Category Archives: Pressure Transducer

5 Things to Consider when Working with a Pressure Transducer

Pressure is crucial in fluid-power circuits. Using a pressure transducer allows you to control your system. Connecting the transducer a power source, while plumbed to a pressure source, generates an electrical output signal that matches the pressure. However, there are many considerations when you use this equipment. Here are some of them:

The Pressure Range

Since pressure transducers are designed to provide electrical output to match any given pressure range, make sure you pick a pressure transducer with a full scale range that is closest expected pressure. Also, check and confirm if the electrical output signal is matched to the needs for your existing system. Compatibility issues can compromise results or damage the pressure transducer.

Quality

Using trustworthy suppliers, such as Validyne, to source a premium-grade pressure transducer should be a priority. In doing so you won’t have to contend with units that exhibit low-quality performance. Opting for the right suppliers will keep those superior-quality transducers coming whenever you need them.

Installation

Be careful not to damage the electrical connector during the installation process. Denting a pin or connector shell could damage your unit. To prevent this from happening precisely follow the installation guidelines. If you have any doubts, ask for an installation professional for help. Be sure to connect the pressure transducer pressure ports with high quality NPT fittings and use Teflon tape on the threads.

System

Accuracy and reliability are important. Check to make sure the cabling, readout devices, signal conditioning, and amplification units are all in good working condition. Don’t forget about the power wiring to the transducer, as that is a common spot for trouble.

Maintenance

You will need to ensure the transducer receives proper maintenance and care. That’s the best way to ensure optimum performance. Regularly check the pressure transducer pressure connections for leaks, and tighten as needed. Also, keep all electrical connections between the system and components free of water and spray.

Keeping your transducer functioning at top form is crucial, so keep these practical tips in mind. If you need to know more about transducers, seek contact us today!

Measuring Vacuum Pressures

A subject that tends to cause confusion when specifying pressure transducers is the  measurement of a vacuum and how it relates to absolute pressure.

Here are some definitions:

Absolute Pressure – A pressure referenced to zero absolute pressure.

Gauge Pressure – A pressure referenced to the local atmospheric pressure

Differential Pressure – The difference in pressure between two points.

Vacuum – A pressure less than the local atmospheric pressure.

From the above definitions we can see that an absolute pressure is measured starting from absolute zero – the complete absence of pressure. The complete absence of pressure exists in space, but on the surface of the earth the atmosphere exerts a pressure of about 14.7 psia at sea level. A barometer is a device that uses a column of mercury to measure the atmospheric pressure and this ranges from about 27 In Hg to 33 In Hg, depending on the weather. In addition to varying with the weather, the local atmospheric pressure also depends on elevation. The atmospheric pressure is about half that at sea level when you are at an altitude of 17,000 feet.

A vacuum is any pressure less than the local atmospheric pressure. It is defined as the difference between the local atmospheric pressure and the point of measurement. A vacuum is correctly measured with a differential pressure transducer that has one port open to atmosphere. If, for example, the negative port is connected to a vacuum and the + port open to atmosphere, the transducer signal will increase as the vacuum increases. It will always indicate the correct vacuum, even when the local atmospheric pressure changes with the weather.

An absolute pressure transducer cannot measure vacuum directly. If connected to a vacuum the signal from an absolute pressure transducer will decrease as the vacuum increases, but you can only know the actual vacuum if you know the local atmospheric pressure because vacuum is always referred to the atmosphere. Another way of thinking of a vacuum is that it is a negative gauge pressure.

The local elevation will affect a vacuum measurement because the atmospheric pressure is affected: no matter how powerful your vacuum pump, you cannot pull a vacuum of 14 psi at an elevation of 6000 feet – because the atmosphere there is only about 12.5 psia and the difference between the atmosphere and a vacuum cannot exceed that pressure.

In summary: a vacuum is best measured with a differential pressure transducer having one port open to atmosphere.

Check out Validyne’s absolute, vacuum, gauge and differential pressure transducers.

measuring vacuum

Application Note: Basics of Air Velocity, Pressure and Flow

Air velocity can be measured by sensing the pressure produced by the movement of the air. This application note will describe the basic relationships between air velocity and the pressure generated by air flow.

Anyone who has put their hand out the window of a moving car has experienced the force applied by moving air. This force can be sensed as a pressure by connecting a tube from the positive port of a differential pressure transducer like the P55 differential pressure transducer and pointing the open end of the tube directly into the oncoming air stream. The figure below shows how this might look.

Differential Pressure Transducer Air Velocity

The tube that is placed into the air stream is called a pitot tube after Henri Pitot, the French engineer of the early 18th century who invented it. As the velocity of the air increases the pressure also increases inside the pitot tube with respect to the ambient atmosphere. A differential pressure sensor like a DP15 Range Changeable Pressure Sensor can be plumbed to measure this when the pitot tube is connected on the + port and the – port is open to atmosphere. Note that the pitot must be pointed directly into the flow – if the tube is mounted at some angle to the direction of flow, the transducer will not sense the full pressure developed by the air velocity.

The pressure developed by the air velocity is called the velocity head, and it is affected by the density of the air. The density of the air, in turn, is a function of the local atmospheric pressure and the temperature. The equations that relate all these factors are:

airflow-3 air velocity

Note that to determine the air velocity the density must first be known. This is the second equation and relates ambient atmospheric pressure and temperature to density. The temperature in degrees Rankine is an absolute reference and is T in degrees F + 460. Assuming average conditions of 70 F and a barometer of 29.92 In Hg, the density of air is 0.075 Lbs/Cu Ft.

If, for example, we measure a differential pressure from the pitot tube of 2.00 In H2O, then the air velocity calculates to 5671 ft/min or 94.5 ft/sec.

Air velocity is a function of air density and differential pressure, but determining air flow requires that the geometry of the piping be taken into account. The pitot tube can be used as before, but  the negative port of the pressure transducer is now connected to the pipe or duct so that the internal pressure is taken into account by the measurement of the velocity head.

Differential Pressure Transducer air velocity

Note that it is still critical that the pitot tube be installed so that it is pointed directly into the oncoming flow stream.

Ideally, determining the flow in terms of volume should simply a matter of multiplying the cross sectional area of the tube or duct by the air velocity. If the dimensions of the ducting are known, then the cross-sectional area can be easily determined and the volumetric flow calculated.

There is a problem with this, however – the velocity of the air is not uniform at all points along the cross-sectional area of the tube. This is because friction between the moving air and the inside surface of the pipe or duct slows the velocity down. The air velocity in a pipe, for example, is highest near the center but slows towards the inside walls. To make things even more complicated, the shape of the velocity profile is also affected by the type of flow – turbulent or laminar – and the proximity of other fittings and protuberances inside the piping.

airflow-4 air velocity

Averaging pitot tubes have been developed that sense the velocity head at several points along the cross-section of the air pipe or duct and deliver a differential pressure that more accurately reflects the average velocity profile.

airflow-5 air velocity

The extent to which the averaging pitot tube differential pressures deviate from the actual velocity profile pressures is expressed by a correction factor supplied by the manufacturer. This factor may depend on duct geometry and the flow regime present – sizing a pitot tube correctly has become an art. But the pitot tube factor must also be included along with the other variables in the final flow equation.

The Validyne P55 Pressure Transducer can be used to measure flow. Click here now to contact us for more information. 

p55