Category Archives: Sensors

Measuring Pressure Drop Across Protective Mask

measuring pressure dropFace masks are often the first line of defense against the spread of infection or damaging particles. A properly designed face mask will stop dangerous materials, but allow normal air flow for breathing. The mask material must be woven tightly enough to trap unwanted particles, but the pressure drop through the mask should be low enough so that breathing effort is normal. Testing protective masks requires measuring pressure drops which equal to just a few millimeters of water.

measuring pressure dropThe Validyne DP103 differential pressure transducer is available in full scale pressure ranges as low as 3.5 mm H2O. This pressure transducer was recently used to measure the pressure drop through prototype face masks that were on the order of 5 to 10 mm H2O. The pressure drop was to be recorded at different flow rates and this required a pc based data acquisition system  for the output of the DP103.

Stand-Alone Pressure Transducer or Sensor + Electronics?


Validyne pressure transducers break down into two general categories:

Type 1 – A complete transducer with integral electronics

pressure transducer






Type 2 – A variable reluctance sensor and supporting carrier demodulator electronics.

pressure transducer







Type 1 category products include models P55, P61, P66, the P895 family and the DR800 and P532 process transmitters.

Type 2 category products include models DP15, DP360/363, DP103 with carrier demodulator models CD15, CD23/223, CD280 and CD17.

The transducers in both categories measure the same pressure ranges – so why would you choose one type over another?

Cost Effective DC Power and DC Signal:

Type 1 category transducers are generally more cost effective per point than are the sensor + electronics (Type 2) category. The Type 1 products come ready for DC power and produce a high-level DC signal, +-/5 Vdc or 4-20 mA. The Type 1 transducers include temperature compensation and are also available with higher accuracy because we can program corrections to sensor errors into the microprocessors in these products.

Type 1 products are generally ‘plug and play’ devices and are ideal for permanent installations.

Type 1 products, however, do not lend themselves to the changing of pressure ranges easily. It is possible to disassemble the sensor on a P55, for example, and replace a damaged diaphragm or install a diaphragm with a new range – but the correction factors and temperature compensation in the microprocessor will not be matched to the new assembly. Validyne can do this – and include new temperature compensation and error correction factors – but this takes time and has a cost.

Easy Range Changing:

The biggest reason to use Type 2 products is for convenient range changing. A DP15, for example, will be easier to disassemble and easier to replace a diaphragm than the Type 1 units. The sensor will be easier to calibrate with the zero and span adjustment ranges built into the external carrier demodulators. If fast frequency is important, the smaller variable reluctance sensors can be more conveniently close-coupled to piping than the larger Type 1 units and the electronics supporting Type 2 sensors have a higher low pass filter frequency available – up to 1 Khz.

Type 2 products are best suited to laboratory settings where pressure ranges are frequently changing, where a digital display is needed and where installation flexibility is important.

Type 2 products, however, do not have built-in temperature compensation, must be calibrated by the user with an appropriate pressure standard and are generally more expensive per measurement point.

Validyne Pressure Sensors “Soar” as an effective and versatile Flight Testing tool


Validyne Pressure Sensors “Soar” as an effective and versatile Flight Testing tool

Northridge, CA – May 29, 2015 – Validyne Engineering sensors improve passenger air safety, and aid in military applications.

Validyne’s P55 Pressure Transducers measure changes in everything from low pressure systems and noise cancellation, to helium pressure in Aerostats and HVAC pressures functions, to drone testing for the military.

According to Paul Muller, Validyne’s Director of Sales & Marketing “With an increased public demand for  passenger safety,  Aircraft and Aerospace companies are searching for better ways to detect environmental changes inside and outside of the craft. Validyne is dedicated to supporting the sensors and related systems that aid in providing more varied data and safer flights.”

A World Leader in Variable Reluctance Technology (VRT) since 1968, Validyne has a rich history of providing standard and custom solutions to the Aerospace industry.

Airbus A380

Validyne pressure transducers have been used in a wide variety of aircraft flight test applications.  Our Model P855 was used in the prototype Airbus A380.  These transducers were designed to measure low pressures, +/-35 mBar, and operate over wide temperature ranges -20 to +70C.  In addition, each P855 communicated over RS485 serial data lines.  The transducers were delivered while the first A380 was built in sections.  When the sections were assembled into the first operation aircraft, the sensors were already embedded inside.  Building the flight test sensors into the prototype saved over a year – normally a prototype aircraft is flown, then test instrumentation installed – a time consuming process.

Bombardier – Noise Cancelling System

The Validyne P55 transducers were installed in short range Bombardier aircraft as part of a noise-cancelling system in the passenger cabin.  The fast response time and high sensitivity to low pressures allowed the detection of noise and air movement.  This was used to trigger the noise cancellation.

