# Resolution and Frequency Response in Pressure Transducers

Resolution and frequency response in pressure sensors and pressure transducers are two performance parameters that are important, but often misunderstood.  This article will describe how each of these parameters relates to Validyne pressure transducers and pressure sensors.

## Resolution

Resolution of a pressure transducer is defined as the smallest change in pressure that can be detected by the transducer.  Validyne pressure transducer are analog devices and the resolution, in theory, is infinite.  As a practical matter, however, the resolution of the analog signal from the pressure transducer electronics is a function of the signal to noise ratio.  All analog signals contain noise and the various carrier demodulator circuits used with Validyne variable reluctance sensors have somewhat different specifications for the noise level, depending on the demodulation scheme employed and the output filtering used.  In general, the noise level of the carrier demodulator signal will be 0.05% or less of the pressure transducer full scale.  So the smallest pressure change that can be detected from the Validyne pressure transducer signal will be less than 0.05% of the maximum pressure range of the pressure transducer.

## Frequency Response

The frequency response of a pressure transducer is a measure of how quickly the pressure transducer can respond to changes in pressure.  There are two ways to define this: response time and flat frequency response.  Response time – sometimes called the sensor time constant – is the time, in seconds, required for a sensor signal to change from 0 to 63.2% of the full scale when the pressure sensor is exposed to an instantaneous full scale pressure change.  Response time is often used for slower pressure transducers that respond to pressure changes as a first-order system.  Knowing the time constant of the pressure transducer allows the user to calculate how the sensor signal will change in response to different applied pressure signatures during operation.

For faster pressure transducers – such as Validyne pressure transducers – the flat frequency parameter is a more accurate way to describe the pressure transducer frequency response.  Flat frequency is the maximum frequency, in Hz,  that the pressure sensor can pass into its signal without distortion.  This depends on the geometry and construction of the pressure sensor, the plumbing leading up to the pressure sensor, the fluid media and the output filtering of the carrier demodulator.

Validyne has tested the standard DP15 family of pressure sensors or the P55 family of pressure transducers types for flat response and this has been found to be  80 Hz in air when the varying pressure source is close-coupled to the sensor port.  That means that the pressure sensor is capable of allowing pressure changes of up to 80 times per second to pass, without distortion when the pressure transducer is close-coupled.

Many times, however, the pressure transducer is connected by a length of tubing to the source of the pressure variations, and this degrades the flat frequency response, as shown in the table below:

Tubing Length, FT            Flat Response, Hz

0                                         80

0.5                                      50

1.0                                      36

2.0                                      25

3.0                                      20

4.0                                      12

5.0                                        8

The output filtering of the carrier demodulator will also affect the system response, but in general most will pass the 80 Hz sensor response frequency.  Those carrier demodulators models that feature selectable low-pass filtering, however, may provide for lower settings that will filter out these frequencies, even when the pressure sensor is capable of passing them into the signal.

The flat frequency response of the pressure transducer/plumbing system will change by ratio of the speed of sound in air to the speed of sound in a liquid.  For water this ratio is about 4X, so the maximum frequency response of the pressure transducer in liquids will be greater than 300 Hz.  For a CD15 demodulator and DP15 pressure sensor, the output filtering will allow frequencies of up to 1 KHz to pass, and this is the fastest system we offer.  The P55 electronics in the P55 pressure transducer, however, has a low-pass cut-off frequency of 250 Hz and so may attenuate very fast pressure changes in liquids.

The response time can be roughly related to the flat frequency response for the purposes of comparing the performance of various sensor types.  Since response time is a measure of how long it takes for the pressure transducer signal to rise from 0 to 62.2% of full scale, the rise time can be assumed to be not more than one quarter of one complete cycle of the pressure sensor maximum flat frequency.  This is simply the reciprocal of 4 times the maximum flat frequency.  Thus the 80 Hz flat response would be 1/(4 * 80) or about 3.2 msec.

