Tag Archives: pressure sensors

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.

Differential Pressure Transducer

Pressure Transducers – Saving Lives, Property and Maximizing Production Capabilities

A pressure transducer can directly measure the force of gas or liquid, and convert the value into an electrical signal. Pressure transducers typically include a sensing diaphragm capable of responding to changes in pressure. Pressure pushes on this diaphragm, changing its position, and this changes the inductance of sensing coils mounted opposite the sensing diaphragm. The coils are excited with an AC waveform and the resulting change in electrical impedance represents the applied pressure. The electrical output of the coils is converted to a DC signal.

Pressure Transducers – Classifications

Pressure transducers come in a variety of shapes, sizes, and output signal types. In addition to a DC voltage output, current signals are often used for electrically noisy environments commonly present in industrial applications. A 4 to 20 mA signal has been adopted as the industry standard and the current signals can be sent accurately beyond 1,000 feet. All pressure transducers are generally characterized by their pressure measurement range. They are also classified by accuracy, errors due to temperature change, and the amount of static pressure the sensor can tolerate. Temperature always affects transducer accuracy and most transducers have a scheme to correct for ambient temperature changes Resolution is another characteristic used to evaluate a pressure transducer, and this is defined as the smallest amount of pressure change that can be detected – typically a function of the signal-to-noise ratio of the output.

Electromagnetic interference (EMI), can also affect transducers. Some units are protected against EMI effects, but only up to certain intensities. Materials used to make the sensors vary, and include plastic, silicon, stainless steel, or epoxies. Epoxies can be adversely affected by certain fluids under pressure.

Some pressure sensors are mounted to a circuit board with contacts to secure a solid connection. Others are designed for industrial environments and sturdily constructed with weatherproof enclosures. If the device is for general use, it is likely to have a standard design that allows it to easily connect to commonly used receiving devices such as computers, programmable controllers and panel meters. The costlier transducers are known for their high-accuracy readings and low rates of error as a percentage of full scale range. At Validyne, we offer many different configurations to meet your exact requirements.

Types of Transducers

At Validyne, we serve several major markets and carry a variety of transducers, which include differential pressure sensors, gage pressure transducers, USB pressure transducers, electronic pressure manometers, low pressure transducers, OEM pressure sensors, and more.

Transducers perform a critical job in every industry, especially in automation and control. Transducers used in aircraft or healthcare applications have lives depending on reliable and accurate performance.

Pressure Transducer Used in Aerostat

The Validyne P55 pressure transducer are used by a major manufacturer of aerostat lifting systems to measure the helium pressure inside the gas bag. An aerostat is a large helium-filled balloon – similar to the old WWII barrage balloons – that are used to quickly deploy radar to remote locations. Lifting the radar antennas several hundred or thousand feet in the air greatly increases the operational range. Aerostat radars are routinely used to survey air traffic in remote areas for the purpose of interdicting drug traffic or keeping track of enemy air movements.

Pressure Transducer

An aerostat can remain on station – continuously in the air – for weeks at a time. The P55 pressure transducer is ideally suited for this application because it can accurately measure the relatively low helium pressures inside the balloon. In addition, the P55 pressure transducer consumes a small amount of power, is small in size and weight and has a robust construction that makes it suitable for field operations.

What is the Difference Between Line Pressure and Overpressure?

The over pressure and maximum line pressure specifications for differential transducers are often confused. This application note will describe the differences and give examples.

Over Pressure:

For all differential pressure transducers, over pressure is defined as the maximum differential pressure the transducer can withstand without compromising subsequent measurements. This is determined by the Validyne pressure range code of the transducer . The range code is given in the transducer model number.

For example, a transducer having a -36 range code will measure from 0 to +/-5 psi, differential
pressure. If the sensor is exposed to up to 10 psi differential, no damage will result and the transducer can make subsequent measurements of 5 psi and below accurately.

Some further examples:

-42 will tolerate a maximum differential pressure of 40 psi
-20 will tolerate a maximum differential pressure of 7 In H2O
-56 will tolerate a maximum differential pressure of 1000 psi

Note that the maximum pressure that the P55 or DP15 sensor can contain is 4000 psig! So the -64 range, with a full scale of 3200 psi differential will likely leak once the over pressure reaches 4000 psig – somewhat less than twice the full scale range of the transducer.

Line Pressure:
The line pressure specification is the maximum pressure that can be applied to both ports at the same time. The maximum line pressure for the P55D, for example, is 3200 psig, and this is the maximum pressure that can be applied to both ports simultaneously. It is often necessary to measure small differential pressures at high line pressures – as in measuring the pressure drop across a high-pressure filter. The filter may operate at 1000 psig, but have less than a 5 psi differential pressure drop across it. A P55D with range code -36 could be used to measure the actual pressure drop across the filter because the + port (upstream side of the filter) might have 1005 psig and the – port (downstream side) 1000 psig. There is a difference of 5 psid but the common-mode line pressure is 1000 psig.

There is a slight error that occurs as a function of line pressure – the zero output will shift as much as 1% per 1000 psig of line pressure. This can be corrected using a 3-valve manifold so that the differential pressure can be equalized across the sensor while the full line pressure is applied to both ports. Turning the Zero adjustment will re-zero the output signal at the operating line pressure.

For any differential transducer operating at high line pressures, care must be taken not to expose one side of the transducer to full line pressure while the other side is at atmospheric pressure – this will result in severe over pressure and require repair.

 

Pressure Transducer