Lockheed Martin Aerostat

Radar systems deployed in remote areas use a large helium balloon – or aerostat – to lift radar antennas to improve range.  The Validyne P55 has been used to monitor the helium pressure inside the aerostat to insure that it will remain aloft.

Gulfstream Private Jets

The Validyne P55 has been used by Gulfstream and Honda aircraft in flight testing to determine HVAC functions and air pressures inside air and heater ducts.

Other Government Programs

The Validyne P55 has been used in classified military flight testing of drones and other aircraft.


Validyne Engineering Inc., headquartered in Northridge, Calif., has a diverse customer base that ranges from automotive and engine manufacturers, to research and development labs, to aerospace companies. For more information on Validyne or to find out more about the new VPAS Pressure System, please visit, send us an e-mail at, or reach us by calling (818) 886-2057



Airbus makes the freedom of flight possible by designing, manufacturing and supporting the world’s best aircraft. Its people around the globe are united by a passion for aviation, as well as their desire to create better, more efficient ways for airlines and passengers to fly.

The company – a division of Airbus Group – helps shape the future of air transportation and drives sustainable growth around the world. Based on a deep understanding of ever-changing market needs, Airbus pioneers technological solutions and seeks the most efficient sourcing and manufacturing possible – so airlines can grow and people can connect.

Airbus’ comprehensive product line comprises highly-successful families of aircraft ranging from 100 to more than 500 seats: the single-aisle A320 Family (including A320neo, the best-selling aircraft in aviation history); the widebody, long-range A330 Family (including the A330-200 Freighter, plus the recently-launched A330-800neo and -900neo); the new-generation A350 XWB Family; and the flagship A380.


Bombardier is the world’s leading manufacturer of both planes and trains. Looking far ahead while delivering today, Bombardier is evolving mobility worldwide by answering the call for more efficient, sustainable and enjoyable transportation everywhere. Our vehicles, services and, most of all, our employees are what make us a global leader in transportation.

Bombardier is headquartered in Montréal, Canada. Our shares are traded on the Toronto Stock Exchange (BBD) and we are listed on the Dow Jones Sustainability World and North America Indices. In the fiscal year ended December 31, 2014, we posted revenues of $20.1 billion. News and information are available at or follow us on Twitter @Bombardier.


Headquartered in Bethesda, Maryland, Lockheed Martin is a global security and aerospace company that employs approximately 112,000 people worldwide and is principally engaged in the research, design, development, manufacture, integration and sustainment of advanced technology systems, products and services. The Corporation’s net sales for 2014 were $45.6 billion.


Gulfstream Aerospace Corporation, a wholly owned subsidiary of General Dynamics (NYSE: GD), designs, develops, manufactures, markets, services and supports the world’s most technologically advanced business-jet aircraft. Gulfstream has produced more than 2,200 aircraft for customers around the world since 1958. To meet the diverse transportation needs of the future, Gulfstream offers a comprehensive fleet of aircraft, comprising the Gulfstream G150TM, the Gulfstream G280TM, the Gulfstream G450TM, the Gulfstream G550TM, the Gulfstream G500TM, the Gulfstream G600TM, the Gulfstream G650TM and the Gulfstream G650ERTM. Gulfstream also offers aircraft ownership services via Gulfstream Pre-Owned Aircraft SalesTM. The company employs more than 15,000 people at 12 major locations. We invite you to visit our website for more information and photos at


Interfacing 4-20 mA Current Loops to the USB2250

Many pressure transducers and other field instruments use the two-wire 4-20 mA current loop for both power and signal. The 4-20 mA current loop is economical to install, using the same two wires for power and signal. It is also ideal for sending a signal over long distances – up to a mile or more – with high resistance to noise. Most data acquisition devices, however, are configured to accept voltage signals. This application note will describe how to interface a standard two-wire 4-20 mA current loop to the USB2250 sensor interface.

A typical 4-20 mA transmitter receives power from an external power supply. The power supply must be able to provide enough voltage and current to power the transmitter under all operating conditions. The transmitter will require some voltage just to produce a signal and the power supply must provide this plus any power needed to overcome any resistances placed in the current loop. The maximum amount of current required by the transmitter will be at least 20 mA, but it is best to select a power supply that will provide for 25 or 30 mA through the loop to allow for over-range indication by the transmitter.

