How to measure water level in a vacuum...

Peter Nachtwey said:

Water boils in a vacuum. This is how naval ships distill water.

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P.S. @Peter Nachtwey was the first to mention real issue, many posts ago.

drbitboy said:

P.S. @Peter Nachtwey was the first to mention real issue, many posts ago.

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Yes, watch video.

https://www.google.com/search?q=Water+boils+in+a+vacuum&rlz=1CAXDKE_enUS957&sourceid=chrome&ie=UTF-8

Hi, Sorry if this is a silly suggestion, because it sounds too simple so i am most likely missing something. Maybe my suggestion will not work in your process conditions.

Can you use a float sensor? Like how a lavoratory cistern controls the water level?

thehoneybadger said:

Hi, Sorry if this is a silly suggestion, because it sounds too simple so i am most likely missing something. Maybe my suggestion will not work in your process conditions.

Can you use a float sensor? Like how a lavoratory cistern controls the water level?

Click to expand...

I have read some things in this thread that imply that discrete point level control is what is required. I don't really know how important an analog signal is to the OP.

one or more float sensors were suggested earlier, and Post #29 from the OP said it might be a workable solution.

I do not know the exact system we are speaking of. Based on the size of the vessel that has been described, i would not necessarily use a float to produce an analog signal that measures liquid level. As i understand it, you need only a device that measures liquid level for the purpose of controlling a large volume of liquid to a set point. For this purpose, i would imagine a float, used to send a digital on/off signal, ie Level below set point ON, Level above set point OFF, would work.

thehoneybadger said:

Hi, Sorry if this is a silly suggestion, because it sounds too simple so i am most likely missing something. Maybe my suggestion will not work in your process conditions.

Can you use a float sensor? Like how a lavoratory cistern controls the water level?

Click to expand...

Yes, using float switches is a good and cheap solution.

Too bad they can't get the WIKA breather tube out to the atmosphere via the gland.

drbitboy said:

View attachment 62537


Too bad they can't get the WIKA breather tube out to the atmosphere via the gland.

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How would it work out if a linear encoder acted as the rail for the float switch? Would finding a device for this temperature range be the biggest hurdle?

Also found these: https://www.sick.com/us/en/encoders/wire-draw-encoders/c/g286652
Wire encoders. If a ball can reliably floated on the water under some cable tension, this is a potential solution.

Or maybe a laser distance sensor, not sure how they work with steam though.

drbitboy said:

View attachment 62537Too bad they can't get the WIKA breather tube out to the atmosphere via the gland.

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The submersible transmitter needs the breather tube inside the vessel, sensing the vacuum pressure, because only configuration that will provide a valid pressure/level reading.


All pressure sensing is inherently differential, meaning comparison to a reference pressure. The reference pressure in that vessel is the pressure above the liquid, whether that pressure be a partial vacuum, a full vacuum or atmospheric pressure.


To bring the breather tube out to atmosphere would put atmospheric pressure on the "low" side of the submersible sensor, which gets subtracted from the "high" side of the sensor, which would result in an error of an unrealistically low reported pressure because atmospheric pressure is much, much higher than the 20 Torr that the vessel runs at while in operation.



The absolute pressure measurement recommended in one post would work because its reference side is sealed at as-close-to-absolute-zero pressure as is commercially feasible. But it requires the measurement of the above-liquid vacuum space, as well.



Submersible pressure sensors are usually used with the reference 'breather' tube exposed to atmospheric pressure because the liquid level is exposed to atmospheric pressure.

danw said:

All pressure sensing is inherently differential, ...

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Mas01 said:

drbitboy said:

Are you measuring the vaccuum level? If yes, is that measurement, or can it be, made available to the PLC?

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The vacuum is measured and goes to the PLC.

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@danw: yes, all pressure sensing is differential; now do you see it? It's not ideal, but it might be good enough.

This ain't my first rodeo.

_

Since we cannot penetrate the pressure vessel, can we instead penetrate the piping near the pump suction, to add a pressure tap (red in the image below)?

thehoneybadger said:

I do not know the exact system we are speaking of. Based on the size of the vessel that has been described, i would not necessarily use a float to produce an analog signal that measures liquid level. As i understand it, you need only a device that measures liquid level for the purpose of controlling a large volume of liquid to a set point. For this purpose, i would imagine a float, used to send a digital on/off signal, ie Level below set point ON, Level above set point OFF, would work.

Click to expand...

Thanks, yes we are thinking about this as a potential solution.

drbitboy said:

View attachment 62537


Too bad they can't get the WIKA breather tube out to the atmosphere via the gland.

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Nice animation!

It sounds like the tank is large. But, would it be possible to weigh the tank to determine how much water is inside?