Topic: 1 hPa Equals What In inHg and mmHg?

[Buford T. Justice]

I am trying to figure out what the exact value of 1 hPa is when converted to inHg and mmHg. There is a lot of disagreement to these values.

1 hPa = ? inHg:

1) 0.02953

2) 0.02952998016471232
http://www.conversion-website.com/pressure/hectopascal_to_inch_of_mercury.html

3) 0.0295299830714
http://www.convertunits.com/from/hpa/to/inhg

4) 0.0295299833
http://www.sensorsone.co.uk/pressure-units-conversion.html

5) 0.029530059
http://www.unitconversion.org/unit_converter/pressure-ex.html

1 hPa = ? mmHg:

1) 0.75

2) 0.75006
http://www.csgnetwork.com/meteorologyconvtbl.html

3) 0.750061561303
http://www.convertunits.com/from/hpa/to/mmhg

4) 0.75006157585
http://en.wikipedia.org/wiki/Talk:Torr

5) 0.750061576
http://www.sensorsone.co.uk/pressure-units-conversion.html

6) 0.7500615781804147
http://www.conversion-website.com/pressure/hectopascal_to_millimeter_of_mercury.html

7) 0.7500616827042
http://www.unit-conversion.info/pressure.html

8) 0.750062
http://www.sensorsone.co.uk/pressure-measurement-glossary/hpa-hectopascal-pressure-unit.html

9) 0.750063755
http://www.unitconversion.org/unit_converter/pressure-ex.html

10) 0.75006375541921
http://www.unitjuggler.com/convert-pressure-from-hPa-to-mmHg.html

Many of the above mmHg links assume mmHg is equal to Torr. THEY ARE NOT EQUAL:

1 mmHg = 1.00000015001 Torr
1 Torr = 0.9999998499900226 mmHg
http://www.convertunits.com/from/mm+Hg/to/torr

So what are the EXACT values of inHg and mmHg which equal 1 hPa that are the accepted scientific values?

[Randall Kayfes]

Did you try NOAA:

http://www.srh.noaa.gov/ama/?n=conversions

The site lists both table and converters

Millibars:1      
Inches:0.02952988424285377      
Millimeters:   0.7500590597684857   
 
This is the site I personally would settle on YMMV...

[92merc]

I use a converter program by Josh Madison.  It converts just about everything to everything else.

http://joshmadison.com/convert-for-windows/

It shows 1 pascal equal to:
.0002952999 inches of mercury
.007500617 mm of mercury

[Buford T. Justice]

@kaymann
Yes I did give that site a try, but looking at the conversion tables, it would appear NOAA converted inHg into hPa which puts it in error.  There are usually 2-3 hPa values rounded to the tenth for every inHg.

@92merc
I think most of the websites I linked to do the same thing with different results.
-----
I just find it odd that some internationally-respected scientific body has not defined the values.  1 hPa should have a precise value to inHg, mmHg, and Torr.  The main purpose of the OP was to point out the oddness of these values not existing and that most are happy with a close equivalent though that equivalent is probably in error.

According to http://www.convertunits.com/ :

1 hPa = 0.0295299830714 inHg
1 hPa = 0.750061561303 mmHg
1 hPa = 0.750061673821 Torr

Though it seems every aneroid barometer has 1000 hPa = 750 mmHg = 29.53 inHg so I guess I will have to join the "good enough for me crowd":

[Scalphunter]

Read this 


http://en.wikipedia.org/wiki/International_System_of_Units

[Beaudog]

Ahhhh I really hate to ask this. But can you explain to me any instance where a .00001   difference in pressure would make any significant difference in the weather.

[Randall Kayfes]

Ahhhh I really hate to ask this. But can you explain to me any instance where a .00001   difference in pressure would make any significant difference in the weather.

Probably none other than for a NASA technician, except that it is a very interesting discussion. BTJ checking the CRC Handbook of Chemistry and Physics reveals:

1 mb = 0.0295300 inches of Mercury at 32F
Apparently temperature makes a difference as there is a complete chart of of the volume of mercury at a given temperature.
If the each conversion calculator uses a different temperature you could end up with the minute differences we are seeing...

[Buford T. Justice]

It's mainly just a curiosity question as to why there is no "set-in-stone" number for what 1 hPa is in inHg or mmHg.  It seems like the numbers my Fine Offset display uses are 1 hPa = 0.02953 inHg = .75 mmHg.  I just found it odd the numbers used on different websites were different.

@Scalphunter
I went through that, but I am not seeing what you are pointing out there in regards to inHg and mmHg other than mmHg is not an SI measure but is accepted.

@beaudog
It probably makes no real difference, but I am curious to know why there is no scientifically accepted number for either (as it seems).

@kaymann
Now that is interesting.  I had heard different numbers are used for 0 °C and 60 °C (I may have read somewhere it was 0 °C and 100 °C but maybe not).

I have an old Airguide barometer that does inHg and cmHg.  It looks like 29.53 inHg is about 74.92 cmHg so 749.2 mmHg?

So 749.2 mmHg / .75 = 998.9333333333333 hPa

So 998.9333333333333 hPa * 0.02953 = 29.49850133333333 inHg

Let me try reversing that:

29.53 = 1000 hPa

So 1000 hPa * .75 = 750 mmHg

That looks like the picture I posted earlier.

This old Airguide barometer might being doing Torr values as the 75 cmHg mark looks like it lines up to around 29.55 inHg.

Here is a picture the cmHg barometer I have:



And here is the exact same thing that does hPa:

[Scalphunter]

Yep you found it. Not reconize by the peop;e who set the standards. Like mention above mercury is subject to  temperature. It does solidify at low temps like -40. Also it has an lot to do with the rounding up/down of the calculator / program one is using. Most people most likily use it to the 3th decimel place. Kinda like use Pi.
no one use this

3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679 
  8214808651328230664709384460955058223172535940812848111745028410270193852110555964462294895493038196 
  4428810975665933446128475648233786783165271201909145648566923460348610454326648213393607260249141273 
  724587006606315588174881520920962829254091715364367892590360011330530548820466521384146951941511609

they just use 3.1416

[Buford T. Justice]

I just thought of something with the barometer I have that does cmHg.  29.53 inches = 75 centimeters.  As you can see, the mark is not in the right place.  75 cmHg on the barometer is closer to ~29.555.  I'd like to get one of those Traceable Barometers by Thomas Scientific (actually Control Company).

[Buford T. Justice]

I agree Scalphunter.  I was thinking there are probably no "set-in-stone" numbers because it is impossible to do.  Mercury would not only be influenced by different temperatures, but also by different elevations and even gravity.

Am I correct to say Pascals, being a measure of pressure, don't suffer from these limitations?

[DaculaWeather]

I'll say this... at 1 hPa... it doesn't mean a hill of beans. At those levels, you look at heights instead.