The Math of hPa to inHG and mmHg

[Buford T. Justice]

Here are a few actual values as shown on my Fine Offset display:

1030.4 hPa
772.8 mmHg
30.43 inHg

1030.5 hPa
772.9 mmHg
30.43 inHg

1030.6 hPa
773.0 mmHg
30.43 inHg

1030.7 hPa
773.0 mmHg
30.44 inHg

1030.8 hPa
773.1 mmHg
30.44 inHg

1030.9 hPa
773.2 mmHg
30.44 inHg

1031.0 hPa
773.3 mmHg
30.45 inHg

I am trying to figure out what the exact math is that the Fine Offset displays use to convert hPa to inHg and mmHg.

The closest values I have come up with based on the above data are these:

inHg = hPa * 0.02953
mmHg = hPa * 0.75

I have seen websites stating the inHg value for 1 hPa is:

• 1) 0.02952998016471232
  http://www.conversion-website.com/press ... rcury.html
  
  2) 0.0295299830714
  http://www.convertunits.com/from/hpa/to/inhg
  
  3) 0.0295299833
  http://www.sensorsone.co.uk/pressure-un ... rsion.html
  
  4) 0.029530059
  http://www.unitconversion.org/unit_conv ... re-ex.html

I have seen websites stating the mmHg value for 1 hPa is:

• 1) 0.75006
  http://www.csgnetwork.com/meteorologyconvtbl.html
  
  2) 0.750061561303
  http://www.convertunits.com/from/hpa/to/mmhg
  
  3) 0.75006157585
  http://en.wikipedia.org/wiki/Talk:Torr
  
  4) 0.750061576
  http://www.sensorsone.co.uk/pressure-un ... rsion.html
  
  5) 0.7500615781804147
  http://www.conversion-website.com/press ... rcury.html
  
  6) 0.7500616827042
  http://www.unit-conversion.info/pressure.html
  
  7) 0.750062
  http://www.sensorsone.co.uk/pressure-me ... -unit.html
  
  8) 0.750063755
  http://www.unitconversion.org/unit_conv ... re-ex.html
  
  9) 0.75006375541921
  http://www.unitjuggler.com/convert-pres ... -mmHg.html

There seems to be A LOT of disagreement as seen above on what the value of 1 hPa is. 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

Along with trying to figure out the EXACT values the Fine Offset displays use, what are the EXACT values of inHg and mmHg which equal 1 hPa that are the accepted scientific values?

[steve]

Cumulus uses 0.02953, which agrees (to 4 sig figs) with the (reciprocal) conventional figure quoted in NIST Special Publication 811

inch of mercury, conventional (inHg) ............... pascal (Pa)............................................. 3.386 389 E+03

[AllyCat]

THEY ARE NOT EQUAL:

1 mmHg = 1.00000015001 Torr
......
Along with trying to figure out the EXACT values the Fine Offset displays use

Hi,

An "error" in the eighth digit is hardly significant. I calculate that it's approximately equivalent to a difference in the elevation of the barometer by 1mm. Have you determined the elevation of your station to an accuracy (not resolution) within +/- 1mm above mean sea level?

I don't know what conversion value FO use, but it's almost certain that they don't do maths to better than 16 bits resolution (probably less), which is more than 100 times worse than the conversion error that you are quoting.

Cheers, Alan.

[peterh]

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

If you consider these (loudly) to be "NOT EQUAL", pray tell, what margin of error do you deem acceptable?
And what are you doing with a Fine Offset weather station? Good Lord in heaven, the margins of error that you are quoting are beyond the most sophisticated equipment currently commercially available! And if they weren't, YOU WOULD BE (shouting intentional), because I doubt that you're going to get your station elevation accurate to a few inches.



Rather than just do a binary compare, think about what you are comparing. Apologies for my bluntness, but this is beyond "anal-retentive"... this is just ...
There is a word for this, but I hesitate to state it. It starts with an S, and rhymes with Cupid.

[Buford T. Justice]

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":

[peterh]

1. Oh, come on. OP, The Original Poster, is you. No reason to NOT phrase "The main purpose of the OP" as "My main purpose". This is not the OP suffering from OCD, this is you.
2. It is completely OK to be "happy with a close equivalent" if our realtime measurements cannot even begin to approach the Obsessive-Compulsive accuracy that you seem to be after.

