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.
A “meter of water @ 4°C” is a unique unit used primarily to measure pressure. At this specific temperature, water reaches its maximum density, and the pressure exerted by a one-meter column of water is approximately 9.81 kilopascals or 0.0979 atmospheres. This scenario represents the optimal conditions for standardizing pressure measurements because the physical properties of water are most stable, reducing variances in calculations.
This unit is especially important in fields like plumbing and hydraulics, where accurate pressure measurements are crucial. Understanding these principles can lead to better designs in water distribution systems, preventing common issues such as pipe bursts or inefficient water flow, ensuring systems operate safely and efficiently.
Inches of mercury | Meters of water @ 4°C |
---|---|
0
inHg
|
0
mH₂O
|
1
inHg
|
0.345316
mH₂O
|
2
inHg
|
0.690631
mH₂O
|
3
inHg
|
1.03595
mH₂O
|
4
inHg
|
1.38126
mH₂O
|
5
inHg
|
1.72658
mH₂O
|
6
inHg
|
2.07189
mH₂O
|
7
inHg
|
2.41721
mH₂O
|
8
inHg
|
2.76252
mH₂O
|
9
inHg
|
3.10784
mH₂O
|
10
inHg
|
3.45316
mH₂O
|
11
inHg
|
3.79847
mH₂O
|
12
inHg
|
4.14379
mH₂O
|
13
inHg
|
4.4891
mH₂O
|
14
inHg
|
4.83442
mH₂O
|
15
inHg
|
5.17973
mH₂O
|
16
inHg
|
5.52505
mH₂O
|
17
inHg
|
5.87036
mH₂O
|
18
inHg
|
6.21568
mH₂O
|
19
inHg
|
6.561
mH₂O
|
20
inHg
|
6.90631
mH₂O
|
21
inHg
|
7.25163
mH₂O
|
22
inHg
|
7.59694
mH₂O
|
23
inHg
|
7.94226
mH₂O
|
24
inHg
|
8.28757
mH₂O
|
25
inHg
|
8.63289
mH₂O
|
26
inHg
|
8.9782
mH₂O
|
27
inHg
|
9.32352
mH₂O
|
28
inHg
|
9.66884
mH₂O
|
29
inHg
|
10.01415
mH₂O
|
30
inHg
|
10.35947
mH₂O
|
31
inHg
|
10.70478
mH₂O
|
32
inHg
|
11.0501
mH₂O
|
33
inHg
|
11.39541
mH₂O
|
34
inHg
|
11.74073
mH₂O
|
35
inHg
|
12.08605
mH₂O
|
36
inHg
|
12.43136
mH₂O
|
37
inHg
|
12.77668
mH₂O
|
38
inHg
|
13.12199
mH₂O
|
39
inHg
|
13.46731
mH₂O
|
40
inHg
|
13.81262
mH₂O
|
41
inHg
|
14.15794
mH₂O
|
42
inHg
|
14.50325
mH₂O
|
43
inHg
|
14.84857
mH₂O
|
44
inHg
|
15.19389
mH₂O
|
45
inHg
|
15.5392
mH₂O
|
46
inHg
|
15.88452
mH₂O
|
47
inHg
|
16.22983
mH₂O
|
48
inHg
|
16.57515
mH₂O
|
49
inHg
|
16.92046
mH₂O
|
50
inHg
|
17.26578
mH₂O
|
51
inHg
|
17.61109
mH₂O
|
52
inHg
|
17.95641
mH₂O
|
53
inHg
|
18.30173
mH₂O
|
54
inHg
|
18.64704
mH₂O
|
55
inHg
|
18.99236
mH₂O
|
56
inHg
|
19.33767
mH₂O
|
57
inHg
|
19.68299
mH₂O
|
58
inHg
|
20.0283
mH₂O
|
59
inHg
|
20.37362
mH₂O
|
60
inHg
|
20.71893
mH₂O
|
61
inHg
|
21.06425
mH₂O
|
62
inHg
|
21.40957
mH₂O
|
63
inHg
|
21.75488
mH₂O
|
64
inHg
|
22.1002
mH₂O
|
65
inHg
|
22.44551
mH₂O
|
66
inHg
|
22.79083
mH₂O
|
67
inHg
|
23.13614
mH₂O
|
68
inHg
|
23.48146
mH₂O
|
69
inHg
|
23.82677
mH₂O
|
70
inHg
|
24.17209
mH₂O
|
71
inHg
|
24.51741
mH₂O
|
72
inHg
|
24.86272
mH₂O
|
73
inHg
|
25.20804
mH₂O
|
74
inHg
|
25.55335
mH₂O
|
75
inHg
|
25.89867
mH₂O
|
76
inHg
|
26.24398
mH₂O
|
77
inHg
|
26.5893
mH₂O
|
78
inHg
|
26.93461
mH₂O
|
79
inHg
|
27.27993
mH₂O
|
80
inHg
|
27.62525
mH₂O
|
81
inHg
|
27.97056
mH₂O
|
82
inHg
|
28.31588
mH₂O
|
83
inHg
|
28.66119
mH₂O
|
84
inHg
|
29.00651
mH₂O
|
85
inHg
|
29.35182
mH₂O
|
86
inHg
|
29.69714
mH₂O
|
87
inHg
|
30.04245
mH₂O
|
88
inHg
|
30.38777
mH₂O
|
89
inHg
|
30.73309
mH₂O
|
90
inHg
|
31.0784
mH₂O
|
91
inHg
|
31.42372
mH₂O
|
92
inHg
|
31.76903
mH₂O
|
93
inHg
|
32.11435
mH₂O
|
94
inHg
|
32.45966
mH₂O
|
95
inHg
|
32.80498
mH₂O
|
96
inHg
|
33.15029
mH₂O
|
97
inHg
|
33.49561
mH₂O
|
98
inHg
|
33.84093
mH₂O
|
99
inHg
|
34.18624
mH₂O
|
100
inHg
|
34.53156
mH₂O
|
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