Level Transmitter Zero Suppression

Garry, welcome to the forum.

>You state the connections are 2000 mm apart - the SG of process is oil/water = 0.8 and 1 respectively.

OP: >SG of fluid/oil/water are 0.93/0.87/0.1025

An SG of 0.87 makes a substantial difference.

>The range of the level is the difference of the process S.G. 1.0 minus 0.8 = 0.2 multiplied by the height of connections 2000 mm = 400 mm. The range of transmitter is 400 mm.

An SG of 0.87 gives a range of 260mm: 1.0 minus 0.87 = 0.130 * 2000 = 260mm

But you're thinking wet leg and water filled impulse tubing/pipes. OP says capillary connections; probably something like this:

The capillary fill exerts a head pressure of its own that must be taken into account because it directly affects the LRV and URV.

Either Syltherm or DC-200 fluid fill is close to an SG of 0.93, so the OP's fluid 0.93 SG is likely the fill fluid in the capillaries.

A capillary extending 2000mm above the transmitter will exert a pressure on the low side port of 2000 mm * 0.93 = 1860 mm w.c.

The high side subtracts the low side pressure, so empty, the LRV will be -1860 mm w.c. (that's negative 1860 mm w.c.)

When the tank is 100% full of oil, 2000mm of oil exerts a head pressure of 2000*0.87, or 1740 mm w.c. head pressure, or a span from the LRV of -1860 up to -120mm (w.c.) [-1860 + 1740 = -120].

When the tank is 100% full of water, 2000 mm of water exerts a head pressure of 2000 mm w.c. or a span from the LRV of -1860 to +140mm w.c.

The span from -120 mm to +140 mm (as the interface level varies from all oil to all water) is the calculated 260 mm span difference between an interface level at the bottom port (100% oil above the port) and an interface level at the top port (100% water above the lower port). All other interface levels between 100% oil and 100% water will be proportional across that span.

If the zero is elevated or suppressed (I can't recall which is which at the moment, I'm sure 17 people will respond with the answer) so that 1740mm w.c. is the 4.0 mA 'zero' output and the 20.0mA is scaled 260 mm above the zero point, then

- when the tank is 100% full of oil, the interface level is at the bottom of the tank at the lower port elevation, the output will be 4.0 mA

- when the tank is 100% full of water, the interface level will be at the top or upper port and the output will be 20.0mA

My original question to the OP about getting interface level was rhetorical, I was hoping to prompt a response, because I've found many people think the transmitter should somehow reflect the interface level directly, when the idea is to span the transmitter's output as above, 0 to 260 mm w.c. (or 1740 to 2000 mm w.c.) which can be scaled in a receiver (meter, DCS, PLC, PAC, indicator, recorder, whatever) to represent the interface level somewhere between the 0 to 2000 mm distance between the ports or 0 to 100%.