Understanding the Differences in Design and Operation when Specifying Pressure Sensor
Key points
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.
Piezo-resistive sensors
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, as used in the Bürkert Type 8323, is very well suited to low pressure measurement, while also being able to withstand high overload factors.
Thin-film strain gauges
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.
Ceramic cell
For applications involving aggressive media or higher pressures, a thick-film ceramic measuring cell, such as the Bürkert Type 8316, 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.
Pressure measurement
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.
Materials specification
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, such as the one produced by Bürkert, 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).
System integration
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.
Kirsty Anderson
Bürkert Fluid Control Systems
Tel: +44 (0)1285 648720 Fax: +44 (0)1285 648721
Web: www.burkert.co.uk
Email: kirsty.andeson@burkert.com
Burkert Fluid Control Systems
- 01285648720
- sales.uk@burkert.com
- http://www.burkert.com/
- Fluid Control Centre 1 Bridge End Gloucestershire Cirencester GL7 1QY GB
About us
Bürkert is present in thirtyfive countries around the world. We also work with a large network of distributors and partners, which means we are as close as possible to our customers. This global presence ensures full service and support to all of our customers in every country around the world. Research is the lifeblood of our company.
At Bürkert, we are never satisfied with the status quo and are continually seeking new technologies and solutions for our customers. Every year, our people develop new and highly advanced products and solutions, ranging from integrated process measurement and control units to the most sophisticated systems used in pharmaceutical research. To be a market leader, we are also an R&D leader.
Therefore, our investment in research & development is one of the highest in our industry. In our research centres in Germany and France, 150 people are committed to working for a common future for our company and our customers. We are committed to offering our expertise wherever it is needed, anywhere in the world. This global presence ensures that our advances in fluid control technology are also global.
What we do in a nutshell
Manufacture of process equipment. One of the few manufacturers to provide solutions for the complete control loop.
Where we supply to
UK Ireland, Europe
Industries we supply to
Food and Beverage, Pharmaceutical Cosmetics Toiletries, Water and Wastewater
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