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The SPD pressure sensors
Gauge, Absolute or Differential
Bridge, Analog or Digital
PRODUCT DESCRIPTION
The Smart Pressure Device SPD series of pressure sensors are silicon based and have resistive bridges inside. The output voltage of the sensor is linear to the applied pressure. The type available are "gauge", as well as "absolute" and differential.
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TECHNICAL DESCRIPTION
The sensors are based on four silicon resistors, that are connected in bridge mode. The value of one of these resistors changes, when pressure is applied. This change of resistance (and therefore the output voltage of the bridge) is lineairly proportinal to the applied pressure. It is expressed in the form of a lineair equation with two parameters: Offset and Span. The offset is the output voltage when zero pressure is applied, and the span is difference between the output voltage when the maximum pressure is applied and the offset.
PROBLEM
Because the resistors are made of silicon, the value of the resistors does not only change with a change in pressure, but also changes considerably with varying temperatures. This effect is expressed in the temperature coefficients of the span and the offset. In order measure the pressure correctly, temperature compensation has to be applied and the values of these coefficients have to be measured as part of the calibration process. Depending on the wanted accuracy, this can be done on a sensor basis or on a series basis. Note: This is a property of silicon based pressure sensors in general!
SOLUTION
It is very well possible to establish the values of offset and span, as well as their temperature coefficients, but this is not always needed. In certain applications, where accuracy is not very important, these pressure sensors can be used without calibration, benefitting from their very competitive pricing. However, if accuracy is important and the process of calibrating is not an option for the user, then choosing a sensor which has been calibrated at the factory and which has an analogue output, may be considered. All the Smartec sensors that have an analogue output, have been compensated for offset, sensitivity, temperature drift and non-linearity.
Note: If the user opts for a - non compensated - bridge type sensor, then the use of the Smartec UTI interface can be a very good interface solution (see below).
SPECIFICATIONS
Many pressure ranges are available; full scale ranges from 0,01 to 20 Bar. The most current sensor types are:
If you want to be able to tell from the name, what the pressure range is, whether it's a Gauge type or Absolute or Differential and if the output signal is Bridge, Analogue or Digital, you may want to read the pressure sensor name convention first. It can be found in a one page document
spdnames.pdf. For the electrical properties, and the other technical details, please refer to the respective datasheets (list above), which can also be found in our SUPPORTSHOP A brochure is available as well, highlighting some of the possibilities and features of these sensors.
BRIDGE TYPE PRESSURE SENSORS AND THE SMARTEC UTI INTERFACE
Because the applied pressured results in an imbalance of the resistive bridge in the sensor, this sensor is especially suited to use in combination with the Universal Transducer Interface (UTI). The pressure sensor can directly be connected to the UTI, which needs only one microcontroller input for collecting the pressure data. Mode 13 of the sixteen available UTI modes supports a bridge with an extra resistive element (Like PT1000 for example), which can be used to measure the temperature at the same time. This is the ideal solution for the phenomena that was explained above: The strong temperature dependance of the silicon resistors.
The absolute version (series SPDxxxA) comes with a reference vacuum chamber, which is formed on the die during manufacturing.
TIP: AVOID A WATERHAMMER
In fluid based systems, a so called waterhammer may be created by stopping and/or starting a liquid flow suddenly. The results of a waterhammer or impulse load are devastating to a pressure sensor. For more information please visit :
OMEGA: WATERHAMMER, a complex phenomenon with a simple solution
SUPPORT
All relevant documentation and files (data sheet, application notes, software
etc.) for our products can be downloaded from the SUPPORTSHOP page.
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