What is a pressure transmitter?
A pressure transducer often called a pressure transmitter,
is a transducer that converts pressure into an analog electrical signal.
Although there are various types of pressure transducers,
one of the most common is the strain-gage base transducer.
The conversion of pressure into an electrical signal,
is achieved by the physical deformation of strain gauges,
which are bonded into the diaphragm of the pressure transducer,
and wired into a Wheatstone bridge configuration.
Pressure applied to the pressure transducer produces a deflection of the diaphragm,
which introduces strain to the gages.
The strain will produce an electrical resistance change proportional to the pressure.
Types of Pressure Sensors
There are different types of pressure transducers based on their design.
These sensors can come in several shapes and sizes,
but the technology inside can also differ.
There 4 main types of pressure sensor based on this:
- Strain Gauge Pressure Transducers
- Capacitance Pressure Transducers
- Potentiometric Pressure Transducers
- Resonant Wire Pressure Transducers
Pressure Transmitter Working Principle
In general, a pressure transmitter consists of three main components:
a pressure sensor, a measuring circuit,
and a process connection.
The main function of the pressure transmitter is to convert the physical parameters of the gas,
liquid and other physical parameters of the pressure sensor into a standard electrical signal.
The standard electrical signal is more convenient for indicating the alarm device,
the adjusting device, the recorder, and the secondary instrument.
Capacitive pressure transmitters measure all sensitive components in an all-welded construction.
The electronic circuit is the peak of soldering and connector mounting.
The overall structure is rugged, rugged and has few faults.
The capacitive pressure transmitter consists of a measuring diaphragm,
and electrodes on both sides of the insulating sheet.
The capacitive pressure transmitters of the measuring medium are divided into high-voltage and low-pressure chambers,
and on the isolating diaphragms on both sides of the sensitive element,
the liquid-filled isolation and components
are transferred to the sides of the membrane for measurement.
When the pressures on both sides are inconsistent,
the measurement of the displacement of the diaphragm,
the displacement and the pressure difference,
are proportional to the two sides of the capacitance range,
through the process of oscillation and demodulation,
and the pressure is converted into a signal.
The working principle of a capacitive pressure transmitter is similar to that of a differential pressure transmitter,
unlike the pressure chamber pressure of a pressure chamber.
The A/D converter of the capacitive pressure transmitter,
converts the current of the demodulator into a digital signal,
and uses the value of the microprocessor to determine the input pressure value.
The microprocessor controls the transmitter.
How does the pressure transmitter work?
The installation of the differential pressure transmitter measurement system,
consists of three parts, namely the laying of the pressure guiding tube,
the laying of the electrical signal cable and the installation of the differential pressure transmitter.
During the operation of the pressure transmitter,
the pressure of the medium is transmitted to the central measuring diaphragm,
through the isolating diaphragm and the silicone oil,
and the pressure difference from the double-sided pressure guiding tube is received on the double-sided isolating diaphragm,
where the membrane is measured.
The sheet functions as a resilient element and is deformed by the pressure difference.
There is a positive proportional relationship,
between the displacement of the measuring diaphragm,
and the differential pressure,
and under the influence of the displacement of the diaphragm,
the capacitance of the differential capacitor also changes,
and the measuring circuit converts it into a DC current signal of 4-20 mA.
Taking the Model SI1151 transmitter as an example,
this type of pressure transmitter has no central axis,
the fixed plate is spherical, and the transmitter has a symmetrical two-chamber structure.
When finished glass and metal body burning after the knot works,
a spherical concave surface is formed.
Finally, a metal film is plated on the surface of the glass so that a fixed plate is formed.
Between the two fixed plates, the measuring diaphragm is welded to form a movable plate.
The isolating diaphragm on the outside of the fixed plate is responsible for transmitting pressure,
from the silicone oil.
When the diaphragm is under pressure,
it is sent to the measuring diaphragm,
and the diaphragm is deformed under the action of pressure,
which causes the relative position of the capacitive movable plate,
and the spherical fixed plate to also change,
and the capacitance changes in the motor.
The line converts it into a 4-20 mA DC signal.
If there is an overvoltage, the first thing to protect is the measuring diaphragm,
which is attached to the spherical fixed plate.
When an overvoltage occurs,
the isolation diaphragm is completely attached to the fixed plate.
The function of the conversion part is mainly to convert the standard electrical signal,
and adjust the signal.
Current control amplifiers, current converters, oscillators,
voltage regulators, etc. together form a conversion circuit.
The block diagram is shown in Figure below.
What is the function of a pressure transmitter?
The pressure transmitter is a commonly used sensor in industrial applications.
