DP Level Transmitter

Here some basics. Hope it helps.

Not very clear:

"that Low side tapping connected at the top and High side tapping at the bottom. while some HIGH side tapping connected at top, LOW side tap at bottom."

Perhaps you could post a sketch or more detail....

It may depend whether low/high refers to liquid level, or pressure. (Since the higher pressure occurs at the lower level, and vice versa.)

In "dry leg" systems where the gas pressure compensating line contains no liquid, the transmitter's "H" port is connected to the bottom tap and the "L" port to the top tap. Since the dry leg sees nothing but gas pressure while the wet leg sees gas pressure plus hydrostatic pressure of the liquid inside the vessel, the "H" side pressure will always be greater than the "L" side pressure, and this pressure difference increases as liquid level increases.

In "wet leg" systems where the gas pressure compensating line is filled full with liquid, it is traditional to connect the transmitter's "H" port to the top tap and the "L" port to the bottom tap. The reason for this has to do with the construction of legacy force-balance DP transmitters, many models of which were unable to measure large negative differences in pressure (i.e. where the "L" pressure was significantly greater than the "H" pressure). Since the wet leg connected to the top tap is guaranteed to have its full height always filled with liquid, but the low-side tap sees a varying liquid level, the DP transmitter's connection to the top tap will always see a greater hydrostatic pressure than the bottom tap connection. Therefore, old-style DP transmitters were connected "backwards" with the "H" port connected to the top tap in order to ensure the "H" pressure was always greater than the "L" pressure. This also meant the transmitter was "reverse indicating:" greater liquid level in the vessel created greater pressure at the bottom tap (greater pressure at the transmitter's "L" port), causing the transmitter to output a decreased signal.

With the advent of electronic DP transmitters capable of measuring positive and negative differential pressures with equal accuracy, there is no longer a need to connect the "H" port to the top and the "L" port to the bottom in wet-leg service. However, there are so many engineers and technicians accustomed to traditional installation standards, we often see the "reverse" convention ("H" to top, "L" to bottom) still applied.

Friend,

A good point regarding this issue is that if you use H side (bottom) and L side (top) you will need use negative range in order to measure the level for example LRV: -150"H2O and URV: -10"H2O, and if you invert the taps, I mean H side (top) and L side (bottom) you will need use positive range for example LRV: 200"H2O and URV: 50"H2O, Whatever you selected the transmitter can be able to measure level the important thing is know how to calculate the range in order to get a correct 4-20mA relation according level decreasing/increasing.

Just remember this: Dp = H - L, the transmitter executed this function.

Few days ago I readed some article about vacuum application using Delta pressure level transmitter but I dont remember very good the recommendations

Regards
JP