For everyday purposes, that sort of accuracy has no value whatsoever. For everyday purposes, we're more interested in CHANGES in the approximate value than absolute numbers in the exact value in the first place. Meteorogically speaking, we couldn't care less if the current barometric pressure is 1013,598 or 1013,604 hPa. What is of interest to us is that it has dropped 2 hPa in the last hour. Whether this is 1.9 or 2.0 hPa is completely insignificant.

And I keep wondering why you purchase an FO device if you're interested in 8 digit accuracy. Are you trolling us? FO devices are not known to be capable to measure to that tolerance.

If you're not trying to troll us, insert coin in slot, put brain in gear, and THINK about this.

[Buford T. Justice]

If you consider these (loudly) to be "NOT EQUAL", pray tell, what margin of error do you deem acceptable?
And what are you doing with a Fine Offset weather station? Good Lord in heaven, the margins of error that you are quoting are beyond the most sophisticated equipment currently commercially available! And if they weren't, YOU WOULD BE (shouting intentional), because I doubt that you're going to get your station elevation accurate to a few inches.



Rather than just do a binary compare, think about what you are comparing. Apologies for my bluntness, but this is beyond "anal-retentive"... this is just ...
There is a word for this, but I hesitate to state it. It starts with an S, and rhymes with Cupid.

1. Oh, come on. OP, The Original Poster, is you. No reason to NOT phrase "The main purpose of the OP" as "My main purpose". This is not the OP being OCD, this is you.
2. It is completely OK to be "happy with a close equivalent" if our realtime measurements cannot even begin to approach the OCD accuracy that you seem to be after.

For everyday purposes, that sort of accuracy has no value whatsoever. For everyday purposes, we're more interested in CHANGES in the approximate value than absolute numbers in the exact value in the first place. Meteorogically speaking, we couldn't care less if the current barometric pressure is 1013,598 or 1013,604 hPa. What is of interest to us is that it has dropped 2 hPa in the last hour. Whether this is 1.9 or 2.0 hPa is completely insignificant.

And I keep wondering why you purchase an FO device if you're interested in 8 digit accuracy. FO devices are not known to be capable to measure to that tolerance.

Talk about "anal-retentive" LOL!

OP = Original Post

I am not interested in 8-digit accuracy. I am interested in why no definitive number seems to exist.

But anyway, the values of 29.53 inHg and 750 mmHg for 1000 hPa work fine for me and the math of using those values always seem to agree with what is being displayed on the FO display.

I do admit a certain desire for the most absolute, attainable accuracy as did one of my heroes; John Harrison.

Now if you seek a definition of troll, I suggest you look at what you wrote, then hobble over and look into the closest mirror, then hobble back to the bridge you reside under.

[peterh]

I am not interested in 8-digit accuracy. I am interested in why no definitive number seems to exist.

Aha! That is a different question altogether. While I wouldn't loose any sleep over it, it is definitely odd.
It may be that the science of meteorology has never bothered to give an answer to this is due to the fact that, for meteorological purposes, the fact that mmHg would not be precisely equal to Torr is completely irrelevant.

And where it gets really hazy for me is your question what calculation the FO device would use. That would redefine the word 'irrelevant', because its measurement is going to be more off than the margins between the various calculations.

Precision measurement is a good thing. I share your desire if it is about my mechanical watches, or even my audio... I *love* certified chronometers. But in meteorology, we appear to be a lot more interested in the changes than in the absolute values themselves. And, as someone else already noted, your margin of error in determining your station elevation is going to cause your device to be more off than the margins between the calculations!

Now if you seek a definition of troll, I suggest you look at what you wrote, then hobble over and look into the closest mirror, then hobble back to the bridge you reside under.

That's not the definition of a troll. That, if your assertion would be true, would be an example of a troll.
More importantly, there is an important difference between wondering if someone's just deliberately, cleverly, and secretly trying to piss people off, usually via the internet, using dialogue (which is a definition for trolling) and insulting someone personally.

I think it would be a wise decision if I would retract from this particular discourse.