liquid level sensing - potential leak 2

[skacur]

we are experiencing what appears to be a slow air leak in the following setup:

low pressure sensor connected to a tygon tube via 1/8" hose barb; the tygon tube is submerged in a liquid; the hydrostatic pressure of the liquid compresses the air in the tube; the sensor measures the air pressure; the linear relationship between hydrostatic pressure and liquid level enables liquid level determination

problem: over the course of several weeks we've noticed a slow, steady drop in pressure while a constant liquid level is maintained. of note, the small "slug" of water at the bottom of the tube climbs slowly and steadily indicating an air leak.

two questions:
1 - if this is an air leak, is it through the sensor, the hose barb seam or the tubing itself (permeation)? we have tested several variations of this with no conclusive answer

2 - if this is not an air leak, what is causing this phenomenon?

thank you in advance

 

[StoneCold]

We have a couple of these units, but we put a small flow of regulated air into them all the time, so they bubble slowly all the time, it help keep them from plugging and supplies air all the time to avoid the problem you are having.

Hope that helps.

StoneCold

 

[moltenmetal]

Sounds like the air in your Tygon tube is simply dissolving over time rather than leaking out.

Solutions are to either ensure your impulse line (your Tygon tube) remains empty by purging it the way StoneCold describes (with a CONSTANT, low flow of air or other gas such that it becomes a "bubbler"), OR ensure that the leg is filled and remains full throughout. The latter is best if you can fill the leg and if the transmitter can handle being directly wetted by the process solution.

 

[monkeydog]

It could not be air absorbing into your liquid?

 

[skacur]

Thank you for the replies.

Stonecold: Great suggestion, but adding this would be cost prohibitive for this application

Moltenmetal/Monkeydog: Air dissolving into the water may be a factor, but I think we are losing more air elsewhere. I say this because we've submerged an inverted glass test tube and there is no evidence of leakage after several weeks (no water slug climb). This is my main reason for thinking we are losing air elsewhere in our setup. Direct liquid contact with the sensor is not possible either.

We've been leaning toward air permeability of the tubing and/or leakage past the barb. A customer of ours actually had a separate application where they were pulling air thru tygon tubing. They switched to Saran tubing which has a lower permeability. Unfortunately, Saran tubing is made to order in large quantities only. Anybody know of a better source for this or an appropriate substitute? Thanks again.

 

[monkeydog]

Without any details of the test set-up kinda hard to provide any more constructive feedback.

 

[bchoate]

skacur

If permeation is the problem, a different tube material may help. Units for a permeability coefficient are as follows:

cc X mil / 100 sqin/day/atm

General purpose grades of tygon have coefficents of 105 and higher. Saran (PVDC) has a coefficient of 0.1. EVOH (used as a barrier layer in PEX radiant floor tubing) has a coefficient of 0.05. A significant reduction in permeation can be obtained by switching to a F-4040-A grade tygon tubing (used for fuel lines) available from Cole Parmer (coefficient = 9.3). Nylon tubing is translucent and less flexible. It's coefficient is about 2.6. PFA (teflon allomer) has a coefficient of 19.3.

If permeation is the problem, then tubing selection can only mitigate its effect. Permeation will occur. Mechanical air losses can be practically eliminated by improving connections. If economics or other factors are not prohibitive then a PT with a diaphragm seal is a better solution.

 

[skacur]

bchoate: Thank you for the suggestions. PEX would be worth trying but we need 1/8" I.D. and I've only been able to find 1/4" I.D. minimum. Also, I found one reference that says PEX should not be exposed to sunlight which could be prohibitive.

We've also tried Fluran F-5500-A grade Tygon tubing which St. Gobain shows as having an even lower permeability than F-4040-A grade. Unfortunately, we've seen no difference between Fluran and Tygon B-44-3 which we use as standard. This made us question if permeability is a main cause.

FEP shows some promise but it and Nylon are a bit more rigid than we would like since we are stuck using the hose-barb connection of the sensor. Saran looks to have the same issue.

If I remember correctly, submersible sensors were outside the budget of this project.

 

[bchoate]

shakur

Permeability coeficients given previously were for oxygen as a reference; other gases co-permeate. If the comparison between the standard tygon and the fluran has been conducted under similar conditions for a period of time sufficient to demonstrate any differences, then perhaps permeation isn't the major contributor to the observed problem. What can we say about a potential hose barb connection leak? A leak through a small hole is essentially an orifice calculation. With air at near ambient conditions in the tube, the DP driving force would not be large. Similarly, wouldn't the leak also be very small from a small hole around the hose barb under such conditions?
To eliminate or at least minimize the hose barb as a potential leak, try wrapping the made-up connection with several layers of teflon tape or using teflon dope.

Under static conditions as you describe above (constant level) there is a slug of water in the end of the tube. You comment that more of the tube fills with water over time under such conditions. If the level is truly constant for a long period of time as stated, what happens to the amount of liquid in the tube if the temperature changes?

One other possibility here is that there is a leak across the sensor. The reference side of the sensor is the atmospheric pressure, yes.

A PT with a diaphragm seal is flange mounted not submerged. There is a diaphragm that isolates the process from a column of fluid in the sensor leg. Pressure is transmitted across this diaphragm to the PT.

 

[skacur]

Does anyone happen to know if there are any coatings that can be used to improve the gas permeability of Tygon? I've been looking into CVD(chemical vapor deposition), PVD(physical vapor deposition) and related technologies. There are plastic beverage bottle manufacturers that are using coatings, but I haven't found any contract manufacturers that do this work. Any ideas?

 

[StoneCold]

How about spray painting the outside of the tubing to create a layer with lower permeability?