Electronic differential pressure transmitters are among the most interesting new level measurement solutions. Today, there is a vast range of dedicated level measurement devices but many companies still prefer to stick to old techniques. Of course, sometimes they have reasons for caution: they need to keep the process running, or they need certain certifications.
Before discussing how electronic DP transmitters avoid the problems of traditional installations, we should note that some vendors have different names for the devices. For example, Endress + Hauser calls it electronic DP transmitters.
To know more about the applications of DP transmitters, you can read the Visaya Artcile on DP transmitters
The electronic differential pressure transmitter’s working principle
The design of the electronic DP transmitter is based on differential capacitance. The sensing element is a taut metal diaphragm located between two stationary metal surfaces, comprising three plates for a complementary pair of capacitors. An electrically insulating fill fluid (usually a liquid silicone compound) transfers motion from the isolating diaphragms to the sensing diaphragm, and also doubles as an effective dielectric for the two capacitors.
Any difference of pressure across the cell causes the diaphragm to flex in the direction of least pressure. The sensing diaphragm is a precision-manufactured spring element, meaning that its displacement is a predictable function of applied force. The applied force in this case can only be a function of differential pressure acting against the surface area of the diaphragm in accordance with the standard force-pressure-area equation F = PA.
In this case, we have two forces caused by two fluid pressures working against each other, so our force-pressure-area equation may be rewritten to describe resultant force as a function of differential pressure (P1 − P2) and diaphragm area: F = (P1 − P2)A. Since diaphragm area is constant, and force is predictably related to diaphragm displacement, all we need now in order to infer differential pressure is to accurately measure displacement of the diaphragm.
The diaphragm’s secondary function as one plate of two capacitors provides a convenient method for measuring displacement. Since capacitance between conductors is inversely proportional to the distance separating them, capacitance on the low-pressure side will increase while capacitance on the high-pressure side will decrease.
Electronic differential pressure transmitter applications and advantages
An electronic differential pressure transmitter is a new addition to an old solution. While level measurement using differential pressure transmitters is a tried-and-true solution, some processes challenge traditional differential pressure transmitters. This is where the electronic differential pressure transmitter comes in.
The biggest advantage of this type of technology are exceptional accuracy and stability. When you have a pressurized or closed vessel, we need to compensate for the pressure or vapor with the low-pressure cell of the transmitter. Depending on the needs, we can use a capillary or impulse line, but either way, we can have problems.
When do we apply a capillary or impulse line? First of all, you can start with impulse lines, but we may need the capillary to solve certain problems. Impulse lines have two segments, called wet leg and dry leg. The wet leg has the line filled with either column product, and the dry leg has a suitable dry gas instead of liquid. When the vapor or gas won’t condense into the impulse line, we can use the dry leg. If you have the possibility of condensation, then you need the wet leg.
These kinds of setups have disadvantages, though. A wet leg may leak or ice up, requiring a special installation to avoid these issues. A dry leg may create condensation in the line. These issues can make your measurements vary and demand extra maintenance. Hence, the capillary and seal system came to eliminate many of these problems. However, the solution also has its own problems!
To know more about the working of differential pressure transmitters, you can read the Visaya Article on Pressure Transmitters
What is a capillary and seal system?
This oil-filled diaphragm mounts directly into the high- and low-pressure cells. When the pressure changes, the diaphragm deflects it and sends the force through the oil system up to the sensor. This effect results in the measurement. Because the capillaries can flex, we don’t need to build a structure for the system, making installation easier than for an impulse line.
We have two capillaries here, one in the high-pressure cell and the other in the low. The capillary construction usually reflects a balanced system, with identical seals and equal lengths of tubing on each side of the transmitter. With the same conditions on both sides, when the ambient temperature changes, both sides will react the same. That means the measurement won’t vary ideally.
In reality, we may still have variance. If the low side gets more heat than the high side, then the density will change on the low side and mess up the measurement. We call this a temperature-induced density effect.
Recommended electronic differential pressure transmitters
The electronic differential pressure transmitter came about to solve all the issues with impulse lines and capillary systems. As we said before, different brands have different names, but they bring the same benefits to the end users.
Now we won’t have to fight temperature changes, condensation, or mechanical issues. We can reduce installation costs too, because these systems go in much easier compared to an impulse line or capillary system. Electronic systems have one drawback, though – they hate high heat. Still, some vendors offer devices that can withstand 410 degrees Celsius and more.
To know more about electronic differential pressure transmitters, you can get in touch with our engineers and we will be happy to help.