I am currently working on a project for which I need reservoirs with electronic liquid level measurement. I have some ideas on what to use, but I would like feedback from other engineers—maybe I can find something which already exists for a similar application and is widely available. The tanks need to satisfy the following criteria:

• Volume of approximately 30 litres
• Under partial vacuum (saturated water/vapour at room temperature)
• Shallow and wide aspect ratio (to limit liquid level variation as liquid volume changes)
• Top and bottom openings
• Continuous liquid level measurement (electronic analogue sensor)

For the tank, a horizontal cylinder with top and bottom openings (in the curved surface) would be ideal, however I don't know where I can get such cylinders. My best ideas so far for the tank are as follows:

• Flow-through expansion vessel with diaphragm removed (however would ideally be shorter than it is wide)
• Automatic ceiling mounted fire extinguisher (however will lack the top opening and will be <30 litres)
• Car fuel tank (however would probably not withstand a partial vacuum)

As for liquid level measurement, I believe ultrasonic sensors won't work because as the fluid will be saturated, any condensation would affect the sensor. Thus my only option seems to be capacitive probes. I was thinking of fitting a riser in parallel to my tank and insert a probe through the top of it.

Any better ideas or improvements are very welcome.