Understanding sensors: Pressure and temperature sensors
Kieran Bennett, process control specialist at Bürkert, concludes our three-part series of articles detailing the range of different sensors for fluid control systems and their contrasting design features.
Pressure Sensors explained
The use of hydrostatic pressure measurements can determine fluid levels within a tank. Essentially, fluid within a tank generates a specific hydrostatic pressure based on level and fluid density.
“By measuring this pressure, with respect to a reference pressure, which is usually ambient pressure, the level can be determined – assuming that the fluid density is known,” says Kieran.
It is also possible to use this method within a pressurised vessel by using a second sensor to measure the gauge pressure above the fluid level.
Kieran adds: “Using these two pressure measurements, it is possible to evaluate an accurate level measurement.
“Accuracy is dependent on the precision of the pressure sensor, however, higher internal pressures within the tank increase the margin of error as will the use of two sensors, which compounds the original tolerance values.”
Various designs of pressure sensor are available to suit a variety of applications:
A piezo-resistive sensor is filled with hydraulic fluid to provide protection for the sensor. External pressure is sensed through the diaphragm by a change in the pressure of the hydraulic fluid.
“The sensor produces a pressure-proportional signal which is converted to the conventional analogue 4-20 mA output signal,” Kieran continues.
This device is very well suited to low pressure measurement, while also able to withstand high overload factors.
A thin-film strain gauge provides very precise measurements as well as very high burst pressure characteristics.
“This design uses a thin-film Wheatstone bridge to measure changes in resistance caused by external pressure and converts these measurements into an analogue output signal which is proportional to the pressure,” says Kieran.
For applications involving aggressive media or higher pressures, a thick-film ceramic measuring cell could be used.
“In this case the Wheatstone bridge is bonded to a ceramic diaphragm, providing greater protection and improved chemical resistance.
“However, the measuring accuracy is not as high as that of the thin-film strain gauge,” Kieran adds.
The technology in temperature sensors has remained steady for some time, the developments have been made in the materials, both for the sensor body and the seals to allow operation in more aggressive fluids.
“The key is selecting the correct design of sensor based on the operating range and the environment in which it needs to perform,” Kieran recommends.
“The crucial aspect to making the correct decisions with regard to sensor selection is to ensure that all the information concerning the process to be controlled is available,” he adds.
Manufacturers, such as Bürkert, have a wide range of products to suit a vast array of industries and applications, with experts on hand to provide technical information and support.
With advances in materials technology and in the communication abilities of modern sensors, it is vital to understand the application and how it is controlled in order to ensure the best decisions are made.
Kieran says: “From the most basic level control systems to fully integrated, PLC controlled production processes, sensors play a vital role in providing essential information to the system controllers, therefore reliable operation is key.”
Thankfully, Bürkert Fluid Control Systems offers free training courses on this and many other subjects to help all those involved in the specification and operation of fluid control systems.
For more information please visit www.burkert.com/en.
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