity and temperature variation of a closed water tank/sump).

Answer 1

Given the working parameters, the Ultrasonic Ranging solution indicates the best maintenance-free option. Depending on the desired MTTF, it need not be an expensive solution either.

The budget approach involves modules such as this one:

_{(From eBay, for under $2)}

Rather than deploying the module as-is, the module can be potted using epoxy potting compounds, covering everything but the tops of the ultrasound transmitter and receiver and the connector pins. Better yet, suitable weatherproof cabling can be connected, and the junction also potted. This promises reasonable longevity.

With a larger budget, other ultrasonic ranging modules can be found, which are IP67 rated, designed from the ground up for extreme environments.

Answer 2

Several effective schemes you could try to sense water level in the tank exist. Here are two schemes that I've worked with in the past. Do note that these are interesting technology ideas to work with and require some design work but could be a fun project to work on.

1) An assembly that has several thermistors mounted on it is hung in the tank. The thermistors are wired up to to some current sources that cause each thermistor to self heat a certain amount. The thermistors are located along the assembly at various levels. When the water level rises up and covers a thermistor it cools the component making it change resistance. A sensing circuit that measures the voltage drop across each thermistor can determine if a particular component is covered by water or not.

2) You can place plastic rods that stick down into the tank at different depths. Each plastic rod has its end cut off at a 45 degree angle from each side and then polished to a very smooth finish. A light beam shines down into the rod from the top (could be an LED visible or infrared depending upon what is suitable to the materials you are using) and normally reflects off the two bevels at then end of the rod and is reflected back up to the top of the rod where an photo diode or photo transistor is used to detect the reflected light. When water in the tank rises up and covers the beveled end of the rod it changes the refraction index at the end of the rod and the light coming down no longer fully reflects back up to the detector. The assortment of rods can tell you where the water level is. The picture below shows the concept involved. It would be fun to experiment with this concept using a cheap laser pointer module as the light source. (Back when I worked with this concept years ago there were no laser pointers!!)

Answer 3

If you use a differential pressure sensor you can build a solution that will be very robust because it has no moving parts, is not impacted by temperature/humidity, water cleanness is irrelevant (both with regards to salts and transparency), and no electrical sensors are in contact with the fluid (so if you want to meassure level in an acid tank, no problem).

Some years ago (hm, digging up the notes reveals 2008) I planned a system for my parents to monitor the water level in a water well 100-150m from their house. The following is from the description of the project:

My suggestion is to use a differential air pressure sensor mounted in a pipe sealed in one end and filled with air, partly lowered into the water to meassure the water level.

Assume initially that the the air pressure is 1.0ATM. Then the pressure in the water at 1m below the surface will be 1.1ATM. By lowering a pipe filled with air and sealed in the top 1m down in the water the water preassure at the bottom of the pipe will compress the air inside the pipe to 1.1ATM.

By mounting the pipe to a fixed position, different water levels will give different pressures inside the pipe. Since the sensor is differential variations in air pressure will not affect the sensor. I have no specific preference on what sensor to use, but the MPX2050DP sensor seems to be usable.

The only thing that could weaken the robustness of this solution would be if the fluid contained large objects that potentionally could block and seal the bottom opening of the pipe, however since the preassure is independent of the volume, you can use as wide pipe as you want to.

The bulk of the cost for this will be the sensor (prices for such sensors seems to be in the range 10-20 EUR). For all the rest you can use whatever plastic/metal pipe you have lying around, some rope and a stick if you want to cut cost.

Answer 4

You mention a float switch, and worries about mechanical components getting jammed. However, you could build a float switch in which all of the critical components are above water, in the style of a classic toilet tank filler: place the float on the end of a long rod which can pivot, and encode the angle at the pivot (using an absolute rotary encoder or a potentiometer) to determine the water level. The pivot need not be anywhere close to the water. Unless the rod is particularly heavy, you could even use a standard toilet float ball.

Since you say you only need 4 water levels, you don't need output linearity but only 4 threshold values, and the float arm and pivot can be shaped and placed wherever's convenient — as long as the arm is free to travel a sufficient range inside your container. If the container is tall and narrow, for example, you might need a linearly moving float instead, which is harder to make robust against friction and to measure.

Advantage: Your contaminants might change the calibration slightly as they stick to the float, but not disable the mechanism until they stop it from floating altogether.

Disadvantage: The cost of robust custom-built mechanical components might well exceed a fancy sensor.

(This came to mind when I read your question because a decorative fountain/pool in my hometown used just such a float switch.)

Answer 5

Use a small air source at the top and run the pipe to the bottom so that the air gently bubbles out of the pipe. With your pressure transducer at the top looking at the pressure in the pipe, you can calculate the height of the water above the bottom of the pipe.

Too much pressure and the pipe is blocked.

Too little pressure and something is wrong with your air source or pipe. So it would be very reliable and difficult to break/block. You mentioned silt and stuff, does this mean it may not be just water in there?

If so anything relying on pressure may not be accurate due to varying densities, however, I have a thought that if you coupled this bubbler technic and maybe an ultrasonic surface detector you could actually also give a % contamination figure for the water as well, particularly stuff like oil floating on the top.