How to measure water level in a vacuum...

[Mas01]

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

The tank is very large, but it's impossible to weight it and its content.

I'm pretty sure we will be going down the route of float switches (digital inputs) to the PLC. E.g. if the lowest Digital input is TRUE, but the rest aren't, then water level =20cm (say). If bottom 2 are TRUE, but others aren't then water level = 40cm say, and so on. The high and low setpoints could still be used against these digital inputs.

 

[goghie]

I doubt that float level will work in satisfying manner. I would opt for force/weigh sensor even if you wrote that it is not option. Whatever mechanical preparation you need to do will be less challenging and expansive than any other solution. It is only long-term solution that gives good precision.

 

[Peter Nachtwey]

I have been watching you guys flail and fail while I have been eating pop corn and enjoying the show.

There are so many question that I have.
Mas01, if the chamber is in a vacuum, why doesn't all the water boil?.
If all the water boils to steam, the mass will still be the same so there is no point in using a weighing system. A float will not work because the density of steam is not higher than that of a float. Obviously, some of the fluid must be a liquid state. BTW, steam and air is a fluid.

If this system has worked before, what is the pressure and temperature inside the vessel? This will determine how much of the liquid/water, I assume it is water, is in a vapor state as opposed to a liquid state. There are steam tables that will determine the state of the fluid.

What kind of pump is doing the savaging? Some pumps are more susceptible to cavitation. drbitboy mentioned NPSH. Pumps will cavitate without a minimum NPSH.

More important, what is Mas01 testing?
There is nothing like finding the right solution to the wrong problem. This is not a PLC problem. It is a thermodynamics problem.

 

[Mas01]

I have been watching you guys flail and fail while I have been eating pop corn and enjoying the show.

There are so many question that I have.
Mas01, if the chamber is in a vacuum, why doesn't all the water boil?.
If all the water boils to steam, the mass will still be the same so there is no point in using a weighing system. A float will not work because the density of steam is not higher than that of a float. Obviously, some of the fluid must be a liquid state. BTW, steam and air is a fluid.

If this system has worked before, what is the pressure and temperature inside the vessel? This will determine how much of the liquid/water, I assume it is water, is in a vapor state as opposed to a liquid state. There are steam tables that will determine the state of the fluid.

What kind of pump is doing the savaging? Some pumps are more susceptible to cavitation. drbitboy mentioned NPSH. Pumps will cavitate without a minimum NPSH.

More important, what is Mas01 testing?
There is nothing like finding the right solution to the wrong problem. This is not a PLC problem. It is a thermodynamics problem.

What I am testing is covered in my previous posts on this thread, especially the ones with sketches.
The water doesn't boil because it's not hot enough.
Pressure inside the vessel is typically 20 Torr.

 

[goghie]

@Peter
Maybe I didn't read well but I understood that there is steam output so that's why it needs level measurement.

@Mas01
With enough vacuum there will be steam even if it is cold. If I converted good 20 Torr is 26mbar and there is no water, just steam.

 

[Mas01]

@Mas01
With enough vacuum there will be steam even if it is cold. If I converted good 20 Torr is 26mbar and there is no water, just steam.[/QUOTE]
Correct, 20 Torr is about 26mbar.
There's a constant flow of water into the vessel, so it won't all be steam. The water that accumulates, needs to be purged when the level reaches the high setpoint (and pump turned off at low setpoint).

 

[drbitboy]

@Peter Nachtwey: if you are mocking anyone about flailing, then why are you bringing steam into this? When 20Torr was first mentioned, it was mentioned as a vacuum, i.e. relative to atmospheric, i.e. the absolute pressure inside the vessel, above the liquid, is (~760 - 20) = 740Torr. That lowers the boiling point by less than 1°C (cf. here). So unless the incoming water is near atmospheric boiling point to begin with, it is not boiling and there is no "steam," in the common sense of the word, in the vessel; there is certainly water vapor, but it should not be called steam.

If the only things that can penetrate the pressure vessel, other than perhaps the existing vacuum measurement (draw?), are electrical signal and power wiring, then a system of floats may be the best option.

If it is acceptable to penetrate the drain piping, outside the vessel, near the pump suction for a pressure tap (atmospheric - suction differential), we can derive and estimate the liquid level from that plus the vacuum measurement; whether that estimate will be accurate enough is TBD, but I suspect it will be okay:

• We only need the high limit when the pump is not running, when the frictional and dynamic head losses in the drain pipe are zero, so it is a direct static head calculation.
• We only need the low limit when draining and the pump is running, and the actual issue is NPSH to prevent cavitation. So a pressure measurement at the pump suction is actually the direct, most relevant measurement; it would be pointless to convert, which involves estimating non-zero frictional and dynamic losses, that direct pressure measurement to level and comparing the level result to some minimum level setpoint, because using a low level to stop a drain pump only works in the first place because level is a cheap, approximate proxy for NPSH.

