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Pressure sensors: 3 industrial applications enabling the smart factory

Real-time sensor data is enabling factories to better understand their own processes and keep them running. And putting that data into the IIoT also helps optimise how raw materials are ordered, handled and consumed. Knowing what to reorder and when can keep continuous processes at high capacity.

Smart manufacturing is changing the way we make, package and distribute just about everything, but Industry 4.0 is revolutionising the way our factories operate. While the Industrial IoT is dependent on connectivity, fundamentally it’s harnessing raw data and turning that into operational intelligence, which means sensors are key to the entire process. While there are many types of sensor at play here, the pressure sensor is probably the most diverse and widely deployed type of sensor in the IIoT.

1. Monitoring process flows

Differential pressure sensors are used extensively in process flows where a fluid needs to pass through some form of barrier, such as a filter. Under normal conditions the pressure difference between the upstream (often called the line or influent pressure) and the downstream (effluent) pressure should be nil or minimal. As the filter becomes blocked with contaminants, the downstream pressure will decrease, which causes the difference measured to increase.

The sensor’s output can be calibrated to show the maximum permissible pressure difference at full scale. For example, a 4-20mA output could be calibrated to show 20mA when the pressure difference reaches the maximum permissible, but read 4mA when the pressure difference is nil.

2. Measuring safe levels in liquid tanks

Submersible pressure sensors that are certified for use in intrinsically safe areas can be used to measure liquid pressures of up to 30 PSI with either a voltage or current (4-20mA) output. Positioning a submersible pressure sensor at the bottom of a tank would provide an accurate reading of the contents of the tank, thereby alerting workers or the process control system when the level in the tank falls below an allowable lower limit.

The pressure at the bottom of a tank – normally called the hydrostatic or head pressure – is measured in units of distance (feet, inches, meters) of water columns. For example, 27.670 inches of water column (WC) is around the same as 1 PSI at 100°F.

The pressure measured is only dependent on the height of the tank (as opposed to its shape) or the volume of the liquid. For this reason it is important that the sensor is placed at the bottom of the tank (instead of half way down).

Process controllers can calculate the level of liquid in a tank by measuring the hydrostatic pressure, which is more accurately measured when the density of the liquid is also known.

3.Managing control loops

As well as being used to monitor processes, pressure sensors are often instrumental in the control loop. This is particularly relevant in the use of hydraulics, where pressurised fluids are used to apply effort in presses or lifts for example.

The sensors are often small, particularly those based on MEMS technology. They can measure less than 2mm on each side yet be capable of measuring absolute pressures in the region of 20 Bar or more. This makes them suitable in a range of applications, including medical and automotive.

Even smarter sensors
Smart factories are now employing smarter sensors, such as pressure sensors with built-in Bluetooth connectivity, allowing them to be monitored wirelessly. These sub-systems feature a gauge pressure sensor, Analog to Digital Converter and Bluetooth radio in a single sealed unit that can be mounted in places where adding wires may be difficult. As they are battery-powered they can operate autonomously for as much as two years without any maintenance, providing accurate pressure sensor readings for gases, liquids and even mildly corrosive fluids.

Pressure sensors are fundamental in the smart factory and in enabling Industry 4.0.