Facts about pressure


Brief facts about pressure

Learn about absolute pressure, differential pressure, positive and negative pressure and atmospheric pressure. Also includes tips for correct pressure measurement and conversion tables for the main pressure units.

Facts about pressure

Besides temperature, pressure is one of the most important physical measurement parameters. It plays a crucial role in many areas such as ventilation and cooling applications, but also power and energy technologies. Common uses are to calculate the air velocity by measuring the differential pressure with a pitot tube (prandtl tube). Other examples are operating rooms and clean rooms where some overpressure is needed for hygiene and safety reasons, to keep contaminants and bacteria etc. outside.

Physical principles.

Pressure is the force per unit area acting at each point inside a closed volume of gas or liquid and also against adjacent solid surfaces, e.g., the ground. the walls of a container.

The variable pressure (p) is defined as a force (F) that is uniformly applied to a given surface (A). This force can be applied by a liquid, by gases or by solid bodies.

The basic and SI unit of pressure is the pascal (Pa), named after the French physicist Blaise Pascal.

The pressure unit Pascal (Pa) is derived from the unit Newton (N). 1 pascal (Pa) corresponds to the pressure when 1 Newton (N) is exerted vertically on an area of 1m² (1 Pa = 1 N/m²).

Pressure pressure formula

Different pressure parameters.

All pressure measurements are relative and thus compared to a reference. A distinction is made between different types of pressure, depending on the reference used. If absolute vacuum is used as a reference, an absolute pressure is measured. If the reference is instead the current ambient pressure, it is a negative or positive pressure (lower or higher than the ambient pressure). The difference between two pressure values is called differential pressure (see diagram below).

Absolute pressure

  • Measured pressure above absolute vacuum
  • Reference: absolute vacuum
  • Measured pressure is always greater than reference pressure

Differential pressure

  • The difference between two pressure values
  • Reference: the two pressure values are measured separately in relation to the ambient pressure.
Pressurize pressure parameters with vacuum

Overpressure (positive pressure)

  • Measured pressure above ambient pressure
  • Reference: current ambient pressure
  • Measured pressure is always greater than reference pressure

Negative pressure (negative pressure)

  • Measured pressure below ambient pressure
  • Reference: current ambient pressure
  • Measured pressure is always less than the reference pressure

Atmospheric pressure

  • Atmosphere (atm) is a unit of pressure of 1013.25 hPa, which corresponds to normal atmospheric pressure at sea level.
  • 1013.25 hPa = 1013.25 mbar

Tips for correct pressure measurement.

Tip 1: Temperature impact

Choose a pressure measuring instrument with temperature compensation to get accurate results in long-term measurements. For instruments without temperature compensation, you should ensure that the instrument has acclimatized to the ambient temperature for up to 30 minutes.

Pressure Temperature effect

Tip 2: Angular impact

When measuring in the low range of 0.1 to 1000 pascals, it is very important that the instrument is held in the same position during measurement as during zeroing. Due to the effect of gravity on the membrane of the pressure sensor, measurement errors of up to almost 10 pascals can occur (depending on the type of instrument).

Pressure Angular impact

Tip 3: Correct connection

Always connect positive and negative pressure to the correct connection. Otherwise, false measurements or none at all may result. That is, the positive nipple for the higher pressure and the negative nipple for the lower.

Press the correct connection

Tip 4: Avoid overloading

Most pressure gauges indicate the maximum pressure to which the instrument can be subjected without damage. This can be 2 to 10 times the maximum measurement range. Check the technical data carefully.

Pressure avoid overloading

Conversion table.

Pressure conversion table

Convert online

There are several websites that offer free conversion of different pressure units. UnitConverter.net is one of the better ones.

Link to UnitConverter

Example of a pressure gauge from Nordtec.

Measurement facts testo 440 dP handheld instrument

Manual instrument

Testo has a wide range of portable hand instruments for measuring differential pressure, absolute pressure and air pressure. Most also include calculation of air velocity in pitot tube measurements.

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Measurement facts cooling applications testo 557s

Cooling applications

The most common measurement parameter in the adjustment of cooling and heat pump systems is pressure. Testo has a number of specially adapted instruments for this application.

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Measurement facts for transmitters testo 6621


Testo has several transmitters for continuous monitoring of differential pressure. All with high accuracy and without zero point drift thanks to automatic zeroing.

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