How to Commission a Bubbler Level System

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Gather What You Need

Assemble the dip tube cut and installed to a fixed, known depth, a clean and dry purge gas supply with a regulator and a needle valve or a self-regulating purge-flow controller, a rotameter or flow indicator to see the bubble rate, the back-pressure transmitter, and a calibrated tape for the verification. Confirm the purge gas is compatible with the process, since a bubbler dumps its gas into the vessel, and instrument air or nitrogen are the usual choices.

Know the liquid density before you configure the transmitter, exactly as you would for any hydrostatic measurement, because the bubbler reads pressure and converts it to height by dividing by specific gravity. The measuring principle is identical to a hydrostatic level transmitter with the dip-tube tip standing in for a submerged diaphragm, and the packaging tradeoffs are covered in the overview of the bubbler air-purge level method. Have the specific gravity from process data or a lab result.

Set the Purge Flow to a Steady Bubble

The purge rate is the heart of a bubbler and the thing people set wrong. You want the smallest gas flow that still produces a continuous, unbroken stream of bubbles at the tube tip under every operating condition, including the highest expected level when back-pressure is greatest. Watch the rotameter and open the needle valve until bubbles form a steady chain, then back off to the minimum that keeps them going. A self-regulating purge controller holds this automatically and is worth using where the level swing is large.

Too little flow and the bubbles stall at high level, so the reading lags or reads low until pressure rebuilds. Too much flow is the subtler error: the gas rushing down a long, small-bore dip tube develops a friction pressure drop that adds directly to the measured back-pressure, so the transmitter reads a level higher than reality. The friction rises with flow, so an over-purged bubbler reads high, and the error grows if someone later opens the purge to clear a suspected blockage. Set it low and leave it.

Configure Density, Range, and Zero

The back-pressure the transmitter sees equals the hydrostatic head from the liquid surface down to the dip-tube tip, so the tip elevation is your zero. Enter the fluid specific gravity so the transmitter converts that pressure to the height of your liquid, then set 4 mA at the lowest level you need to resolve above the tip and 20 mA at the highest expected level. Any liquid below the tube tip is invisible, so treat the tip as the bottom of the measurable range and account for it when you map 4 mA.

Zero the transmitter with the tube open to atmosphere or at a known empty condition where the head is zero, then confirm the span with the range endpoints. This is the same zero-and-span logic as any pressure loop, described in span and zero, and because the bubbler transmitter is just reading pressure you can inject a known pressure to prove the electronics separately from the tube. Record the as-found and as-left values on the commissioning sheet.

Verify Against a Hand Dip

Prove the whole system with an independent measurement: drop a calibrated tape into the vessel and compare the physical liquid surface against the level the bubbler reports. Convert both to the same reference by working from the dip-tube tip elevation, since that is the bubbler's zero. If the tape and the transmitter agree within tolerance at the operating level, the purge rate, density, and range are all working together.

Check at a low and a high level if the process allows, because the high point exposes any purge-starvation lag and the comparison across two points confirms the span. If the low point matches but the high point reads high, suspect over-purge friction; if it lags at high level, the purge flow is too low. Log both tape readings alongside the transmitter values. Feeding the verified tag into a monitoring history means the periodic dips become an ongoing check that catches a slowly plugging tube or a drifting purge regulator early.

Avoid the Common Mistakes

The purge-flow errors dominate. An over-purged tube reads high from friction and tempts people to re-zero it, which hides the real fault. An under-purged tube lags and reads low at high level. A partially plugged dip tube raises the back-pressure and mimics a high level, and a fully plugged one freezes the reading. Wet or dirty purge gas can corrode the tube or leave deposits that slowly restrict it. Keep the gas clean and dry and the flow minimal.

A bubbler also depends on the dip-tube tip staying exactly where it was set, because the tip is the zero of the whole measurement. Any change in tube depth shifts every reading. Trending the level tag continuously turns these slow faults visible: a plugging tube shows as a creeping high bias against the hand dips, and a purge regulator drifting shows as a step. The monitoring history flags the change; the tape confirms which way the error runs.

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