How to Zero a DP Level Transmitter on a Sealed Tank

Back to Blog

Identify the Reference-Leg Arrangement

A sealed tank puts vapour pressure on the liquid, so a DP transmitter references its low side to that vapour space to cancel the pressure and measure only the liquid head. How that low side is connected decides everything about the zero. A dry leg leaves the low-side impulse line empty of liquid, suitable where the vapour will not condense; a wet leg deliberately fills the low-side line with liquid, needed where condensate would otherwise collect unpredictably. The two behave oppositely at the zero, so identify which you have before touching the transmitter.

The distinction is the crux of sealed-tank level and is covered in the wet leg versus dry leg note. With a dry leg, the low side sees only vapour pressure, so the empty-tank differential is a small positive mounting head. With a wet leg, the filled reference column adds its own head to the low side, making the empty-tank differential negative. The underlying measurement is the standard differential-pressure level measurement; the leg type just sets the sign of the zero.

Apply Suppression for a Dry Leg

With a dry leg, the transmitter usually sits below the lower tap so its high-side impulse line stays filled, which puts a fixed liquid head on the high side even when the tank is at empty. That mounting head is a positive offset at the empty condition, so you suppress it: set the 4 mA point to that positive differential rather than to zero, so an empty tank reads 4 mA instead of reading the mounting head as a false level. This is zero suppression, and forgetting it makes an empty tank read well up the range.

Compute the suppression from the geometry, the fixed head from the transmitter's mounting offset to the empty level, and enter the 4 mA point at that value. The full-scale calculation that produces both endpoints for a sealed tank is worked in calculating the DP level range for a sealed tank. Enter the suppressed zero and the corresponding span, then confirm rather than assume, since a suppression error is one of the most common sealed-tank level faults.

Apply Elevation for a Wet Leg

With a wet leg, the filled low-side reference column presses on the low side, so at empty tank the low side is heavier than the high side and the differential is negative, equal to minus the reference-leg head. You elevate the zero: set the 4 mA point to that negative differential, so an empty tank reads 4 mA rather than pinning below range. This is zero elevation, the mirror image of suppression, and both endpoints of the range sit negative on a wet-reference sealed tank.

The cleanest way to capture the wet-leg zero in the field is to equalize the transmitter with the reference leg filled and the tank at a known condition, so the transmitter measures and trims the actual leg head. The mechanics of doing this with the manifold are the wet-leg DP zero trim procedure. Confirm the reference leg is genuinely full first, because an under-filled or gassed leg gives a wrong leg head and a zero that will not hold as the leg condition changes.

Confirm the Zero Against a Known Level

Prove the zero rather than trusting the calculation. The strongest confirmation is at a known level: with the tank at a genuine empty or a known measured level, confirm the transmitter reads the expected current for that condition. Where you can equalize the manifold with the legs in their operating state, the equalized reading should correspond to the configured offset, which directly checks that the suppression or elevation is right. A wandering or unexpected equalized value points to a gas pocket, a wrong leg fill, or an arithmetic error.

Then reconcile against a hand dip at an operating level to confirm the whole loop, not just the zero. Record the as-found and as-left values. When the loop feeds a monitoring history, a wet-leg zero that drifts, from the reference leg slowly losing fill or gaining condensate, shows as a steady bias against periodic dips, distinguishing a leg problem from a real level change. The trend flags the drift; an equalized check and a look at the leg confirm whether the reference column is the cause.

Avoid the Common Mistakes

The signature mistakes are confusing suppression and elevation, so a wet-leg tank gets a positive zero and reads backward, or a dry-leg tank gets no suppression and reads full when empty. Zeroing a wet-leg transmitter before the reference leg is properly filled bakes a wrong leg head into the zero. Ignoring the mounting head on a dry leg leaves a fixed offset. And forgetting that both endpoints go negative on a wet reference leads people to reject a correct configuration as impossible.

Because a wrongly zeroed sealed-tank transmitter still produces a smooth, believable signal, the error hides until a dip or an equalized check exposes it. Trending the level against periodic dips in a monitoring platform makes a zero error visible as a consistent offset, and a wet-leg fill problem visible as a slow drift. The history reveals the pattern; matching an offset to a suppression or elevation mistake, versus a drift to a reference-leg fault, points at the specific cause.

Frequently Asked Questions