pressure & air pressure

 
absolute pressure
A pressure being referred to the reference pressure "0" (vacuum) is named absolute pressure. In order to distinguish it from other types of pressure it is marked by the index abs (p_abs).

atmospheric air pressure
The pressure type being most essential for measuring the ambient conditions on earth is the atmospheric air pressure pamb. It is generated by the weight of the atmosphere surrounding the earth up to a height of approx. 500 km. Up to this height where the absolute pressure is p_abs = 0 the atmospheric air pressure is permanently being reduced. At sea level the average air pressure p_amb is 1,013.25 hPa. The air pressure related to sea level is the effective air pressure at the measuring site converted to sea level. It is converted by the summation of the pressure difference being generated by the weight of the air column between measuring site and sea level. By this it is reached that air pressures can be compared independently on sea level. At the so-called high and low pressure conditions the air pressure is fluctuating for about ±5 %. Assumed that there is the same temperature at various distances from the earth, the air pressure is decreasing at increasing height acc. to an exponential function. Here the so-called international height formula is valid :

by means of which the effective air pressure in a defined height h is determined.

relative pressure
The relative pressure is the difference to the current ambient pressure. When measuring the relative pressure a change of the air pressure which would cause a fault, when measuring the absolute pressure, is compensated. For this for long-termed measurements frequently a relative pressure measuring sensor is used.

differential pressure
The difference between two pressures p₁ and p₂ is called pressure difference p = p₁ - p₂. In cases when the difference between two pressures corresponds with the measuring size, we are talking about the pressure difference p₁₂

atmospheric pressure difference, over pressure
At technical applications the atmospheric pressure difference pe is measured most frequently. This is the pressure difference between pabs and p_amb.This pressure difference is called over pressure.The over pressure is positive if the absolute pressure is higher, it’s negative if it is lower than the atmospheric air pressure.

piezo-electric effect
At the surface of certain materials an electrical charge is collected in dependence on the influencing weight. This charge being proportional to the weight can be used for measuring pressure.

measuring principles and pressure sensors
When measuring pressure you are distinguishing between measuring instruments determining the measuring variable directly from one of the both basic relations:
  or  
and measuring instruments converting changes in length resp. electrical, optical and chemical effects into correspondent signals.

direct pressure measuring instruments
liquid pressure measuring instruments
The pressure p to be measured is compared with the height h of a liquid column. Acc. to the formula noted above the pressure is determined (ρ_m: density of medium to be measured, g: gravity acceleration)

pressure scales / piston pressure measuring instruments
Pressure scales or piston pressure measuring instruments are operating acc. to the basic definition of pressure. The pressure acts on a defined space A and effects a force F. This force is compared e.g. with the one of a spring or of a counter-weight. The way of the spring or the mass of the weight is a measure for the pressure then.

indirect pressure measuring instruments
mechanical pressure measuring instruments
The mechanical pressure measuring instruments used most frequently are the ones with spring elastic measuring elements (Bourdon tube). Here the pressure arrives into a defined pressure space of the measuring organ of which two or several walls are deformed proportionally to the pressure.

electronical pressure sensors
Numerous electrical pressure sensors are exisiting with very different measuring principles. We are introducing you in some important processes only:

Strain gauges, semi-conductor strain gauges (piezoresistive effect) etc.

By pressurization the length and by this the value of an electrical resistance acc. to the following formula is changed:

whereat ρ reprensents the specific resistance, l and Q length and cross section of the resistance.

This resistance change is evaluated by a special evaluation procedure, the so-called Wheatstone measuring bridge and converted into an output signal being dependent on pressure.

Further strain gauges, besides the semi-conductor technology named, are thick film and thin film strain gauges as well as film strain gauges.

Hall effect sensors
A hall effect sensor defines the change of a magnetic field in dependance on the deflection of a membrane.

capacitive sensors
When measuring pressure by means of capacitive sensors you are using the change of distance of both of the condensator plates dependent on pressure.

conversion of important pressure units:

	SI units			technical units
	bar	mbar	Pa	mmHg	kp/cm²	atm
1 bar	1	103	105	750.064	1.01972	0.986923
1 mbar	0.001	1	100	0.750062	0.00101972	0.986923 x 10-3
1 Pa	0.00001	0.01	1	0.00750062	10.1972 x 10-6	9.8693 x 10-6
1 mmHg	0.00133322	1.33322	133.322	1	0.00135951	0.00131579
1 kp/cm²	0.980665	980.665	98,066.5	735.561	1	0.967841
1 atm	1.01325	1,013.25	101,325	760	1.03323	1

further conversions:
hPa = 1 mbar
1 Pa= 1 N/m²
1 mmHg = 1 Torr
1 kp/cm² = 1 atü