Bubbler for level indication

Hello friends,

I have been tasked to verify if a bubbler system is operating correctly.

The transmitter is a Foxboro IDP20 with a range of 0 to 120 in H2O. The structure with the process water in it is 8 feet in depth. I'm applying 20 psi of air at 1.5 liters/min. Whats got me concerned is that the display is showing negative value of 1.4 in of H2O and as I reviewed the setup there is a negative .4 in of H2O offset programmed in. Can someone help me understand why this -.4 offset might be used. It seems like such a small negligible value. Also a negative offset is something I don't completely understand.

I'm not convinced this xmtr is setup correctly, but I need to understand a few things to verify the setup.

Thank You for any input you can offer.

If you are dealing with a vented tank, it could be the off-set needed to bring the Xmtr reading inline with the actual level per level gauges or obtained by "sticking" the tank.

If gas blanketing is used, the low side of the xmtr will be connected above the high liquid level to off-set the gas pressure.

In the first 10 google hits a spec sheet for IDP20 did not appear.

What is an IDP 20? A DP or gauge pressure transmitter? If DP which side is connected to the bubbler dip tube? Is the other side connected to the vapor area at the top of the tank or open to atmosphere? What is the transmitter's range?

My mistake the pressure transmitter is a Foxboro IDP10 with a range of 0-200 inches of water. This transmitter is used in conjunction with an identical IDP10 as a differential level indication. The transmitter I'm concerned about I'll call X2 the low side of the weir and the other transmitter will be X1 the high side of the weir.

X1 high side piping has its dip tube connected to it's high side, which is in the high side of the weir and a tap off of it going to the high side of X2. X1 low side is open to atmosphere. X2 low side is connected to the dip tube in the low side of the weir. The weir is in a open chamber vented to atmosphere.

Where do you read the -1.4"?
xmtr's local indicator?
loop meter?
DCS/PLC/HMI?

What is the measurement associated with the -1.4?
- X1 weir high side depth?
- X2 which is X1 weir high side septh minus X2 weir low side depths
- something else
What measurement value do you expect this to be, if not -1.4"

Where do you read the -0.4" offset?
xmtr's LRV?
loop meter tag's zero?
DCS/PLC/HMI tag's zero?

Where can a 0.4 offset come from in bubbler systems

I know of one source. Typical dip tube construction has a notch ground into the bottom of the dip tube, an inverted V. The bubbles come out at the top of the notch. So the true level zero measurement starts at the elevation of the top of the notch. But the water depth includes water to the bottom of the dip tube pipe (which is not included in the bubbler measurement without an offset). A 0.4" notch height would be very typical.

Supposing the dip tube is 10 feet long for an expected maximum 96 inch (8 foot) depth from the bottom of the dip tube where the dip tube has a 0.4" notch. Whoever installs it would use an LRV of -0.4" and a URV of 95.6" for a 4-20mA span of 96" (8 ft). The -0.4 offset takes into account the 0.4" of water between the top of the notch and the bottom of the dip tube. 4.0mA is the water level 0.4" below the top of the notch. The notch is at +0.4", the elevation above the bottom of the dip tube. Sometimes that offset is larger; it can be multiple inches to account for water depth beneath the bottom of the dip tube that isn't measured by the bubbler.

Whether someone cares about the difference between the top of the notch and the bottom of the dip tube depends on how they're using the bubbler.

What happens if you turn the air right off for a few minutes? It should hold the level. Otherwise, you may have a leak in the tubing between transmitter and tube. You might also have too much flow causing a pressure drop if you are injecting air at the transmitter. It's always a tradeoff between injecting it at the transmitter to sweep the impulse line OR at the top of the bubble tube to eliminate the effect of flow.

Merriam used to make a small glass pot usually filled with antifreeze. They were very good, because you could set the the bubble rate with a needle jet and they would hold steady for years at a time. I believe they were called bubblers as well.