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Pressure units cross reference chart


by Frank L. Preuss


Force (weight) per unit area or pressure or stress.

1 Pa = 1 N/m2       1 kPa = 1 kN/m2       1 MPa = 1 MN/m2       1 kgf = 9.806 65 N      


Pressure units cross reference chart
UNIT barmbarkbar PakPaMPa psiatm.ft.Hd. H2O at 20o C in H2Okg/cm2t/m2 Metres H2O Kilo- metres H2O in.Hg. at 20o C mm.Hg. cm.Hg.
1 bar 110000.001 1051000.1 14.5030.987033.514 402.1641.020010 10.21100.01021129.625 752.47075.247
1 mbar 0.001110-6 1000,110-4
1 kbar 10001061 108105100
1 Pa 10-50.0110-8 10.00110-6
1 kPa 0.011010-5 100010.001
1 MPa 101040.01 10610001
1 psi 0.069068.947 6.895 10.06802.310 27.7200.07000.7 0.7040.0007042.043 51.8845.188
1 atm. 1.01301013.0 101.325 14.569133.659 407.5131.033010.3 10.3510.01035130.019 762.48076.248
1 ft.Hd. H2O at 20o C 0.030029.837 2.984 0.4330.02901 12.0000.03000.3 0.3050.0003050.884 22.4522.245
1 in H2O 0.00252.486 0.249 0.0360.00250.833 10.00250.025 0.0250.0000250.074 1.8710.187
1 kg/cm2 0.9810980.662 98.066 14.2330.968032.867 394.408110.0 10.0180.01001829.054 737.95973.796
1 t/m2 1
1 Metre H2O 0.098098.066 9.807 1.4220.09703.287 39.3790.09901.0 10.0010002.905 73.7967.379
1 Kilo- metre H2O 1
1 in.Hg. at 20oC 0.034033.753 3.375 0.4890.03301.131 13.5750.03400.340 0.3400.0003401 25.4002.540
1 mm.Hg. 0.00101.329 0.133 0.0190.00130.045 0.5340.00140.014 0.01360.0000140.039 10.100
1 cm.Hg. 0.013313.290 1.328 0.1930.01310.445 5.3400.01400.140 0.1360.0001360.393 10.0001
UNIT barmbarkbar PakPaMPa psiatm.ft.Hd. H2O at 20o C in H2Okg/cm2t/m2 Metres H2O Kilo- metres H2O in.Hg. at 20o C mm.Hg. cm.Hg.
Examples of stress in loaded materials:
ground 0.25 35 2.525 250.025
timber 8.5 1200 85850 8500.850
brick- work 1.2 170 12120 1200.120
con- crete 8 1120 80800 8000.800
steel 140 20000 140014000 1400014




1. Comment

One of the reasons why I worked on this above chart, was to get a feeling, how, for example, stresses in different situations relate to each other. When there is, for example, a vertical steel bar in a suspension bridge between the arch of the bridge and the deck of the bridge carrying the load from the bridge deck up to arch, then the stress in this bar might be 1400 kg/cm2, which is equal to 14 kilometres H2O. When I now compare this stress to that stress, that water imposes on the ground of the ocean at a very deep location, let us say at the depth of 10 km, then that stress would be 10 kilometres H2O. And that would be approxiamately in the same order of magnitude as that stress in that steel bar.


2. Comment

When I now use the previous example again, but place the bridge deck above the arch, then the steel bar would be more like a steel column, carrying the load from the deck down to the arch. In the first example the steel bar would be in tension, and in the second example the steel column would be in compression. But steel can take about as much tension as it can take compression. With timber there is about the same situation, assumed the force is in the direction of the grain. With other materials there can be a great difference between the capacity to take compression and the capacity to take tension. Brickwork, for example, is normally not used to take tension. But both brickwork and concrete must have some tension strength in order to prevent the material from disintegrating under load; this can be seen, when one imagines just having sand. A brick made of sand only, without any binding material between the individual particles, would be useless, because sand has high compression strength, but no tension strength.

The ability of water to bear pressure is very high, the ability to bear tension very low, one can see this, when a tap is leaking; as soon as the droplet becomes a drop, separation takes place, because the tension ability of the water is very low. But this discussion shows, that the word "stress" is the more appropriate word to be used, in connection with our above chart, because "pressure" refers only to the one side of the values given there. So the word "stress" is more suitable here, because it covers not only pressure, but also tension. But I have used some of the values from a chart used by people working with control instruments, and there it is more about pressure, and not so much about tension and that chart used the word "pressure".


