Answer / carl_ellis

A 2 wire, loop powered transmitter can not output a negative current, like -13mA. The 4-20mA output is always positive.

However, a remote seal DP transmitter used for level will 'see' a negative pressure with nothing in the vessel and the LRV (lower range value) for the transmitter will be a relatively negative value, close to the (negative) span of level range.

A DP transmitter with remote seals used for level measurement will ALWAYS have a zero level reading that is a relatively large negative number because of the head pressure of the fill fluid in the capillaries.

The reason for the negative value is that the fill fluid in the high side capillary (which connects to the lower flange) pulls away from the transmitters high side diaphragm as it presses down on the lower seal by the force of gravity.

Oppositely, the fill fluid in the low side capillary (which connects to the upper flange) exerts a positive static pressure on the transmitter's low side diaphragm because the seal is up high and the fill fluid presses down on the low side transmitter diaphragm, again by the force of the acceleration of gravity.

A differential transmitter works by subtracting the low side pressure from the high side pressure:

High side minus low side = differential

As an example, in a vessel 7m tall, the transmitter has two remote seals, each 4 meters in length. The upper seal has to be above the liquid level in the head space above the liquid. The medium is water, SG = 1.00.

Assume a fill fluid with a Specific Gravity of 1.00, 4m of high side capillary and 4m of low side capillary. With nothing in the vessel (zero level), the applied pressures would be:

high side pressure = approx - 3.75m H2O (negative 3.75 meters, water column)
low side pressure = approx 3.75m H2O (positive 3.75 meters, water column)

high side minus low side = differential
-3.75m minus 3.75m = - 7.5m (negative 3.75m minus 3.75m = negative 7.5m)

The LRV (lower range value) would be approximately -7.5m H2O.

The transmitter needs to be ranged properly, with the appropriate lower range value for zero level.

Carl Ellis
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