Pressure is defined as the force applied at the right angles to the surface of an object per unit area over which that force is distributed. When selecting devices for pressure measurement, it might not be immediately obvious which equipment should be selected for your application. In this article, we look at absolute, gauge and differential pressure measurement to highlight where they work best.
Absolute Pressure Measurement
Absolute pressure uses absolute zero, or full vacuum, as its zero point. Who doesn’t like talking about the weather so you’ll be pleased to know that the best example of an absolute referenced pressure is the measurement of barometric pressure. In order to produce an absolute pressure sensor the manufacturer will seal a high vacuum behind the sensing diaphragm. Therefore if you hold open the process pressure connection of an absolute pressure transmitter to the air it will read the actual barometric pressure. Other common applications are found in automotive, medical and building automation.
Gauge Pressure Measurement
In comparison to absolute pressure, gauge pressure (also known as relative pressure) uses atmospheric pressure as its zero point, meaning changes in atmospheric pressure doesn’t influence the net measurement. This allows gauge pressure to be used for level measurement applications as changes in atmospheric pressure won’t influence the overall pressure.
Differential Pressure Measurement
Differential pressure (dP) is a type of gauge pressure that is the difference between the two pressures. It helps to measure the pressure difference between two different points. Differential pressure is commonly used for flow and level measurement applications. It can also be used for level measurement on pressurised tanks, such as when Nitrogen blanketing is used to reduce the oygen content in the vapour space of a tank, as it subtracts the applied pressure at the top of the tank from the pressure being applied by the liquid column. dP is also ideal for monitoring blockades across filters and heat exchangers.
Metallic vs Ceramic Diaphragm Cells – Which one?
The ideal diaphragm material very much depends on the application. Metallic sensor can be used for high pressure and temperature applications. The metallic diaphragm isolates the sensor and deviates according to the pressure being applied. Behind the diaphragm is an oil filling and an integrated resistance bridge. The deviations cause the bridge to give a variable output voltage that is measured and processed. For corrosive applications, there are many different types of metal cell to select, including Stainless Steel and Hastelloy C (which are usually standard) to titanium, tantalum, monel and even gold.
Ceramic cells work on a capacitance principle and have no fluid filling so won’t drift with temperature changes. They are very abrasion resistant and the diaphragm is not easily damaged so are commonly found in applications requiring a robust solution including in the mining industry and also oil and gas.
At EMC we provide a full range of pressure measurement instrumentation, from cost effective basic functionality devices to those where a sophisticated solution is required.
We’ve been measuring New Zealand for fifty years! Contact us to get your quality pressure measurement equipment today.