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Pressure sensors are used in a vast array of applications, from medical environments to safety-critical pressure measurement in hydraulic brake systems. In industrial applications where processing takes place, it is often important to measure pressure accurately to control the quality of the final product, and sometimes to control the process itself. The market has various types of pressure sensors available, not least because they are used in so many different ways, however, within this article, we will take a look at some of the variables that are accounted for by different types of pressure sensors and the factors to consider when selecting one.

What different types of pressure are there?

Starting with the very basic principles, pressure is the perpendicular force applied to an object over the surface area of the object. Pressure = Force/Area. PSI, for instance, is pounds per square inch. Or a Pascal, which is one newton per square metre. There are three different types of pressure:

Gauge pressure: this is the most common type of pressure when dealing with engineering applications. Gauge pressure is the difference between a given pressure and atmospheric pressure. A positive overpressure is referred to when the absolute pressure is greater than the atmospheric pressure. If the measured gauge pressure is negative, it is called under pressure or partial vacuum.

Absolute pressure: this is the point above a perfect vacuum. Usually, it is the sum of gauge pressure plus atmospheric pressure.

Differential pressure: this is the difference between two points when neither is known and neither is a perfect vacuum.

All other ‘types’ of pressure (e.g. hydrostatic, negative, deflagration) are merely one of the options above with a name directly referencing the context of the pressure.

What pressure sensors are available?

 

Types of pressure sensors vary widely, however they can generally be categorised by the type of pressure (as outlined above), combined with the method of sensing, the output signal type and the media that they are measuring. Looking at each of those in more detail:

Sensing method: the sensor technique has a simple aim, to translate the pressure applied on the sensor mechanism into an electric signal as an output. Types of sensor options can include resistive, capacitive, resonant, piezoelectric, optical and MEMS. The sensor method used can affect accuracy, reliability, measurement range and whether it is appropriate for the operating environment.  

Output signals: these are generally either transmitters, which create an output current or transducers, which create an output voltage which is variable in accordance with the experienced pressure.

Media type: the operating environment will impact the type of pressure sensor that you choose. For example, if your pressure sensor will be working with corrosive media or within clean in place systems or other hygienic environments, you will need to carefully select a solution that not only maintains the stringent hygiene levels employed but which won’t be damaged by the solutions that it is measuring. Other considerations in terms of media include whether the flow is air, gas, liquid, hydraulic or pneumatic. 

There are so many types of pressure sensors available that there is likely one on the market which is perfect for your application. However, if you are finding the array of options daunting, or would like advice in selecting the appropriate sensor for your type of pressure sensing requirement, why not get in touch? Call us on 01254 390555 or email [email protected]

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How is flow measured?
Flow rate measurement is employed widely across many industries, and is used to measure the volume of a liquid or gas passing through a specific cross-sectional area per unit of time. Flow rate measurement helps us to control and monitor the rate of liquid flow within applications and ensure that fluid control processes are running safely and cost-effectively. We will take a look at how flow is measured, some of the flow control options available and what it is important to consider when thinking about flow rate measurement.

Why is flow rate measurement important?

People have been measuring flow rates since 5000BC, with the earliest known device used by the Mesopotamians. They developed a crude plumbing system within their Sumerian cities to deliver water to each household from the nearby rivers and used flow measurement techniques to monitor the flow rate from the river into their channels. Since the early 18th century, scientists and mathematicians have been developing theories and devices which, over time, have led to the development of the technologies that we use today for flow rate measurement. These devices employ a number of different techniques and systems which allow us to monitor and control flow rates. All industries that process liquids and gases or have a requirement for fluid control use flow rate measurement tools – from utility companies measuring the flow of wastewater moving into water filtration systems, to power generation businesses requiring deionised flow measurement; from oil and gas industries measuring the rate of flow of crude oil, through to wine producers measuring the fill rate during the bottling process. Wherever fluid moves through a system, there is likely to be a need for flow rate measurement.

How is flow measurement affected by the fluid?

Within industrial settings there are a number of options available for flow rate measurement. In fact, there are over 100 different types of flow meters employed globally, each offering a different solution depending on the application parameters and other influencers such as budget. Some of these include:
  • Coriolis meters: one of the more popular flow rate measurement devices, this flow meter uses motion mechanics principles, utilising a vibrating tube that is caused to twist in proportion to the mass flow rate. This response is subsequently monitored to provide an output.
  • Magnetic flow meters: these use Faraday’s Law, whereby a magnetic field is passed through the media which is flowing through the pipe causing a voltage signal to be transmitted to electrodes. This voltage is directly proportional to the velocity of the fluid.
  • Differential pressure flow meters: a constriction is created within a pipe that results in a pressure drop. The flow meter measures this pressure drop and, using Bernoulli’s equation, calculates the flow rate on the basis that the pressure drop across the constriction is proportional to the square of the flow rate.
  • Turbine flow meters: these are mechanical flow meters that use a rotating turbine within the flow stream. This uses the velocity of the flow to rotate the turbine which sits directly in the flow, enabling a flow to be calculated using a continuity equation.
One of the most important factors to consider when looking at flow rate measurement is the type of media flow, because how flow is measured can depend on what the flow consists of. For example, gas can be measured using Coriolis, differential pressure and positive displacement flow rate measurement devices, but cannot use electromagnetic or ultrasonic flow meters. If you are looking for the right flow rate measurement tool but are unsure of which flow meter would be most suited to your application, Best Pneumatics has a range of flow sensors and switches available. For more information on how flow is measured or flow rate measurement options, contact us on 01254 390555.
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How to Select the Right Flow Meters & Switches
With such a variety of flow meters available on the market – over 100 different types – it can be daunting when you embark on a flow meter selection process, but making sure that you choose the right one for your application is extremely important. We take a look at how to select the right flow meters and switches to ensure accurate outputs and reliable control.