To interface to a data acquisition device such as the USB2250 a resistor is placed in the loop and the voltage drop across the resistor will be connected to the USB2250 as a single-ended voltage input. To see how this works, assume the following conditions:

Minimum voltage required by the transmitter = 12 Vdc
Maximum loop current = 25 mA
Interface Resistor = 250 Ohms
Wire or other miscellaneous resistances in the loop = 20 Ohms

To calculate the voltage required for the power supply, we add up the voltage drops in the loop:

12 Vdc for the transmitter
Voltage drop through the wire = 0.025 * 20 = 0.5 Vdc
Voltage drop through the interface resistor = 0.025 * 250 = 6.25 Vdc

Adding these voltage drops together, the minimum voltage provided by the power supply must be 12 + 0.5 + 6.25 = 18.75 Vdc to push 25 mA through all the resistances in the loop. For a single loop the power supply will need to be rated for at least 18.75 * 0.25 = 0.47 W, but typically a single power supply will power many loops before wattage ratings become an issue.  We can use a 24 Vdc power supply – just as long as it is greater than 18.75 Vdc.

The diagram below shows how the current loop is interfaced to the USB2250 terminal block. Note that the power return is common to the USB2250 signal ground. The voltage drop across the resistor is 1 Vdc when the signal is 4 mA and 5 Vdc when the signal is 20 mA, being proportional in between.

4-20 mA USB2250

USB2250 scale and offset factors in Easy Sense software are used to convert the 1 to 5 Vdc input into engineering units. If, for example, the 4-20 mA signal is for a 100 psig pressure transducer, then at an input of 1 Vdc the pressure is 0 psig and at 5 Vdc the pressure is 100 psig.

The algebra is simplified by first determining the scale factor = change in pressure/change in voltage = 100/4 = 25.

Multiply the scale factor by -1 to obtain the offset factor: 25 * -1 = -25.

Check by determining the readings at each end point:

At 1 Vdc the reading will be R = (25 * 1) -25 = 0 psig

At 5 Vdc the reading R = (25 * 5) -25 = 100 psig

The USB2250 will read the input from the current loop and provide readings in psig or any other engineering units.

Absolute or Gage Sensor – Which is Best to measure Absolute Pressure?

If you want to make an absolute pressure measurement, you have two choices of sensor type.  The first and most obvious choice is to specify an absolute pressure transducer.  Another choice is to use a gage pressure sensor and offset the output signal to exclude the local atmospheric pressure.

An absolute pressure transducer is referenced to absolute zero and such transducers are manufactured to incorporate a vacuum chamber into the sensor body. This adds to the cost of an absolute transducer and so a more economical approach is to use a gage pressure sensor that is open to the atmosphere and then offset the output signal by 14.7 psi.

A gage pressure transducer with a fixed offset for atmospheric pressure, however, has the disadvantage of being affected by the variations in atmospheric pressure due to the weather. It is possible to quantify this error and determine if a gage-referenced sensor with an offset will result in a satisfactory measurement.  For historical records we can see the range of barometric pressure variations due to the weather.  These vary depending on geographic location.

The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above 30.01 in Hg, with record highs close to 32.04 in Hg .  The lowest measurable sea-level pressure is found at the centers of tropical cyclones and tornadoes, with a record low of 25.69 in Hg).

Honolulu, Hawaii is the place in the US with the overall smallest range of changes in barometric pressure, ranging from 29.34 to 30.32 in Hg. San Diego is the city with the smallest range of changes in the continental US, with an average range of about 29.37 to 30.53 in Hg.

As for the places with the greatest range of pressure changes, St. Paul, Alaska ranges from 27.35 to 30.86 in Hg. In the contiguous US, Charleston, South Carolina has the largest range of changes, with a 27.64 to 30.85 in Hg.

Choosing between an absolute transducer with a full vacuum reference and a gage-referenced transducer with an offset is simply a matter of determining how much error will result from typical ambient atmospheric pressure changes due to the weather.

Here is a table to help guide the selection process.

absolute pressure


Here are two examples:

You want to make a 200 psia pressure measurement and you are in the southeast US. The weather will likely be similar to Charleston, SC and so looking at the chart you would expect an error of 0.788% to be the result of ambient atmospheric changes if you were to use a gage pressure sensor offset for atmospheric pressure. This error is excessive so the extra expense of a true absolute transducer would be warranted.

You want to make a 2000 psia measurement in Los Angeles. San Diego has a similar weather pattern so from the chart you can see that the error incurred by using an offset gage pressure sensor would be just 0.029% FS. This is very small compared to the 0.25% accuracy of the transducer and so would be the most economical choice.

Look at Validyne’s offerings of absolute and gage sensors and transducers

Test and Measurement Grade Pressure Transducers (P895,P896,P897V)

General Purpose Pressure Transducers

AP10 Variable Reluctance Absolute Pressure Sensor