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water main break

Measuring Water Main Leakage

If a large water main is leaking, how would you know? What part of the city consumes the most water, and when? These are questions often asked of the city water engineer. In order to give an accurate answer, direct measurement of the flow in a large water main is often the best way to verify metering station data. How can the flow in a water main be observed directly?

Fluid flow in a water main can be measured using a pitot tube and differential pressure transducer. Water mains are usually accessible from underground vaults in city streets. A pressure tap is made, while the main is still under pressure, and a pitot tube inserted into the water flow. A cross-sectional survey is made by traversing the pitot tube across the ID of the water main pipe. The differential pressure is recorded at each point across the survey. Calculations are later made to convert the differential pressure readings into water velocity, and the flow rate calculated from the velocity profile.

Water in large mains may be moving very slowly, so the differential pressure developed across the pitot tube will also be small. The pressure transducer must be capable of measuring low differential pressures while operating at the static pressure of the water main. The analog output of the pressure transducer can be sent to a portable data logger for long-term studies.

Pressure Sensors and External Carrier Demodulators

The most popular Validyne pressure transducers are the P55/P61/P365 series.  These all include a pressure sensor, carrier demodulator electronics package, a high level output signal, temperature and linearity correction as well as a compact form factor.  There are applications, however, where a better solution might be to separate the pressure sensor from the electronics, with the two connected by a cable.  This article describes when this approach makes the most sense.

Validyne offers the sensors and electronics package from the P55/P61 available as stand-alone components.  The DP15 series of pressure sensors is identical to that used in the P55 and P61, while the DP360 and DP363 are high pressure variants the same as used in the P365 and P368.  The CD16 standard analog output electronics or the CD17 USB-based electronics can be used with any of these sensors, and standard cables are available in a variety of different lengths to connect the two.

Pressure Sensors

When should a sensor be separated from the electronics?  The biggest reason to do this is to allow convenient re-ranging of the pressure sensor.  The full scale pressure range of Validyne sensors can be changed by replacing the sensing diaphragm.  There are 23 different full scale ranges available for the DP15, for example, and these run from a few inches of water to 3200 psi. Changing the diaphragm is straightforward; the connector and four body bolts must be removed to gain access to the sensing diaphragm, and the DP15 sensor makes this easy, requiring just a torque wrench and a vise.  With a little practice, the diaphragm in a DP15 can be replaced and re-calibrated with the CD16 or CD17 electronics in about 20 minutes.  The DP360 and DP363 high pressure sensors are similar in construction and also lend themselves to straightforward diaphragm replacement. Frequent re-ranging of the full scale of a Validyne transducer is common in laboratory situations where pressure measurements vary widely from day to day.  Test labs and university labs are typical places where a separate sensor and electronics package are used to best advantage.

Another reason for separating the pressure sensor from the electronics is to conserve space or limit the weight at the measurement point.  In tight locations, such as aircraft compartments or in submersible vehicles, the pressure connection may be in a relatively inaccessible space and the smaller footprint of the DP15 sensor, might fit better than the full P55.  If mass or weight is important, the sensor will be lighter than the full transducer and this will relieve any stress on the piping connections in areas where shock and vibration are a consideration.

It is important to realize that separating the sensor from the electronics will compromise the temperature correction as the temperature sensor is located on the electronics package and not at the pressure sensor.  A pressure sensor such as a DP15 used with a remote electronics such as the CD16 will be most effective in applications having a stable temperature environment.


pressure sensor

Pressure Sensor Accuracy

When using a pressure sensor to measure pressure, one of the first questions that comes up is accuracy: How closely does the value reported by the pressure sensor come to the actual pressure?  We use the term accuracy, but what we really want to know is the error of the pressure measurement.  This article will describe standard specifications and methods for calculating sensor accuracy.

The definition of sensor accuracy actually defines error, and the two terms are used more or less interchangeably:  The accuracy of the sensor is the simply the difference between the pressure reported by the sensor and the actual pressure, expressed as a percent of the sensor full scale.  For example, if a pressure sensor with a full scale range of 100 psi reports a pressure of 76 psi – and the actual pressure is 75 psi, then the error is 1 psi, and when we divide this by the full scale and express it as a percentage, we say that accuracy (or error) of the sensor is 1%.  Most industrial sensors are better than that, with specified accuracies of +/-0.25% or +/-0.1% of full scale (FS).  So the error of a 100 psi FS sensor with an accuracy of +/-0.1% FS will not exceed +0.1 psi or -0.1 psi – at any point in the measurement range of the sensor.

