Mark the level on the outside of the glass at some determined interval. Or get a cheap dp cell and sit it in the bottom, it would be sensitive to density changes as well though.
Could you sit the glass bottle on a weight measuring scale with a read out system attached - this will allow you to measure the weight of the bottle and contents throughout the process which you could then convert to level measurements at what ever setpoints you need.
A dp is differential pressure switch, which could also be used measure the bottle and contents mass or use the method as suggested by ash9144
I tried marking the outside of the glass bottle, but it is not accurate enough. We did think about placing it on a scale, but there are many tube coming in and out of the bottle, even with hanging it in a way that will not touch the bottle, it would not be accurate to at least 200ml. Do I put the dp switch inside the bottle? is there a way to measure using hydrolic pressure? and specific gravity.
You would want a dp cell not switch. You could then place the high pressure cell in the bottom of your tank with low cell outside tank. Then calibrate with solution density of choice. The dp then changes based on liquid height and liquid density.
The dp cell measures the differential pressure at bottom of liquid compared to the pressure of vapor over the liquid surface. This is used with density of the liquid to derive the liquid height. If density of liquid changes during the reactions, then there will need to be some factor to alter the calculation of liquid height. The density change could be measured, and incorporated into the calculation for the liquid height - you must decide whether it is required. The reaction mass (reactor + contents weight) might be more important than the absolute liquid height measurement.
Thinking about vapor pressure over the liquid, if it is open to atmosphere, then the dp cell only needs to be referenced to the atmosphere. But if your glass 5 gallon bottle is pressurized, then there are other concerns.
A pressurized reactor must have some pressure safety relief system. A glass bottle would not be a good container for any more than a few psi as pressure vessel. Do you have any scenario where a run-away reaction might result in extra pressure or extreme heat release that would greatly increase the pressure inside your glass bottle?
Make the tank narrow and tall. If the changes in level are too small to measure with your eye, then a low cost dp cell may not give you the results you need either.
