Differential pressure transducers measure the difference in pressure between two points. Through various calculations, differential pressure transducers can also be used to monitor flow and density. In selecting a differential transducer, the details of the application as well as the installation become top priority. For example, what happens to a differential pressure transducer if the valve connected to P2 is shut off? What can I expect to happen if I open full pressure to P1 before P2?
A basic understanding of the measurement principles is an excellent starting point.
Line pressure is the system pressure that can be applied to both pressure ports (P1 & P2).
Differential pressure is the difference in pressure between both pressure ports (P1 - P2). In flow and filtration applications, P1 and P2 will see the same process media and line pressure. The differential pressure is calculated as the drop in pressure across a filter or orifice plate. In pipeline monitoring, differential pressure transducers can monitor the rupture of a pipe and decrease in pressure.
Common mode is the maximum pressure P1 and P2 can handle at the same time.
In many applications there is a large "turndown ratio" or the ratio of the line pressure in relation to the differential pressure. A differential transducer with a 100 PSI line pressure and 1 PSID measurement range would have a 100:1 turndown ratio.
The installation and operation are two factors that are commonly overlooked by not understanding the sensor technology. In liquid filled sensor capsules, a diaphragm is located at P1 and P2 and the pressure is applied to each with the difference being the amount the fluid is compressed. While the transducer line pressure might be capable of handling 1,000 PSI, in many cases, the pressure must be applied equally to both sides. If the pressure is not applied equally at start up, more pressure is applied to one side over the other. Since there is no differential pressure from the opposite end, the diaphragm experiences deformation, thus creating a permanent shift in output, or worse, a rupture of the diaphragm and fluid in the process. While AST always recommends equal pressure at start up, our differential transducers are designed to account for this instance by removing fluid filled cavities and protecting against pressure application on either side of the diaphragm.
AST uses two sensor technologies for its differential products: MEMS strain gages and LVDT's. Our MEMS based AST5400 uses two independent pressure sensors to measure differential pressure where as the AST5100 and AST5300 utilize linear position sensors to monitor the deflection of the diaphragm. Below is a brief overview for each product.