Sidebar: I am looking at my dad's 1967 ASME Steam Tables on my bookshelf, complete with Mollier diagram inside the back cover, the formulation from which my dad converted from Fortran to Basic on both IBM PC and Timex 2068. He had me fix one of the successive substitution convergence routines, which failed when using single-precision floating-point (4 bytes on the PC; 5 bytes on the Timex) in one regions of the H-S diagram. When Dad, with an automotive tech associate's degree from LIATI Farmingdale, interviewed for the GE Test Program for technicians in the 1950s, he was asked what he knew about the Mollier diagram. He said, "I can spell it." The interviewer replied, "That's better than some of my engineers."

 

[I_Automation]

I just thought of the perfect smart-a** solution.


Mark the inside of the tank with a tape ruler.
Give a worker a scuba outfit, flashlight and waterproof remote tablet and tell him to see what the level is every (xx seconds/minutes) and enter it in the tablet. Just make sure he has a scuba tank big enough for a 40 hour workweek.

 

[Mas01]

@Peter Nachtwey: if you are mocking anyone about flailing, then why are you bringing steam into this? When 20Torr was first mentioned, it was mentioned as a vacuum, i.e. relative to atmospheric, i.e. the absolute pressure inside the vessel, above the liquid, is (~760 - 20) = 740Torr. That lowers the boiling point by less than 1°C (cf. here). So unless the incoming water is near atmospheric boiling point to begin with, it is not boiling and there is no "steam," in the common sense of the word, in the vessel; there is certainly water vapor, but it should not be called steam.

If the only things that can penetrate the pressure vessel, other than perhaps the existing vacuum measurement (draw?), are electrical signal and power wiring, then a system of floats may be the best option.

If it is acceptable to penetrate the drain piping, outside the vessel, near the pump suction for a pressure tap (atmospheric - suction differential), we can derive and estimate the liquid level from that plus the vacuum measurement; whether that estimate will be accurate enough is TBD, but I suspect it will be okay:

• We only need the high limit when the pump is not running, when the frictional and dynamic head losses in the drain pipe are zero, so it is a direct static head calculation.
• We only need the low limit when draining and the pump is running, and the actual issue is NPSH. So a pressure measurement at the pump suction is actually the direct, most relevant measurement; it would be pointless to convert, with non-zero frictional and dynamic losses, that direct pressure measurement to level and comparing the level result to some minimum level setpoint, because using a low level to stop a drain pump only works because level is a cheap proxy for NPSH.

Sidebar: I am looking at my dad's 1967 ASME Steam Tables on my bookshelf, complete with Mollier diagram inside the back cover, the formulation from which my dad converted from Fortran to Basic on both IBM PC and Timex 2068. He had me fix one of the successive substitution convergence routines, which failed when using single-precision floating-point (4 bytes on the PC; 5 bytes on the Timex) in one regions of the H-S diagram. When Dad, with an automotive tech associate's degree from LIATI Farmingdale, interviewed for the GE Test Program for technicians in the 1950s, he was asked what he knew about the Mollier diagram. He said, "I can spell it." The interviewer replied, "That's better than some of my engineers."

No, no...the absolute value is 20 Torr inside the vessel.
Outside the vessel is atmospheric, ie 760 Torr.

 

[drbitboy]

What kind of pump is doing the savaging?

LOL. Gotta love auto-correct!

 

[drbitboy]

No, no...the absolute value is 20 Torr inside the vessel.
Outside the vessel is atmospheric, ie 760 Torr.

Whoops! I take it all back. LOL.

I'm a flailing idiot: you did not say 20Torr absolute until now, but a six-inch thick vessel is not required for a 20Torr differential pressure, so I should have known.

And the water pump is positive displacement, not centrifugal?


How far is the pump elevation below the bottom of the vessel, or how high is the vessel elevated?

 

[Peter Nachtwey]

No, no...the absolute value is 20 Torr inside the vessel.
Outside the vessel is atmospheric, ie 760 Torr.

you did not say 20Torr absolute until now,

@Mas01, when you said vacuum in your first post I thought you meant near perfect vacuum. My assumption matched what you said in the quotes.
That is close to perfect given the fluid inside. There will be some vapor pressure

Now why doesn't the water boil again?
https://www.engineeringtoolbox.com/water-evacuation-pressure-temperature-d_1686.html
Now what is the boiling point of water at 20 torr absolute?

Time to pop another bag of popcorn.

 

[drbitboy]

According to the link posted earlier, vapor pressure of water is 20Torr at around 22°C/72°F.

The critical number is the NPSH required for the pump.

As @danw stated, all pressures are differential, and even though I misunderstood the reference for the 20Torr measurement, everything else I said about using a pressure tap at the pump suction, to control when to turn the pump on and off, is valid.

 

[NetNathan]

Gonna need some sort of level switches (for high and low), if analog not needed.

 

[Regular_Jim]

@Peter Nachtwey why are you being such a jerk? The man says the tank is at 20 torra and he is trying to measure level of water. If they're designing a test system, do you really think the mechanical/chemical engineer involved doesn't know his/her steam tables? It's worth it to bring it up, but harping on it non-stop, obviously not reading the rest of the thread, and then condescending not only OP but everyone trying to help him isn't a good look.