One of the most common and helpful pressure measuring instruments used in industry is called a differential pressure transmitter. This device measures differential pressure. In addition to that, it is one of the most widespread.
This device is able to detect the pressure difference that is present between two ports and produce a signal that is representative of that pressure in relation to a calibrated range. It can also produce a signal that is representative of that pressure in absolute terms.
Because differential pressure transmitters can be based on any of the pressure-sensing technologies that have been discussed up to this point, this section will concentrate more on application than theory. This is because differential pressure transmitters can be based on any of the pressure-sensing technologies.
The building of DP transmitters and the ways in which they behave
Differential pressure transmitters that are constructed for use in industrial measurement applications typically consist of the following elements: a sturdy (forged metal) body that houses the sensing element(s); a compartment that houses the mechanical and/or electronic components that are required to translate the sensed pressure into a standard instrumentation signal (for example, 3-15 PSI, 4-20 mA, or digital fieldbus codes); and a lid that protects the sensing element(s) and the compartment from external elements.
Both the Rosemount model 1151 (shown on the left) and the model 3051 (shown on the right) are examples of electronic differential pressure transmitter price, which are depicted in the following photographs:
The Yokogawa EJA110 (on the left) and the Foxboro IDP10 (on the right) are two more examples of electronic differential pressure transmitters, and they are shown in the following picture:
In each of these examples of differential pressure transmitters, the pressure-sensing element is housed in the lower half of the device (the structure made of forged steel), and the electronics are housed in the upper half of the device (the colored, round structure made of cast aluminum). This arrangement ensures that the differential pressure readings are accurate.
Every differential pressure (DP, d/p, or P) transmitter, regardless of make or model, has two pressure ports that are able to detect different pressures that are being applied to various kinds of process fluids. These pressure ports can also sense the pressures that are being applied to the fluids.
Both of these ports have different labels; one of them says high, and the other says low. The designation of one port as high and another as low does not necessarily imply that the former must always have a higher pressure than the latter. These ports typically feature female NPT threads of 1/4 inch, which makes a connection to the process simple and uncomplicated. Both of these ports have different labels; one of them says high, and the other says low.
These labels indicate the influence that an increase in the fluid pressure that is applied to that port will have on the progression of the change that can be observed in the output signal. This change can be observed as a change in the progression of the change that can be observed in the output signal.
The diaphragm is the sensing element that is used by the vast majority of modern DP transmitters. The pressure of the process fluid is applied to one side of this diaphragm by the port that is designated as high, and the pressure is applied to the other side of the diaphragm by the port that is designated as low. In this way, the pressure across the diaphragm is proportional to the difference between the high and low ports.
After that, the flexing is converted into an output signal utilizing one of a number of different technologies, which vary according to the brand and model of the transmitter:
Labeling the input terminals of an operational amplifier with the terms "inverting" and "noninverting" is conceptually very similar to labeling the port of an instrument that measures differential pressure.
The signs and do not imply the polarity of the voltage (or voltages) that are being input into the circuit. It is therefore not necessary for the + input to have a higher positive value than the input. These symbols simply represent the various directions in which each input has the potential to drive the signal that is being output. This means that it is not necessary for the + input to have a higher positive value than the input.
To describe this phenomenon using the terminology of closed-loop control systems, we could say that the input labeled + is direct-acting, whereas the input labeled is reverse-acting. This is because an increasing potential that is applied to the + input causes the output of the operational amplifier (opamp) to be positive, whereas an increasing potential that is applied to the input causes the output of the operational amplifier (opamp) to be negative.
Also, the labels H and L that are located on the ports of a DP transmitter do not imply the magnitude of the pressures that are being input into the device. Because of this, it is not required that the pressure that is coming out of the H port be higher than the pressure that is coming out of the L port.
When more pressure is applied to the high port of a DP transmitter, the output signal will be driven to a higher level (up), whereas when more pressure is applied to the low port of a DP transmitter, the output signal will be driven to a lower level (down): These symbols simply represent the various effects on the output signal that are caused by applying pressure to each port in a different way. When more pressure is applied to the high port of a DP transmitter, the output signal will be driven to 
The ability to arbitrarily connect a DP transmitter to a process in such a way that the connection results in the process being either direct-acting or reverse-acting is an important benefit. This ability also enables the process to be controlled in either direction.
In the field of electronics, the ability of a differential voltage sensor (such as an operational amplifier) to ignore large potential differences when measuring voltage with reference to ground while still being able to detect minute differences in voltage is referred to as "common-mode rejection." This term is used to describe the capability of the sensor to detect minute voltage differences.
The amount of voltage that is shared by both input terminals is completely disregarded in a perfect operational amplifier, and the device only reacts to the difference in voltage that exists between those two terminals. This is exactly what a well-designed DP instrument does, with the exception that it measures fluid pressure rather than electrical voltage. In a perfect operational amplifier, the device only reacts to the difference in voltage that exists between those two terminals.
To put this another way, a differential pressure instrument should (ideally) respond only to differential pressure while remaining unaffected by common-mode pressure. The gauge pressure that is shared by both ports is disregarded by a DP instrument, which instead pays attention only to the differences in pressure that exist between the two ports.
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