3. Comment

The upper part of the above chart has the same units and also in the same sequence of units from left to right as from the top down to the bottom. This results that the values of the diagonal from the top left to the bottom right are always "1". And also that values opposite this diagonal are always the reciprocal values. So, for example, 1 mbar = 0.001 bar and 1 bar = 1000 mbar. Or 1 psi = 0.0690 bar and 1 bar = 14.503 psi (pound per square inch).


4. Comment

When I have a glass of water and then put a straw into it and suck the water out, then actually the atmospheric pressure, atm., to which the surface of the water is exposed, and which presses the water down, due to its weight - the weight of the air - pushes the water up inside the straw, because I have reduced this pressure inside the straw, by me reducing this pressure inside my mouth.

When I have a hole in the ground and I want to pump the water down in the hole out to the surface, then I can put a pump on the ground, next to the hole, and use a hose from the pump down to the water, to pump the water to the surface. This hosepipe must be rigid, so that it does not collape, when suction is applied to it. Also then the atmosheric pressure is actually pushing the water up the hosepipe, because inside the pump the atmosheric pressure is reduced, and the pressure difference causes the water to be pressed up the hosepipe. And this pressure is 1 atm. and that corresponds to 10 m water, 10 Metres H2O, as it is shown in the above cross reference chart. And this means, that this pumping of water will only work, whenn the hole is less than 10 m deep. If the hole is, for example, 14 m deep, then the water will only rise maximal 10 m, and not more.

On the moon this sucking would not work, because the moon has no atmosphere. For the man in the moon this sucking would only work, when he does it inside his spacesuit or his spacecraft, which has artificial atmospheric pressure.

When I drink with a straw, then the drink is pushed into my mouth by the atmospheric pressure, not by any kind of pulling, and this is confirmed by the fact, that air has no tension strength. So nothing is pulling the drink up into my mouth.

And if air would have tension strength, then the water would not follow, because it also has no tension strength.


5. Comment

Atmospheric excess pressure is used, when only that pressure is of interest, that is above the atmospheric pressure, for example the pressure in a tyre. The German word for "atmospheric excess pressure" is "Atmosphärenüberdruck" or "atü". So 1 atü would be 2 atm.

1 atü would be the pressure 10 m below the surface of a body of water: 1 atm. from the air plus 1 atm. from the 10 m water.


Explanations for some of the above mentioned abbreviations:

atmospheric pressure (Phys.). The pressure exerted by the atmosphere at the surface of the earth is due to the weight of the air. Its standard value is 1.01325 x 105 N/m2, or 14.7 lbf/in2. Variations in the atmospheric pressure are measured by means of the barometer.

bar (Meteor.,Phys.). Unit of pressure or stress, 1 bar = 105 N/m2 or pascals = 750.07 mm of mercury at 0oC and lat. 45o. The millibar (1 mbar = 100 N/m2 or 103 dyn/cm2) is used for barometric purposes. (N.B. Std. atmos. pressure = 1.01325 bar.) The hectobar (1 hbar = 107 N/m2, approx. 0.6475 tonf/in2) is used for some engineering purposes.

H2O - Hydrogen oxide - water.

Hg (Chem.). The symbol for mercury.

lb (Genrl.). Abbrev. for pound.

newton (Elec.Eng.). Symbol N. The unit of force in the SI system, being the force required to impart, to a mass of 1 kg, an acceleration of 1 m/sec2. 1 newton = 0.2248 pounds force.

pascal (Genrl.). The SI derived unit of pressure or stress, equals 1 newton per square metre. abbrev. Pa.

pound (Genrl.). The unit of mass in the old UK system of units established by the Weights and Measures Act, 1856, and until 1963 defined as the mass of the Imperial Standard Pound, a platinum cylinder kept at the Board of Trade. In 1963, it was redefined as 0.453 592 37 kg. The US pound is defined as 0.453 592 427 7kg.


Force (weight) per unit area or pressure or stress

1 pdl/ft2       =1.488 16 N/m2
1 lbf/ft2       =47.8803 N/m2
1 mm Hg       =133.322 N/m2
1 in. H2O       =249.089 N/m2
1 ft H2O       =2989.07 N/m2       =0.029 890 7 bar
1 in. Hg       =3386.39 N/m2       =0.033 863 9 bar
1 lbf/in.2       =6.894 76 kN/m2       =0.068 947 6 bar
1 bar       =105 N/m2
1 std. atmos.       =101.325 kN/m2       =1.013 25 bar
1 ton/ft2       =107.252 kN/m2
1 tonf/in.2       =15.4443 MN/m2       =1.544 43 hectobar


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