It is widely used in various industrial self-control environments,
involving water conservancy and hydropower,
railway transportation, intelligent building, production automation,
aerospace, military, petrochemical, oil well, electric power, ship.
In many industries, such as machine tools and pipelines,
the main role of pressure transmitters is to transmit pressure signals to display pressure on the computer.
The principle is rough as follows:
the mechanical signal of the pressure of water pressure is converted into a current (4-20 mA),
and the electronic signal pressure has a linear relationship,
with the magnitude of the voltage or current,
which is generally proportional.
Therefore, the voltage or current outputted by the transmitter increases with the increase of the pressure,
thereby obtaining a relationship between pressure and voltage or current.
The two pressures of the measured medium of the pressure transmitter are high and low,
and the low voltage is low.
The chamber pressure is applied to the isolating diaphragm on both sides of the δ element
(ie, the sensitive element) by atmospheric pressure or vacuum,
and is transmitted to both sides of the measuring diaphragm,
through the spacer and the filling liquid in the element.
What is differential pressure transmitter working principle?
Differential pressure transmitters measure the difference between two pressures.
Differential Pressure Transmitter converts pressure measurements,
into a proportional 4-20 mA or a 1 – 5 Vdc output signal,
that functions as the input to a controller, recorder, indicator or similar device.
These transmitters find application in the gas, water,
and process industries that need accurate measurements,
over a wide range of environmental conditions.
This equipment will sense the difference in pressure, between two ports and produce an output signal,
with reference to a calibrated pressure range.
The industrial differential pressure transmitters are made of two housings.
The pressure sensing element is housed in the bottom half, and the electronics are housed at the top half.
It will have two pressure ports marked as “High” and “Low”.
It is not compulsory that the high port will be always at high pressure and the low port always at low pressure.
This labeling has its relation to the effect of the port on the output signal.
With its 4 … 20 mA, 4 … 20 mA HART®, PROFIBUS® PA or FOUNDATION Fieldbus™ output signals,
combined with either intrinsically-safe or flameproof ignition protection
the DPT differential pressure transmitter is
suited to applications requiring these features.
The electronics of all transmitters with explosion protection, even for the flameproof variant, are safe.
Thus it is possible to make adjustments on the instrument in EX areas while the instrument is live.
What is the output of pressure transmitter?
Pressure transducers are generally available with three types of electrical output:
millivolt, amplified voltage, and 4-20mA.
In this article how to wire different types of pressure transducers based on its output is explained.
Afterward, it is necessary to convert an electric output into an engineering unit such as PSI or bars.
Below is a summary of the outputs and when they are best used.
Millivolt Output Pressure Transducers
Transducers with millivolt output are normally the most economical pressure transducers.
The output of the millivolt transducer is nominally around 30mV.
The actual output is directly proportional to the pressure transducer input power or excitation.
If the excitation fluctuates, the output will change also.
Because of this dependence on the excitation level,
regulated power supplies are suggested for use with millivolt transducers.
Because the output signal is so low,
the transducer should not be located in an electrically noisy environment.
The distances between the transducer and the readout instrument should also be kept relatively short.
Voltage Output Pressure Transducers
Voltage output transducers include integral signal conditioning,
which provide a much higher output than a millivolt transducer.
The output is normally 0-5Vdc or 0-10Vdc.
Although model specific, the output of the transducer is not normally a direct function of excitation.
This means unregulated power supplies are often sufficient,
as long as they fall within a specified power range.
Because they have a higher level output these transducers are not as susceptible to electrical noise as millivolt transducers,
and can therefore be used in much more industrial environments.
4-20 mA Output Pressure Transducers
These types of transducers are also known as pressure transmitters.
Since a 4-20mA signal is least affected by electrical noise and resistance in the signal wires,
these transducers are best used when the signal must be transmitted long distances.
It is not uncommon to use these transducers in applications where the lead wire must be 1000 feet or more.
How to Choose Pressure Transducer
There are multiple types of pressure transducers for a variety of applications.
Each pressure transducer has different aspects,
that will impact how it works and the applications the pressure transducer works best for.
When selecting a pressure transducer, keep these 6 criteria in mind:
1. Application and measurement type
2. Pressure range
3. Process media
4. Temperature range and installation environment
If you still don’t know how to choose the pressure transmitter,
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How much does it cost a pressure transducer?
There are a number of factors,
that will impact the price of a pressure transducer.
The biggest differentiator is whether you can use a standard,
off-the-shelf pressure transducer or if you need a custom pressure transducer.
For an off-the shelf pressure transducer,
pressure transducer prices will be most affected,
by the level of accuracy required for your application.
The more accurate, typically the more expensive the pressure transducer.