How to measure water level in a vacuum...

For part of our test system, we need to measure water level inside a chamber, which is under vacuum. We were successfully using one of these by WIKA (see link). it has a 'breather' tube. This breather tube must be placed above the level of the water, but still inside the vessel. The sensor uses differential pressure to calculate the depth of water. It's stopped working because (we believe) that some water has entered into the open end of the tube. The problem is that the tube is in a hostile environment with water spray and we think that some drops of water have gone inside the tube,. possibly blocking it and now it reads erratically.

The big problem is that the water level was being used to control a scavenge pump, when the level reached say 50cm (a setpoint, ie user-defined level), the scavenge pump would turn on, so because of this problem the pump won't be controlled properly.

Can anyone suggest another water level sensor that doesn't need a breather tube?
https://www.alphacontrols.com/IL-10-IS-Submersible-Pressure-Transmitter/model/2743

PS the vacuum chamber is not transparent, nor can we add an external vertical tube to it . The signal must go back to the PLC

You could try an ultrasonic sensor on top of tank.
What is vacuum level, must not be much?

Check Automation Direct, they have a few different designs, that may work with vacuum connection.

These may work with one for pump on and one for pump off....
https://www.automationdirect.com/adc/shopping/catalog/process_control_-a-_measurement/level_sensors_-a-_controllers/vibration_fork_level_switches#sort=undefined%20desc&start=0

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

What is the desired accuracy?

The breather tube, if there is one, needs to extend downwards a long way before its opening to the volume at vacuum, and the end should be open downward, to keep water from getting back to the sensing element. Even so, it may still be possible for water vapor to condense in the tube. It may also be possible for surface tension to move water into the tube; what was the diameter of the breathing tube? A slow bleed from the outside could be implemented to keep water from moving toward the sensing element, but would use up some of the capacity of the vacuum pump/system.

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

Mas01 said:

nor can we add an external vertical tube to it .

Click to expand...

Can you add an internal vertical tube to it?

NetNathan said:

You could try an ultrasonic sensor on top of tank.
What is vacuum level, must not be much?

Check Automation Direct, they have a few different designs, that may work with vacuum connection.

These may work with one for pump on and one for pump off....
https://www.automationdirect.com/adc/shopping/catalog/process_control_-a-_measurement/level_sensors_-a-_controllers/vibration_fork_level_switches#sort=undefined%20desc&start=0

Click to expand...

Ultrasonic sounds an interesting idea. I shall read more about that.
It's not a perfect vacuum. It's about 20 Torr (27 mbar)

drbitboy said:

A slow bleed from the outside could be implemented to keep water from moving toward the sensing element, but would use up some of the capacity of the vacuum pump/system.

Click to expand...

Even better, a slow bleed of water into the tank via the breathing tube, pulled by the vacuum and overflowing the upward-facing end of the tube, could also be used.

Peter Nachtwey said:

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

Click to expand...

Indeed, hence the 'hostile' environment I referred to. The water gets hot and steamy.

drbitboy said:

Can you add an internal vertical tube to it?

Click to expand...

Possibly. What did you have in mind though?

Mas01 said:

Ultrasonic sounds an interesting idea. I shall read more about that.
It's not a perfect vacuum. It's about 20 Torr (27 mbar)

Click to expand...

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?

drbitboy said:

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

What is the desired accuracy?

The breather tube, if there is one, needs to extend downwards a long way before its opening to the volume at vacuum, and the end should be open downward, to keep water from getting back to the sensing element. Even so, it may still be possible for water vapor to condense in the tube. It may also be possible for surface tension to move water into the tube; what was the diameter of the breathing tube? A slow bleed from the outside could be implemented to keep water from moving toward the sensing element, but would use up some of the capacity of the vacuum pump/system.

Click to expand...

The vacuum is measured and goes to the PLC.
Desired accuracy should be say to the nearest cm, so we can use the high and low setpoint of the water level to turn on/off the scavenge pump.

Absolutely, drbitboy, I think water has condensed in the tube, but I can't confirm it. The tube itself is only a couple of mm in diameter, like the diameter of a refill of a biro pen.

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.

drbitboy said:

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?

Click to expand...

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