Pressure Transducers and transmitters

The Electrical Output of Pressure Transducers

Pressure transducers are generally available with three types of electrical output; millivolt, amplified voltage and 4-20mA. Below is a summary of the outputs and when they are best used.

Millivolt Output Transducers

Transducers with millivolt output are normally the most economical pressure sensors. 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 transmitter 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 Transmitters

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.

PC Board Mountable Pressure Transducers
PC board mountable pressure transducers are generally compact economical pressure transducers designed to mount on an electrical PC board and be integrated into other products.

General Purpose Transducers
General purpose wheatstone bridge pressure transducer are the most common since they are designed to fit the broadest set of applications.

Heavy Duty/Industrial Pressure transmitters
Heavy Duty/Industrial digital pressure sensors feature a much more rugged enclosure than other transducers. They are designed to accommodate heavy industrial environments. They also often feature a scalable 4-20mA output that provides much greater immunity to electrical noise which is not uncommon in industrial environments.

Submersible pressure transducers
Submersible pressure transducers are designed to make precision level or depth measurements in fresh water or liquids for years in harsh industrial environments. They are also ideal for environmental monitoring of job sites and field locations.

Differential pressure transducer
Differential pressure transducers are designed to measure the pressure different between two media. The media can be wet (liquid) or dry (air). These transducers often are used in flow measurement too.

High Stability/High Accuracy Pressure sensors
Most pressure transducers feature an accuracy of 0.25% of full scale or higher. High stability and high accuracy pressure transducers can offer errors as low as 0.05% of full scale, depending on model. Although more expensive than general purpose transducers, they may be the only option if high precision is required.

Flush Diaphragm Pressure Transducers
With flush diaphragm pressure transducers, the diaphragm is flush to the process. This eliminates a cavity above the diaphragm that could collect fluid matter from the process. In certain applications, this may be very undesirable. Those applications include monitoring the pressure of foods or liquids that have very high viscosity.

Special Purpose Transducers
OMEGA offers a variety of pressure transducers with special features. These include pressure transducers designed for pressure measurement in very high or low temperatures, submersible pressure transducers, barometric pressure transducers and pressure transducers with digital communications output or wireless outputs.

What is the most common indicator that a transducer has been overpressured?

The most common indcation that a transducer has been overpressured is a shift in the zero reading usually in an increasing direction. It may read 5-6 ma or even higher. It can even saturate at a maximum value which is typically around 24 ma.

Selection criteria

Still wondering how to decide what type of pressure transducer or pressure transmitter you need? Our interactive pressure sensor selection tool will take you through all of the requirements for your application and provide a part number and price for the correct transducer.

The pressure transducer housing should be selected to meet both the electrical area classification and the corrosion requirements of the particular installation. The corrosion requirements of the installation are met by selecting corrosion-resistant materials, coatings, and by the use of chemical seals, which are discussed later in this chapter.

If the installation is in an area where explosive vapors may be present, the transducer or transmitter and its power supply must be suitable for these environments. This is usually achieved either by placing them inside purged or explosion-proof housings, or by using intrinsically safe designs.

Probably the single most important decision in selecting a pressure transducer is the range. One must keep in mind two conflicting considerations: the instrument's accuracy and its protection from overpressure. From an accuracy point of view, the range of a transmitter should be low (normal operating pressure at around the middle of the range), so that error, usually a percentage of full scale, is minimized. On the other hand, one must always consider the consequences of overpressure damage due to operating errors, faulty design (waterhammer), or failure to isolate the instrument during pressure-testing and start-up. Therefore, it is important to specify not only the required range, but also the amount of overpressure protection needed.

Most pressure transducers are provided with overpressure protection of 50% to 200% of range (Figure 3-12). These protectors satisfy the majority of applications. Where higher overpressures are expected and their nature is temporary (pressure spikes of short duration--seconds or less), snubbers can be installed (as the one in the image). These filter out spikes, but cause the measurement to be less responsive. If excessive overpressure is expected to be of longer duration, one can protect the sensor by installing a pressure relief valve. However, this will result in a loss of measurement when the relief valve is open.

If the transmitter is to operate under high ambient temperatures, the housing can be cooled electrically (Peltier effect) or by water, or it can be relocated in an air-conditioned area. When freezing temperatures are expected, resistance heating or steam tracing should be used in combination with thermal insulation.

In this white paper we studied both approaches to measure pressure in high temperature media. When high process temperatures are present, one can consider the use of various methods of isolating the pressure sensor from the process. These include loop seals, siphons, chemical seals with capillary tubing for remote mounting, and purging.

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:

Y=MX+B

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

EXAMPLE

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