The Electrical Output of Pressure Transducers
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.
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.
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
In this article the differences between these pressure transducer types are explained and here how a pressure sensor works.
Choosing Pressure Transducer
Still wondering how to decide what type of pressure transducer or pressure transmitter you need?
To learn more about the criteria you should think about when making your decision, this article, Practical Considerations when Choosing a Pressure Transducer will help you pick out the right device for your application.
How much does it cost a pressure transducer?
There are a number of a 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.
To learn more about the pricing of custom pressure sensors click here.
The History of Pressure TransducersAs far back as Galileo Galilei, scientists have been trying to measure pressure. For almost the next three hundred years, scientist would make discoveries that helped inform their understanding of pressure measurement. However, it wasn’t until 1930, with the invention of an unbonded strain gauge by Roy Carlson, a civil engineer, that big strides were made with pressure transducer technology.
1930: Roy Carlson designed the first unbonded wire strain gauge to measure the strain inside a concrete structure.
1938: Arthur Ruge, Massachusetts Institute of Technology, and Edward Simmons, California Institute of Technology, independently but simultaneously develop a process for a bonded wire strain gauge. The two men ended up applying for a patent together.
1952: Foil strain gauges were invented by Peter George Scott Jackson of Saunders-Roe Ltd. Foil strain gauges provided advantages over bonded wire gauges, including better heat dissipation, better thermal stability, better reproducibility, and lower production costs.
1954: The capacitive pressure transducer was developed by Carl Spaulding. These sensors provided a way to measure very small pressures with sufficient accuracy.
1960s: The first thin-film transducers with good stability and low hysteresis were developed. The technology remains in use today for measuring high pressure. Meanwhile, silicon diaphragm and silicon sensor patents were also developed in the late 1960s by Art Zias and John Egan of Honeywell Research Center.
1973: William Polye of Bendix Corporation designed a capacitive transducer using quartz. This made it possible to measure lower pressure ranges.
1979: Robert Bell of Kavlico built on the capacitive transducer technology to replace the quartz body with a ceramic one. This design remains commonly used today.
Converting Current and Voltage Inputs To Engineering Units Such As PSI
It is very often necessary to convert a voltage, millivot or current reading into a more useful value such as PSI, GPM, LBS, etc. For example, if measuring force using a load cell, it would be much more beneficial to the user if they could read and record the data in LBS (pounds) instead of millivolts, which is what the load cell typically produces. Other examples would be using a pressure transducer to measure PSI, a flow sensor to measure GPM and a relative humidity sensor to measure RH units.
It is very simple to scale any sensor, and the same equation applies to all methods of data display and acquisition. First, the formula:
Where Y is the output or ENGINEERING UNITS
Where M is the slope or the SCALE FACTOR
Where X is the INPUT (millivolts, volts, etc) and
Where B is the OFFSET
Here is a typical example where a pressure sensor is used to measure 0-500 PSI and the output is 1-5Vdc.
First, using the Y=MX+B formula, we determine what each value is in order to calculate for Y.
X = 4 (since 1-5V has a span of 4 volts. If it was a 0-10Vdc output, X would be 10)
M = 125 (use divide the Units by the Voltage or Current - 0-500/1-5 = 125) which results in PSI/Volts
B = -125 ( since the output starts at 1 volt, there is an offset. We calculated a value of 125 PSI/Volt, therefore, 1V = -125) If the output of the sensor was 0-5Vdc, then there would be no offset.
To test that the values are correct, put them in the equation. 5 volts out should give us 500PSI and 1 volt out should give us 0 PSI.
Y=125(5) + (-125) = 500PSI
Y=125(1) + (-125) = 0 PSI
Simply insert these values in your data acquisition software where prompted and your readings will now be acquired in PSI instead of Volts. Of course, the software that you are using must support scaling, or at least support the calculation Y=MX+B View this page in another language or region