Under Pressure – sensors in the spotlight
Published: 08 July, 2015
Pressure sensors are used in a whole range of applications, but understanding the differences in design and operation is crucial when specifying a new sensor. These vital components can facilitate a variety of tasks from displaying process information and performing control functions to operating alarms. Kieran Bennett, of process control specialist Bürkert, looks at the range of sensors available, the different technologies used in their design and how the application affects the specification.
When it comes to fluid control systems, very little can be achieved without the raw data supplied by sensors, which need to be carefully specified to ensure that they are suited to the application. Pressure sensors can be constructed using various designs depending on the target application. From simple and cost effective to bespoke, ultra high pressure components, it is important to make the right choice.
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. This type of design is very well suited to low pressure measurement, while also being 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.
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.
When specifying a pressure sensor it is important to ensure that it will give you the readings you expect and this means understanding the terms used in pressure measurement.
Absolute pressure is measured relative to a perfect vacuum. One example is atmospheric pressure.
Gauge pressure is measured relative to ambient pressure. Blood pressure is a good example. Intake manifold vacuum in a car engine is an example of a vacuum gauge measurement (vacuum is negative gauge pressure).
Differential pressure is the difference in pressure between two points of measurement.
The use of hydrostatic pressure measurements can also 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.
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. 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.
While the design of the sensor measuring apparatus is crucial, it is also important to consider the media being measured and the conditions in which the pressure sender is expected to operate. Temperature and pressure are often closely linked, so understanding the operational parameters of each sensor is vital.
The next consideration is the sealing material that is used and its suitability to the media being measured. It is advisable to consult a chemical resistance chart which gives details of the suitability of many sealing materials against a large number of base chemicals, commercial products and liquids found in the food and beverage industry.
In some applications, such as food and beverage or pharmaceutical production, hygiene standards are of paramount importance. This requires the materials used to make the sensor body and any electrical interface to meet certain standards, such as those produced by the EHEDG (European Hygienic Engineering & Design Group).
The final aspect of specifying a pressure sensor is its compatibility with the surrounding control infrastructure. This may be a simple, single sensor, closed loop control system or it may be part of a much larger control system using bus communication and PLC modules. Whatever the situation, there will be a solution available.
Understanding the circumstances in which a pressure sensor is expected to operate is the most important step in finding the most suitable product. This information can then be used by an application engineer to determine the operating characteristics of the ideal pressure sender. This may even help when replacing a sensor, especially if these considerations were not followed in the first instance.
For further information please visit: www.burkert.co.uk