DP Transmitter Under Closed Vaccum Column

Yes, the DP transmitter will measure head pressure from which you can infer level in a closed tank with applied vacuum if the tank port in the area of the upper vacuum is piped to the transmitter's low side port. The DP transmitter doesn't care whether the applied pressure (from the top of the closed tank) to its low side is positive or negative (vacuum), it corrects the head pressure measurement accordingly.

The URV and LRV depend upon the position of the lower tap (do you want it to read zero at the lower tap? If so, URV = 0) and the specific gravity (SG) of the material.

The transmitter's engineering units for level are typically head pressure in water column (mm H2O) or mercury column (mmHg). The DP head pressure measurement is ignorant of the density of the material, so the URV must take the SG into account, by multiplying the actual level by SG to obtain a URV; but dividing the measurement value by the SG at the receiver to obtain actual level.

That means that the head pressure reading from the transmitter will not be the actual level reading unless the material is either water or mercury with the corresponding engineering units.

For example, if the URV will be the upper fill limit of 1700mm (actual) and the material has a SG of 0.90, then the URV would be 1700 (actual mm) * 0.90 = 1530 mm (measured head pressure) w.c.

An indicator or receiver device, (PLC, DCS, whatever) converts the measured signal back to actual level by dividing by the specific gravity; a full tank at 1700 mm would have a measurement signal of 1530mm: 1530/0.90 = 1700 mm

When the density of the material changes, the indicated head pressure will change proportionally. Conversion calculations to volume or mass at the receiver are equally affected by density changes.

The boiling action is likely to produce noisy readings as the sensed pressure is affected by the bubbles from the boiling action. The damping setting can average readings to get a smoother output.

Any brand transmitter has two temperature limits, ambient (electronics) and the medium temperature. Is the steam going to cook the transmitter, as well as the stuff in the vessel?