Pressure Transmitters

Hi All - I am using some Rosemount Pressure transmitters to measure the volume of product (liquid) in a tank. Question I have is when setting up a transmitter I set the amount of head pressure in mm of the product, what point do I use on the sensor, top of it, middle or bottom as the reference point?
Hopefully this makes sense, at the moment I am using the top of the sensor and I am seeing some inaccuracies against the strapping table.

Sensors are 3051SAM ERS.

Cheers
Peter

I generally use the centerline of the transmitter as the reference point. Note that if the sensor is not mounted horizontally from the tap in the tank you have to add or subtract the difference in elevation from the reading.

1) The reference point is the centerline of the threaded port which is also the centerline of the sensing diaphragm

2) Have you zero trimmed?

STEP 7: CALIBRATE THE 3051S ERS SYSTEM

After the 3051S ERS system has been installed and wired, either a zero trim or a lower sensor trim should be performed on each sensor to compensate for installation effects.

• A zero sensor trim should be performed after installing a gage sensor on a vented tank. A
zero sensor trim should not be performed on an absolute sensor or on a gage sensor that is
at line pressure.

• A lower sensor trim should be performed after installing an absolute sensor or a gage sensor
that is at line pressure.

Additionally, a “System DP Zero” trim should be performed to establish a zero-based DP reading.
The System DP Zero trim should be performed after a zero/lower trim has been performed on each sensor.

(procedure follows)
http://www2.emersonprocess.com/siteadmincenter/PM Rosemount Documents/00825-0100-4804.pdf

3) >I set the amount of head pressure in mm of the product
The head pressure is always measured by the pressure transmitter in units of water column, which relates to level of a column of water, in units of inches of water or mm of water.

Corrections to the level/head pressure for product specific gravity/volume have to be made in either the 'receiver' (PLC) or some very high end transmitters have a scaling function that can multiply by a factor to correct for density/specific gravity.

Thanks Guy's - center makes sense.

Cheers

Yep, center! They work beautifully too! I've been using them for a while in pharmaceutical grade applications with a tri-clamp mount. The Center line is really more obvious on those!