Selection Criteria for Differential pressure transmitter
Differential pressure measurement could be considered for most applications with liquid–gas or liquid-liquid interface level measurement.
High static pressure can create a measurement zero and full-scale drift. This can be compensated as required, by measuring and compensating the static pressure.
For low range (e.g. below 300 mm) or similar densities between two liquids (for an interface measurement), particular attention should be paid to sources of measurement error, such as:
temperature/density variation of capillary fluid
measurement resolution error due to 2″ or 3″ nozzle and diaphragm
zero error due to air/liquid pockets in the hook up/transmitter or fouling of the diaphragm
Uncertainties of the transmitter when maximum possible calibration range of the cell is much greater than actual calibrated range.
The impulse line is used to interface the tool with the attachment to the method. There are two techniques that can be used to link the process to the tool:
Using a wet leg
Using a dry leg.
The wetted leg liquid should be selected for avoiding the risk of evaporation and leakage.
A field trifoliate tag should be attached to the three-way multiplier block, highlighting "This level obligation is on a moist leg scheme. Equalization of the transmitter block will result in loss of the wet leg.”
Gas compatibility should be regarded with dry leg material. Gas changes or the presence of liquid in the dry leg should be addressed carefully.
Where use of Differential Pressure dry leg system are deployed on a closed tank, they should be assessed to ensure no excess fluid or condensate can build up in the low pressure (dry) impulse leg.
Dry legs should include an insulating drainage pot at its smallest point (below HP tap) to allow drainage of the condensates.
Remote diaphragm seal:
The capillaries of the diaphragm seal filled with oil require a specialized range setup with zero drift.
Two distinct distant sensors can be used in the event of a high measuring range (e.g. above 6 m). The measuring principle is based on a capillary replacement remote sensor. A thorough calibration method (including the zero change) should be researched in this scenario.
Symmetric and asymmetric capillaries:
Differential pressure seal scheme is typically defined on both elevated and low pressure process links with identical capillary lengths and sealing settings. This sort of scheme is traditionally defined as it compensates for mistakes caused by temperature.
The capillary oil quantity will grow and contract, causing changes in the capillary system’s inner pressure. This mistake will be cancelled because symmetrical building will result in the same growth and contraction of the quantity of oil on both the elevated and low sides of the transmitter.
Electronic DP level system:
This principle of measurement is based on autonomous measurements of stress. Instead of using a single DP transmitter with a mechanical impulse tube or capillary, the electronic DP level scheme utilizes two direct mounting detectors or absolute sensors linked to a non-proprietary electrical wire.
The electronic DP level scheme replaces the long lengths of oil-filled capillary and impulse piping with an electrical wire immune to the impact caused by temperature as well as the long capillary. This implies that precise measurements can be obtained over a wide spectrum of ambient temperatures without affecting the reading of fill-fluid density or quantity modifications.
High and low pressure measurements are fully synchronized to ensure that the differential pressure measurement is accurate.
Electronic DP Level system solves many of the problems traditionally seen when measuring DP on tall ships or towers. Typical issues are the following:
Mechanical installation constraints : two remote seals + capillaries
Ambient Temperature effect on the capillaries (fill fluid dilatation/contraction and density variation) results of inaccuracy : insulation or heating tracing of capillaries
Plugging condensation/evaporation of reference column
Tall measurement range (e.g. above 6 metre).