Differential pressure transmitters have two pressure ports. The pressure ports are connected to the process with piping or tubing, called impulse piping/tubing. The ports and the connection piping/tubing are not technically 'probes', they're the process connection(s) for the transfer of pressure to the transmitter's pressure heads.
When the low side port is connected to the vapor/air pressure above the liquid, the effect of the applied air pressure is subtracted out and the result is the hydrostatic head pressure of the liquid column.
The problem with sewage is that it is sewage with solids. The sewage solids can clog the pressure ports inside the transmitter and clog the impulse tubing resulting in slow response or even no response due to blocked pressure transfer through the tubing.
Diaphragm seals are used to isolate the process medium, sewage, from the measurement device. The diaphragm has a relatively large exposure area. The connection between the diaphragm seal and the transmitter is a capillary tube, filled with fluid, which transfers the pressure from the diaphragm to the process heads. Filling seals requires hard vacuum evacuation and back-filling the fluid, a skilled task and is done by vendors who specialize in it. It is not typically (if ever) done at the plant level is the USA.
Diaphragm seals some issues. The biggest issue is that a pressure transmitter with diaphragm seals is also a temperature transmitter, because it is a 'filled system', which by Boyles or Charles law changes pressure with temperature changes. There is no compensation for the temperature effect on pressure. It's error.
With dual diaphragm seals, the hope and prayer is that both capillaries are at the same temperature so that the effect of temperature related pressure change on the high side cancels the effect of temperature related pressure change on the low. Rarely does that happen. The best results are inside temperature controlled buildings, the worst results are outdoors where sunlight or shadow can substantially affect the temperature of the capillary tubing.
When the medium, sewage, can solidify or crust up against the exposed diaphragm a flushing ring is installed between the mounting flange and the diaphragm seal flange. The flush ring has a threaded port which can be piped and valved to a source of flushing water to clean away deposits and solidification.
The weight of the fill fluid in the capillaries is part of the hydrostatic measurement. That means that the range of the DP transmitter with dual capillary seals has an LRV (lower range value, what the 4.0mA value represents) that is large negative number. Those unfamiliar with capillary seals do not expect that and can get a unit that will zero or span across the range needed for capillary seal measurement. Pmin and Pmax can be calculated with arithmetic, as the illustration below shows.
Vendor specs or 'engineering guide' for remote seals have a table of fill fluid specific gravities.
Specific gravity of the 'solution' that changes over time will produce a proportional error in any hydrostatic head measurement, it's the nature of the beast.
Diaphragm seals are 60 year old technology. An experienced vendor can get the right unit and at correct right range, and I'd even have them range it at the factory, because ranging dual seal capillary DP's is not hard, but there's a lot of misunderstanding at the local level about how to calculate the LRV-URV range.
My suggestion is that you use draw out the tank, its dimensions, the mounting points, the maximum level and calculate the range yourself. Then inquire of vendors as to an appropriate model AND what range they calculate for this specific application. If the vendor doesn't get the right range, find another vendor (sometimes DP level gets shuffled to the entry level new hire who doesn't know)
The attached ISA DP flange level and DP remote seal spec sheets both lack the critically needed area for a drawing.
.... so i am trying to see if this acceptable.