Where to start with flow meter selection

The most common place that people start when looking at flow meter selection is usually with their own budget and therefore the price of a flow meter. Secondly, people tend to explore the most commonly used or popular flow meters within their industry. Whilst budget must be a realistic consideration, some of the higher priced options are low maintenance and provide better longevity, and therefore the initial outlay isn’t necessarily the full picture in terms of cost. Many flow meters are designed for use in specific industries, and these are often a good place to start your research – but you must also consider the application demands when looking at flow meter selection because the specific requirements of your application and intended usage should ultimately dictate how to select the right flow meter and switch.

Which parameters to consider in how to select the right flow meters and switches

A flow meter reports the amount of media passing a point in a system. They are designed to be passive and manually checked by the user. A flow switch is, in effect, a flow meter with a sensor that can be used to trigger an external event based upon specified readings. When going through the process of flow meter selection, these are some of the areas that you will need to consider:
  • What is the intended job of the flowmeter? Are you simply looking for a one-off measurement or continual monitoring of the flow rate within a system, or do you also want to control the flow based upon the output reading? The answer to these will help to guide you as to the kind of configuration that you need.
  • What media are you measuring? You will need to consider whether your media flow is steam, gas or liquid, and, if it is a liquid, whether it is conductive and whether it is clean or dirty (slurry). For example, if you are looking to measure gas, you could opt for differential pressure, turbine or thermal mass flow meter (amongst others) but an ultrasonic or electromagnetic flow meter would not be suitable. Similarly, if your flow is corrosive or toxic, there will be safety considerations and you will want a flow meter that will not be damaged by your media flow.
  • What are your operating parameters? You will need to look at the minimum and maximum pressures and temperature ranges as well as standard expected operating levels. Similarly, whether the flow is continuous or variable and whether inconsistencies such as pulsation, in-line air or slug flow is likely to be an issue. 
  • What output do you require? You will need to decide whether your information should be output in mass or volumetric units; the type and density of the flow will help to determine which would be most suitable. You may also be deciding between a digital or analogue flow meter and whether you need that information available to a shared source.
  • Installation and location considerations. One of the key aspects to consider in flow meter selection is the area in which the flow meter is to be fitted; the available space, accessibility, pipe run length and whether it is up- or down-stream, amongst other things. If it is a hygienic processing application then it may be required to undergo harsh cleaning methods, or perhaps cannot come into contact with the media flow at all.

If you are looking for guidance on how to select the right flow meters and switches, then why not talk to BEST Pneumatic Systems? With over 30 years’ industry experience and a range of flow meters and flow switches, we are ideally placed to assist with flow meter selection for a wide range of applications. Call us now on 01254 395 000 or email [email protected] to speak to one of our experts. 

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How does a flow switch work
Flow switches are critical in adjusting flow rates to maintain safe and manageable rates of flow within a processing system. A flow switch is a commonly used device within any system where liquids or gases are being transported or worked with. We take a look at a flow switch working principle and discover in more detail just how a flow switch does work.

What is the flow switch working principle?

A flow switch is a device used to monitor the flow rate and pressure of liquids, air or other gaseous media through a system. How a flow switch works does depend on the media type and the output measurement required. They usually consist of a primary mechanism, such as a paddle or magnetic trigger, which is connected to a sensor. This mechanism is placed in the media flow. The flow causes the switch to be displaced or the paddle to be rotated, sending a signal to a secondary component known as the transducer. The transducer takes the signals from the primary mechanism, converts it to a more usable signal and passes it on to a transmitter. The reading is then compared against a set of pre-configured values and triggers whatever signal or action is required to adjust the behaviour of components and mechanisms elsewhere. Flow switches are used to monitor, report and control the flow rate of media through either a specific part of a system or through the entire system, ensuring flow stays within those pre-set parameters. If the rate exceeds or drops below what is required, it can be configured to trigger an external alarm, power-on a pump, activate a solenoid valve to divert flow or even isolate parts of the system.

Where are flow switches used?

You will find a flow switch anywhere that the media flow in a system needs to be monitored and/or controlled. There are many industrial applications where flow control is important, some of the critical areas including oil and gas, pharmaceutical and water treatment. On a more domestic scale, air conditioning systems, central heating systems and pool pumps would include flow switches within their operating systems.

Why are flow switches so important?

Flow switches exist to provide damage protection or to adjust flow rates that are undesirable for the system. They are required for the safe and optimal operating of most liquid or gas systems. BEST pneumatics has a range of flow switches available in stock and can cater for all applications and working conditions. If you are looking for a flow switch and would like to talk to one of our experts about the right one for your application, call us now on 01254 390555 or email [email protected].
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