The “inch of mercury” (inHg) is a unit of pressure commonly used in aviation, meteorology, and refrigeration. It originates from the barometric pressure measurement practice using a column of mercury. This unit became a staple due to its early adoption in weather forecasting and its accurate and consistent measurements. An interesting fact is that an inch of mercury is approximately equal to 33.86 millibars or 3386 Pascals, but what truly distinguishes it is its historical significance in the development of modern atmospheric science.
In the 17th century, Evangelista Torricelli, a student of Galileo, first discovered that atmospheric pressure could sustain a column of mercury, laying the groundwork for this unit. His invention, the barometer, revolutionized how we understand and predict weather, marking the inch of mercury as more than just a unit—it’s a nod to the evolution of scientific inquiry and practice.
Pounds per square inch (psi) is a crucial unit of pressure measurement, widely used across various industries, including automotive, aviation, and even space exploration. One interesting fact is that a typical car tire is inflated to around 30 psi, but did you know that a commercial jetliner’s cabin is pressurized to a reduced level of 11 psi to simulate being at a comfortable altitude of about 6,000-8,000 feet? This helps passengers avoid the stress of low oxygen levels while flying at over 35,000 feet, where the atmospheric pressure is a meager 4 psi.
Moreover, psi is pivotal in assessing the force exerted on surfaces. If you’ve ever wondered why deep-sea submarines have thick hulls, it’s because they must withstand pressures of around 8,800 psi at depths of 6 miles beneath the ocean’s surface! This ensures safe exploration of the mysterious deep-sea world, showcasing psi’s significance in enabling human feats in extreme environments.
Inches of mercury | Pounds per square inch |
---|---|
0
inHg
|
0
psi
|
1
inHg
|
0.491154
psi
|
2
inHg
|
0.982308
psi
|
3
inHg
|
1.47346
psi
|
4
inHg
|
1.96462
psi
|
5
inHg
|
2.45577
psi
|
6
inHg
|
2.94692
psi
|
7
inHg
|
3.43808
psi
|
8
inHg
|
3.92923
psi
|
9
inHg
|
4.42039
psi
|
10
inHg
|
4.91154
psi
|
11
inHg
|
5.40269
psi
|
12
inHg
|
5.89385
psi
|
13
inHg
|
6.385
psi
|
14
inHg
|
6.87616
psi
|
15
inHg
|
7.36731
psi
|
16
inHg
|
7.85846
psi
|
17
inHg
|
8.34962
psi
|
18
inHg
|
8.84077
psi
|
19
inHg
|
9.33193
psi
|
20
inHg
|
9.82308
psi
|
21
inHg
|
10.31423
psi
|
22
inHg
|
10.80539
psi
|
23
inHg
|
11.29654
psi
|
24
inHg
|
11.7877
psi
|
25
inHg
|
12.27885
psi
|
26
inHg
|
12.77
psi
|
27
inHg
|
13.26116
psi
|
28
inHg
|
13.75231
psi
|
29
inHg
|
14.24347
psi
|
30
inHg
|
14.73462
psi
|
31
inHg
|
15.22577
psi
|
32
inHg
|
15.71693
psi
|
33
inHg
|
16.20808
psi
|
34
inHg
|
16.69924
psi
|
35
inHg
|
17.19039
psi
|
36
inHg
|
17.68154
psi
|
37
inHg
|
18.1727
psi
|
38
inHg
|
18.66385
psi
|
39
inHg
|
19.15501
psi
|
40
inHg
|
19.64616
psi
|
41
inHg
|
20.13731
psi
|
42
inHg
|
20.62847
psi
|
43
inHg
|
21.11962
psi
|
44
inHg
|
21.61078
psi
|
45
inHg
|
22.10193
psi
|
46
inHg
|
22.59308
psi
|
47
inHg
|
23.08424
psi
|
48
inHg
|
23.57539
psi
|
49
inHg
|
24.06655
psi
|
50
inHg
|
24.5577
psi
|
51
inHg
|
25.04885
psi
|
52
inHg
|
25.54001
psi
|
53
inHg
|
26.03116
psi
|
54
inHg
|
26.52232
psi
|
55
inHg
|
27.01347
psi
|
56
inHg
|
27.50462
psi
|
57
inHg
|
27.99578
psi
|
58
inHg
|
28.48693
psi
|
59
inHg
|
28.97809
psi
|
60
inHg
|
29.46924
psi
|
61
inHg
|
29.96039
psi
|
62
inHg
|
30.45155
psi
|
63
inHg
|
30.9427
psi
|
64
inHg
|
31.43386
psi
|
65
inHg
|
31.92501
psi
|
66
inHg
|
32.41616
psi
|
67
inHg
|
32.90732
psi
|
68
inHg
|
33.39847
psi
|
69
inHg
|
33.88963
psi
|
70
inHg
|
34.38078
psi
|
71
inHg
|
34.87193
psi
|
72
inHg
|
35.36309
psi
|
73
inHg
|
35.85424
psi
|
74
inHg
|
36.3454
psi
|
75
inHg
|
36.83655
psi
|
76
inHg
|
37.3277
psi
|
77
inHg
|
37.81886
psi
|
78
inHg
|
38.31001
psi
|
79
inHg
|
38.80117
psi
|
80
inHg
|
39.29232
psi
|
81
inHg
|
39.78347
psi
|
82
inHg
|
40.27463
psi
|
83
inHg
|
40.76578
psi
|
84
inHg
|
41.25694
psi
|
85
inHg
|
41.74809
psi
|
86
inHg
|
42.23924
psi
|
87
inHg
|
42.7304
psi
|
88
inHg
|
43.22155
psi
|
89
inHg
|
43.71271
psi
|
90
inHg
|
44.20386
psi
|
91
inHg
|
44.69501
psi
|
92
inHg
|
45.18617
psi
|
93
inHg
|
45.67732
psi
|
94
inHg
|
46.16848
psi
|
95
inHg
|
46.65963
psi
|
96
inHg
|
47.15078
psi
|
97
inHg
|
47.64194
psi
|
98
inHg
|
48.13309
psi
|
99
inHg
|
48.62425
psi
|
100
inHg
|
49.1154
psi
|
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