Sensor accuracy is comprised of two error modes: Non-linearity and Hysteresis.  Ideally, pressure sensors are perfectly linear – the output signal or reading is directly proportional to the applied pressure.  Because sensors are mechanical devices, however, they are not perfectly linear, and this error is called non-linearity.  Non-linearity is determined by the five-point calibration method.  A pressure standard (a device know to be at least five times more accurate that the pressure sensor) is used to apply pressures at 0%, 50%, 100%, 50% and 0% of the sensor full scale.  A best-fit straight line is fitted to these points, and the maximum deviation of any of the five points from the value predicted by the best-fit line, is defined as the non-linearity error.

Hysteresis is simply the difference in the value of the readings at the same pressure along these five calibration points, up-scale and down-scale.  Hysteresis can be measured from the readings at the 50% points and at 0% FS pressure.  The greatest difference in any of these is defined as the hysteresis.

The accuracy of the sensor is defined to be the sum of the non-linearity error and the hysteresis error.

Note that the sensor accuracy calculation is pretty much the worst case error that can be determined from the calibration data, and it may not occur at every point along the pressure sensor FS range.  But this is considered to be a conservative method for calculating the error present at any pressure over the range of the sensor.

For differential pressure sensors – those having a plus and minus full scale range, the same techniques are used to calculate accuracy with the addition of -50%, -100%, -50% and 0% calibration points required (9 all together).

Spreadsheets that automatically calculate non-linearity, hysteresis and accuracy are available from Validyne for gage, absolute and differential pressure sensors.  Factory calibration sheets showing these errors are shipped with most sensor models.

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 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.

Corrosion Resistance of Validyne Pressure Transducers

Pressure transducers are exposed to a wide variety of fluids and gases when used to measure pressure.  Corrosion of the pressure transducer sensor body will shorten service life and lead to costly downtime if material selection is not carefully considered.  This article will cover the basics of material selection for Validyne pressure transducers so that the best possible performance and adequate corrosion protection can be ensured prior to purchase.

Validyne offers three types of sensor body materials that will provide appropriate protection for most pressure measurement applications: 410 SS, 316 SS and Inconel.  316 SS and Inconel pressure transducers are supplied with a teflon-coated 410 SS sensing diaphragm needed for the Validyne inductive sensing technology to operate correctly.

410 SS

410 is the standard material for Validyne transducers and does best when used with air, inert gases, or hydrocarbon-based fluids.  Oxidizing environments – or fluids containing chlorides – will cause 410 SS to corrode and pit, sometimes rapidly.  Water-based fluids, fluids containing salts, or corrosive chemicals should not be used with 410 SS.

Validyne offers the P365 High Pressure Transducer and the P368 Digital High Pressure Transducer with the 410 SS option.

316 SS

316 SS is the standard steel for instrumentation and has a high degree of resistance to water-based fluids and mildly corrosive chemicals.  316 SS also does well in fluids with low concentrations of chlorides, but is attacked by nonoxidizing acids such as sulfuric and hydrochloric acid in most concentrations.  316 SS has good resistance to alkaline solutions, organic acids, and other organic compounds.

Validyne offers a wide range of pressure sensors and pressure transducers with 316 SS.  The DP15 Variable Reluctance Pressure Sensor Capable of Range Changes, P55 Pressure Transducer and P61 USB Pressure Transducer are some of the pressure transducers that can be ordered in 316 SS option.


inconel pressure transducers

Inconel is a superior material, ideal for corrosive applications, and is best reserved for systems containing high concentrations of chlorides such as salt water or brine.

Validyne offers a full range of pressure sensors and transducers in inconel. From low-static pressure transducers to high-static pressure transducers, we offer the DP15 Variable Reluctance Pressure Sensor Capable of Range Changes, P55 Pressure TransducerP61 USB Pressure TransducerP365 High Pressure Transducer and the P368 Digital High Pressure Transducer.

Fluids to Avoid

Fluids containing hydrogen or hydrogen sulfide should not be allowed to come in contact with Validyne transducers. Almost all metals lose ductility when they absorb hydrogen, especially at temperatures below 100 °C. Hydrogen molecules can enter the sensor body metals at the grain edges and this will cause embrittlement of the metal that can lead to pressure boundary failure.  Additionally, hydrogen sulfide is poisonous and hydrogen gas is extremely explosive.

Corrosion of Validyne transducers is not covered by the warranty. The proper choice of sensor body material will enhance pressure transducer performance and increase the life of the pressure transducers.

Click here to contact us today to find out the different solutions we can provide for your pressure measurement application.