How to measure water level in a vacuum...

[Mas01]

Ultrasonic is not a player. Ultrasonic depends on a known sonic velocity through a gaseous medium (air) at a known temperature. Any level of vacuum takes ultrasonic off the table.


Radar, on the other hand, doesn't care about the atmosphere or lack of it, above the liquid level.

Radar sounds great, and probably expensive LOL.
Good suggestion for sure.

 

[Saffa]

If you are actually measuring the vacuum pressure, then you could use an absolute pressure sensor inside the water and do the math in the PLC to compensate for the pressure / vacuum above the water level.

We do this on all our new deep well bore level sensors... we used to use the hydrostatic level sensors with the compensation tube, but even with the little filter on the end of the tube we found they would fail regularly with evidence of condensate in the tube. At around 150m deep, there was pretty much always a difference in temperature across the cable / tube.

We now have an absolute sensor in atmosphere measuring atmospheric pressure, then a fully sealed absolute sensor down the hole, and just subtract atmospheric from the bore sensor reading and convert to metres water head

 

[drbitboy]

Possibly. What did you have in mind though?

This. The up and down section could also be almost entirely inside the tank. It still needs to have a bleed or disconnect at the top of the sensor to check for condensation occasionally.

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[Mas01]

20Torr is less than a foot of water (20*13.6÷25.4 = 10.7"). That is not a lot. What is the range of the water level in the tank?

The range is 0-800cm.
The WIKA submersible sensor measured up to 1m depth.

 

[danw]

Preventing condensation in the atmospheric reference capillary tube of a submersible pressure transmitter is difficult even under 'normal' atmospheric conditions, where a desiccant filter is typically used to attempt to trap moisture before it enters the reference tube.

When moisture in vapor state inside the reference tube gets cooled below its dewpoint, it will condense in the reference tube, and eventually collect at the bottom of tube where it exerts a pressure on the 'low' side of the pressure sensor creating an error in the reported pressure, where error is a lower-than-expected pressure compared to the actual pressure.

The dynamics of operating in a vacuum are likely causing a changing level of condensation as water condenses and then boils off again.

 

[drbitboy]

Water is pumped into the tank, and sprayed at high pressure onto rotating blades. The water level will rise.

yes, but what is the range of level of that water at the bottom of the tank? Upper limit is 50cm; what is the lower limit?

 

[Mas01]

This. The up and down section could also be almost entirely inside the tank. It still needs to have a bleed or disconnect at the top of the sensor to check for condensation occasionally.

View attachment 62509​

I wish I could do this, but our vessel is big. It will easily fit a commercial aircraft engine inside it and is probably made from 6"thick steel. It's also circular in profile to reduce stress levels when pulling a vacuum.

 

[drbitboy]

The range is 0-800cm.
The WIKA submersible sensor measured up to 1m depth.

800cm is 8m; did you mean 80cm?

 

[danw]

>Radar, on the other hand, doesn't care about the atmosphere or lack of it, above the liquid level.

I was thinking "vacuum" when I wrote that.

However, steam, being water, has a relatively high dielectric constant value (as does water) and that can affect radar because radar works on the principle of reporting distance (or level) at the point where there is a change in dielectric constant. The difference in dielectric between steam and water might be close enough for radar to struggle. Radar level transmitter manufacturers have reported some success in steam vapor applications so it might worth inquiring about.

 

[Mas01]

yes, but what is the range of level of that water at the bottom of the tank? Upper limit is 50cm; what is the lower limit?

Well 50cm was an example. Realistically more like 80cm (scavenge pump on) & low level is around 10cm (scavenge pump off - to prevent damage to pump running dry).

 

[Mas01]

800cm is 8m; did you mean 80cm?

My mistake. I meant 80cm.

 

[Mas01]

If you are actually measuring the vacuum pressure, then you could use an absolute pressure sensor inside the water and do the math in the PLC to compensate for the pressure / vacuum above the water level.

We do this on all our new deep well bore level sensors... we used to use the hydrostatic level sensors with the compensation tube, but even with the little filter on the end of the tube we found they would fail regularly with evidence of condensate in the tube. At around 150m deep, there was pretty much always a difference in temperature across the cable / tube.

We now have an absolute sensor in atmosphere measuring atmospheric pressure, then a fully sealed absolute sensor down the hole, and just subtract atmospheric from the bore sensor reading and convert to metres water head

Sorry, I missed this post earlier. Sounds like a potential solution. Could you post a link to some suitable sensors?

 

[drbitboy]

Sorry, I missed this post earlier. Sounds like a potential solution. Could you post a link to some suitable sensors?

It doesn't need to be absolute pressure; same idea, see below.

I probably got at least one sign wrong, but the concept is solid. The concern would be the penetrations of a large pressure vessel; radar or other non-contact methods, or even float, might be less expensive than drilling holes through 6" of metal.

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[Mas01]

It doesn't need to be absolute pressure; same idea, see below.

I probably got at least one sign wrong, but the concept is solid. The concern would be the penetrations of a large pressure vessel; radar or other non-contact methods, or even float, might be less expensive than drilling holes through 6" of metal.

View attachment 62510
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Again, nice idea and I appreciate it, but drilling through the vessel is a no-no. We don't want anything that could potentially compromise the integrity of the vessel. A series of float switches, though, would be a cheap and cheerful solution. It wouldn't give us as fine a control of the setpoints but we might be able to live with that.

 

[drbitboy]

Same concept, possibly a simpler install.

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[Update: corrected typos]