Monthly Archives: July 2017

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.