Pressure Measurement Glossary-Pressure Unit

Bar - is a widely used metric unit of measure for pressure and 1 bar converts precisely to 100,000 Pascals.

Kilopascal (kPa) - is a 1000x multiple of the Pascal unit which is the SI unit for pressure. 1 Kilopascal equals 1,000 Pascals.

Megapascal (MPa) - is a 1000000x multiple of the Pascal unit which is the SI unit for pressure. 1 Megapascal equals 1,000,000 Pascals.

Millibar (mbar, mb or mbr) - is a metric unit of pressure mainly used in European countries and is derived directly from the Bar pressure unit which equals 1,000 mbar. In SI units 1 mbar equals 100 Pascals.

Pascal (Pa) - is the SI unit for pressure and is derived from other SI units using the following relationships: Pa = (kg.m/s²)/m² = kg/m.s² = N/m². Since 1 pascal is a very low pressure being 1/100 of a millibar it's use is limited to ultra low pressure applications such as measuring the pressure differences in ventilation systems.  

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Pressure Measurement Glossary-Pressure Type

Absolute Pressure - is one which is measured relative to a perfect vacuum. 

Barometric Pressure or Atmospheric Pressure - is the total outside air pressure measured with reference to absolute vacuum. The pressure varies depending on geographical location, altitude and local weather conditions. For weather reporting purposes the barometric pressure is normally adjusted to a sea level value so that all locations can be compared independent of the altitude at each location

Differential Pressure (DP) - is the difference in pressure between two separate points. 

Gauge Reference Pressure or G - is a pressure measured relative to atmospheric or barometric pressure. 

Hydrostatic Pressure - is what is exerted by a liquid when it is at rest. The height of a liquid column of uniform density is directly proportional to the hydrostatic pressure.

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Pressure Measurement Glossary-Accuracy and Precision

Does Precision mean the same as Accuracy for Pressure Measurement Devices?

In the context of a pressure measurement device, precision refers to how well it provides repeatable and linear results with increasing or decreasing pressure over many cycles. It is therefore an indication of the quality of measurement technology utilised and is a major factor effecting the cost of an instrument.

Accuracy is very closely connected with precision, but unlike precision it is constrained by a series of fixed reference points or absolute values that all readings are compared with to define the exactness of the instrument. Typically these comparison absolute values are read from a more accurate instrument ideally with a 10 to 1 ratio difference in accuracy to ensure it does not contribute significantly to the overall error calculation.  However in practise it is not always possible to achieve such a high ratio difference in accuracy. When this is the case the overall accuracy should also include the reference instrument uncertainty which better reflects the true accuracy of the device.

A sensors measurement error is normally defined as precision because tight electrical tolerances are difficult to achieve with an analog output signal. Once the zero and full scale points have been ascertained, device scaled via a digital panel meter or other analog to digital converter instrument, the error can then be expressed as accuracy rather than precision. 

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Why Pressure Sensors are designed with stainless stell

There are many types of materials used for constructing pressure sensors in a variety of applications, but those made from stainless steel are considered the most reliable and durable.

High Tensile strength
In hydraulic applications where pressure overloads are commonplace a stainless steel pressure sensor can withstand a high number of pressure cycles without fatigue damage.

Durability and Hardness
In demanding environments a high reliability and durability is required to withstand the extremes of shock, vibration and temperature. An all stainless steel design is ideal for these applications due to its high toughness.

Corrosion Resistance
For the majority of fluids and gases, stainless steel provides excellent protection against short and long term corrosion due to a naturally occurring resistant surface layer formed by a self passivisation process.

Welded Seals
Over many pressure and temperature cycles an all welded mechanical assembly provides a high integrity pressure seal for very high pressure and vacuum measurement applications.

Cleanliness and High Purity
For food, beverage and pharmaceutical industries, stainless steel is the preferred material because of its high polished surface which makes it easy to clean and discourages bacterial growth during use. The other benefit is that all the mechanical piece parts exposed to the media can be made from one material and welded together so that there are no crevices to attract contaminants or process media residues.

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