Dear Friends,

I am trying a simple method to sense liquid level in an open tank using hydro-static principle. Though it resembles bubbler/purge method of level sensing, it avoids the need of external purging thus making it simpler. The setup consist of a hollow SS tube of particular diameter which is running till the bottom of tank. The top end of tube is attached to a U-tube manometer via a narrow pipe. Manometer show zero deflection until the liquid level in the tank touches the bottom tip of SS tube. But once the liquid level crossed the bottom tip, a particular volume of air is trapped between the liquid in the SS tube and the manometric fluid.The pressure of this trapped air increases proportionally with the rise in liquid level of tank, which shows a deflection in manometer. The Δh of manometer can be calibrated to liquid level H and the level can be measured.

Now the issue is temperature compensation. The trapped air expands and contracts w.r.t ambient temperature. For a particular liquid level H, the deflection in manometer is constant provided a constant ambient temperature. If the temperature is increased the pressure also increased showing a positive deflection in Δh and vice verse. This variation Δh vs temperature is linear, but the slope is different for different liquid level H, so a direct compensation is not possible. With the change in temperature the volume, pressure and density of trapped air is changing. I am stuck up with this problem, how to compensate level w.r.t. change in temperature and I am not getting the right way to continue.

 

FYI, a year ago, maybe two years ago, a forum topic involved diminishing air pressure in a closed column over water. The consensus seemed to be the air was absorbed gradually by the water. So you've got temperature compensation and air loss by absorption to deal with. (I can't recall which forum it was, this one or another, sorry).

 

Why are you so against adding some air to maintain the level?
To keep it simple use a simple restriction like a sintered gauge snubber, you only need a bubble every minute or so.

 

I have to agree with Roy. I cannot see the benefit of a measuring system like this. Also making use of a compressible medium to act on a incompressible medium and then expecting a linear result, might also not be the best of ideas. I also think you will have a lot more to worry about with a system like this than the effects of temperature changes.

Anyway that is just my opinion. If you want to carry on and play with this, use lagging on the S/S tube going down into the tank. Lagging comes in various form and you can even make your own depending on what you have to work with. The idea behind lagging on a pipe or vessel is to keep the temperature inside the vessel or pipe constant.
In short lagging is when you insulate the pipe or vessel with a neutral material like asbestos or glass fiber cloth that are not influenced by temperature changes and will therefore not transfer temperature changes from its surface to the pipe or vessel. Look at what they use on refrigerator piping or on steam pipes and you will get the idea. If you still have problems you can even install heating elements inside the lagging and control the temperature.

 

FWIW ...

I recently dismantled a discarded washing machine, mainly to get at the motor for use as a generator. The level sensor was as described - a tube running from bottom of the machine to the top, with a pressure sensor at the top. In the environment, this is quite adequate - after all, does it matter if the level is not sensed to the exact mm? And the environment at the top of the machine with the other electronics is a lot better than that at the bottom.

 

In the washing machine application the gradual absorption of air into the water is not a problem because a washing machine spends most of its time empty. Each use starts with a full and fresh column of air to compress. But long-term tank level measurement is another story entirely.

Robert Scott
Real-Time Specialties
Hopkins, MN

 

I wonder why you couldn't simply use a submersible air-bladder suspended near the bottom of the tank and plumbed to a gauge and simply read the pressure change in it as the water level changes. It seems that it would easy enough configure something like that.

Thanks
Mark Allen

 

It also undergoes a fairly rapid change in temperature if the machine is filled with cold water and then heated. A good example of matching the measurement tool to the application - in practice, I'd be surprised if it's used for anything more than a variable-setting switch during filling operation.

But as a one-time instructor, this would have made a very good exercise to illustrate the importance of an understanding of the gas laws. Not quite as spectacular as http://www.driedger.ca/dp-1/Tank-1Profile.jpg though!

 

Thanks for your information.

The absorption of air by water can decrease the pressure inside the tube. But to my knowledge it is a reversible process and the solubility is less for a water column of 1m and temperature range of 20 -40 which of my interest. While calibrating liquid level vs Δh this absorption will be taken care to some extend. I have done some experiments in 1m tank where Δh holds good for a particular level nearly for a month provided the setup is leak proof.

 

I am trying this method to avoid purging of air. There are some application where purging of air/gases is not recommended.

 

Thanks for you suggestions. The pressure exerted by an in compressible medium is transferred via a compressible medium and I am not expecting a linear result. it can be achieved by other means. Yes there are other issues than temperature effects. But now the bottle neck is temperature compensation. Lagging is one method to make the temperature of trapped air constant. But it make the system isolated from environment, which can invite some other issues to the frame. The system in equilibrium with atmosphere is highly reliable where I can measure the air temperature and compensate level for its change.

 

You are correct. Similar approach is used in washing machines to sense the level. But there the measurement is coarse, to sense hardly 3-4 discrete levels, and error due to temperature is tolerable.

 

> I wonder why you couldn't simply use a submersible air-bladder suspended near the bottom of the tank and plumbed to a
> gauge and simply read the pressure change in it as the water level changes. It seems that it would easy enough
> configure something like that.

Thanks for your suggestion. Suspending an elastic diaphragm near the bottom of tank solves the issue, but it reduces the sensitivity drastically. The change in pressure w.r.t. change in liquid level will depend on the capability of diaphragm to detect hydro-static head.