Water Level Control Without Moving or Electronic Parts

A constant water feed and an overflow from the tank.
no moving parts, no electric, just a waste of water.

Unmovium parts are made out of unobtanium.

Just modify, (if necessary), the same apparatus that keeps the tank on the back of a toilet full. Cheap and easy.

Sorry kamerat, that's got moving parts.

Which reminds me I've got to fix the ball valve in the upstairs toilet.

You're right, probably too complex. I was just thinking about what I would use. A simple float valve using combination of gravity/existing vacuum to feed water to container from reservoir. Oh well, I tried.

Since you give NO information on what you are really trying to do, just use a continuous supply of water and control the level with a weir.

Weir - Wikipedia, the free encyclopedia

Fill container to required level, plug all holes. It should stat at that level.

I see several possible ways of doing it:
1. measure distance from the above till the surface of liquid. Here can be used radar type, guided wave, laser, ultrasonic.

2. use hydrostatic method, but there a pressure different from the atmospheric must be compensated.

3. put an external side tube with a float inside with magnetic coupling indicator.

My suggestion is somewhat like Ilmaris.

A very simple solution I am giving. (I presume the water is at normal temperature.) Here we will use a small electronic part called REED SWITCH. It is a small glass cartridge part having dia. of around 6-8mm and length 20-30mm with two contacts inside. The property of this switch is, the two contacts housed inside are normally open, but closes when comes near any magnetic field. This Reed switch will have to be placed inside a glass tube of sufficient length and inserted from the top of the container upto certain depth of the water( lower than the lowermost level of water). A small float has to be constructed out of some nonmetallic part having annular space inside and capable of sliding up and down the glass tube. A small magnet will be placed inside this float(this can be easily constructed with some plastic tubes and sheets guled together with adhesives like 'Araldite' or something which is unaffected by water. The float must be water tight). The two contacts of the Reed switch will be connected to two wires and they will come out of the container through the glass tube. The glass tube connection to the container must be air tight to maintain the vacuum.

Installation: The Reed switch will be positioned at such a depth inside the glass tube that it coincides with the level of water to be maintained in the container. In this position it will be glued again with 'Araldite' in sufficient quantity so that water doesn't enter the glass tube. The two wires coming out of the glass tube now has to be connected to any suitable circuit able to covert the signal for controlling the flow. Whenever the float comes near the reed switch the contact will close and that signal will be processed.

With a small magnet inside the float an accuracy of 15-20mm can be achieved.

(This is a proven design though appears to be crude).

Thanks

I believe this is what you are looking for although it does require a float.

first what you mean of vacuum from an engine? a vacuum pump or a vacuum is created in the tank due to engine operation or the water reservoir is used to seal the vacuum in the engine?

the vacuum itself can be control to maintain the water level of a tank. just vacuum pressure against weigth of water. Say a relief valve set to open if the set vacuum pressure is reach. hope you have enough height for a water column.

There are several types of solid state level detectors on the market. Her are two examples of those used in bilge systems:

These solid state switches certainly beat the older float type switches