# Pressure Measurement in Engine Test Cells

Does synthetic oil reduce bearing wear? Do gasoline additives really improve combustion? These questions are investigated in a special laboratory known as an engine test cells. There are many pressure measurements needed on an engine: oil pressure, exhaust pressure, coolant pressures and any number of emission pressure measurements. Pressure transducers must be protected from the ambient environment of the test cell and still provide useful signals to a data acquisition system.

An engine test cell is a noisy, dirty and cramped room containing a running engine and everything needed to conduct the test and make the required measurements. The engine must have a sturdy mounting and a dynamometer to simulate loads. There must be piping to remove the exhaust gases and to bring in clean air. There must be adequate ventilation so that heat exchanged by the radiator can be removed from the test cell.

Pressure transducers used inside the engine test cell are mounted in a large protective enclosure that usually hangs from the wall or the ceiling. Sometimes a portable enclosure is used that has wheels so the sensors and transducers can be moved for different engine configurations.There are often several dozen transducers inside the enclosure and all of the power and signal wiring must be brought out to the data acquisition system outside the test cell. The plumbing for the transducers is a series of hoses that are connected to the outside of the enclosure and run to various points on the engine.

Because the enclosure holding the transducers is tightly packed, the ideal transducer must be compact and easy to install. The cramped conditions in the typical test cell also require that the transducer be highly reliable; changing out a pressure transducer during a long-term engine test will inevitably create gaps in the test data. A high-level output from the transducer reduces cost for external signal conditioning for the data acquisition system.

The Validyne P55 has been successfully used in test cell environments for many years and provides the rugged stability needed for this challenging environment.  New versions of the P55 with a CAN Bus interface are now able to connect directly into the data stream from the engine’s processors.

# Measuring Pressure Drop Across Protective Mask

Face 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.

The 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.

# Validyne “Senses” the Change in the Pressure Sensor Market & Reveals New Innovations

FOR IMMEDIATE RELEASE:

Validyne “Senses” the Change in the Pressure Sensor Market and Reveals New Innovations

Northridge, CA – June 30, 2015 – Validyne Engineering releases two new product lines for hydraulic pressure and low pressure air flow systems.

Validyne is proud to announce two new product lines to meet the demands of the evolving sensor market.

The two new product lines will give OEM users of pressure sensors the option to choose variable reluctance sensing technology for their applications at an attractive price point. The first new product line offers feature-rich enhancements for our OEM clients. The second new product line offers a streamlined set of features compared to their traditional line, but a lower cost.

Compact form factor combined with a rugged sensor that accepts air, inert gases and hydrocarbon-based liquids are important elements in this new low cost product line. Target audiences include those requiring hydraulic pressure measurement, low pressure air flow systems and any application involving pressures below 5 In H2O.

According to Paul Muller, Validyne’s Director of Sales & Marketing “Validyne is reaching out to the OEM market to provide low cost differential pressure sensors capable of accepting liquids and gases.”

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

Technical Specs for New Products:

Model P17: For air and inert gas applications Full Scales as low as 0.1 In H2O DC Power/DC Signal 0 to +5 Vdc signal

Model P20: For Liquids and Gases Static Pressures to 1000 psig Differential Ranges as low as 0.1 psi full scale DC Power/DC Signal

Model P25: For Liquids and Gases Static Pressures to 10,000 psig Differential Ranges as low as 5 psi full scale DC Power/DC Signal

Models P117, P120 and P125 are similar in functionality from those listed above, but are constructed without a housing for electronics – for use inside the customer’s own unique enclosure.

VALIDYNE ENGINEERING

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 www.validyne.com, send us an e-mail at sales@validyne.com, or reach us by calling (818) 886-2057

# Stand-Alone Pressure Transducer or Sensor + Electronics?

Introduction:

Validyne pressure transducers break down into two general categories:

Type 1 – A complete transducer with integral electronics

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

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.