A small number of conversion problems show up when tested against the NIST data.

**1. â€˜ofhgâ€™**

The value of 'ofhg' appears is off by 10 compared to the NIST values and other sources. Moreover, a more exact value can be derived from the density of mercury (in kg/m^3^) and the value of *gravity* in the constants we defined. Fixing that would result in the following change:

<convertUnit source='ofhg' baseUnit='kilogram-per-square-meter-square-second' factor='10132500000/760000'/>

=>**<convertUnit source='ofhg' baseUnit='kilogram-per-square-meter-square-second' factor='13595.1*gravity'/>**

[Note: sdiff = 2*(a-b)/(a+b)]

Errors:

inch-ofhg âŸ¹ kilopascal

expected 3.386389

actual: 0.3386388157894737

sdiff: 1.63636371856976

xdata: PRESSURE or STRESS (FORCE DIVIDED BY AREA)

Â«inch of mercury, conventional (inHg) 12 kilopascal (kPa) 3.386 389 E+00Â»

millimeter-ofhg âŸ¹ pascal

expected 133.3224

actual: 13.33223684210526

sdiff: 1.63636371466497

xdata: PRESSURE or STRESS (FORCE DIVIDED BY AREA)

Â«millimeter of mercury, conventional (mmHg) 12 pascal (Pa) 1.333 224 E+02Â»

**2. calorie, BTU, and therm**

There are many different definitions of calorie, BTU, and therm, which are interrelated units.

That is, they should have the following relationships:

Unit | Definition | |
---|---|---|

1 calorie | energy required to raise the temperature of one gram of water by 1 Â°C | |

1 BTU | energy required to raise the one pound of water by 1 Â°F | |

1 pound-fahrenheit | 2,267.96185/9 gram-celsius | (exact) |

1 therm | 100,000 British thermal units |

However, there are many ways to measure, depending on the starting temperature of the water, etc. See https://en.wikipedia.org/wiki/Calorie and other sources. So from a given value for calorie, we can precisely calculate the BTU and therm values. We chose the Thermochemical calorie, where 1 calorie (th) â‰¡ 4.184 joule.

We don't actually support the plain therm, just the therm-us, so that can be independent. But we should align the calorie and BTU. Secondly, in reviewing the situation I found that there was a mistake in transcribing the therm (US) conversion rate.

So the following are proposed:

<convertUnit source='british-thermal-unit' baseUnit='kilogram-square-meter-per-square-second' factor='1055.06' systems="ussystem uksystem"/>

âŸ¹**<convertUnit source='british-thermal-unit' baseUnit='kilogram-square-meter-per-square-second' factor='4.184*2,267.96185/9 systems="ussystem uksystem"/> <!-- th -->**

[we could also introduce 1 or more constants to make the derivation clearer]

<convertUnit source='therm-us' baseUnit='kilogram-square-meter-per-square-second' factor='105506000' systems="ussystem"/>

âŸ¹**<convertUnit source='therm-us' baseUnit='kilogram-square-meter-per-square-second' factor='105480400' systems="ussystem"/>**

Errors:

british-thermal-unit âŸ¹ joule

expected 1054.35

actual: 1055.06

sdiff: 6.731740154830024E-4

xdata: ENERGY (includes WORK)

Â«British thermal unitth (Btuth) 9 joule (J) 1.054 350 E+03Â»

therm-us âŸ¹ joule

expected 1.054804E8

actual: 1.05506E8

sdiff: 2.426696697038293E-4

xdata: ENERGY (includes WORK)

Â«therm (U.S.) 24 joule (J) 1.054 804 E+08Â»

Note that in the future we could introduce more unit variants if we find a need. We would just qualify the unit ID appropriately, eg calorie-mean, calorieit (for International Steam Table calorie), calorie-15C, etc.

**3. fahrenheit-hour-square-foot-per-british-thermal-unit-inch **

We also have the following error, which hasn't been analyzed yet, but is another off-by a factor of 10 bug. Might be a software bug.

fahrenheit-hour-square-foot-per-british-thermal-unit-inch âŸ¹ meter-kelvin-per-watt

expected 69381.12

actual: 6.938111789203321

sdiff: 1.999600040008151

xdata: Thermal Resistivity

Â«degree Fahrenheit hour square foot per British thermal unitth inch [Â°F Â· h Â· ft2/(Btuth Â· in)] meter kelvin per watt (m Â· K/W) 6.938 112 E+04Â»

Â

**Update on #3, looks like an error in the NIST document, so no action is needed**

The 3rd one was a puzzler for me. I gnawed away at it until I finally figured out that the NIST line was faulty. They have the following on https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9

degree Fahrenheit hour square foot per British thermal unitIT inch [Â°F Â· h Â· ft2/(BtuIT Â· in)] | meter kelvin per watt (m Â· K/W) | 933 472
| E+00 |

degree Fahrenheit hour square foot per British thermal unitth inch [Â°F Â· h Â· ft2/(Btuth Â· in)] | meter kelvin per watt (m Â· K/W) | 938 112
| E+04 |

Â

Note that the two values are off by a factor of E+04. That doesn't make sense, since the difference between British thermal unitIT and British thermal unitth is not very big.

As it turns out, https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8 (which is supposed to differ just by order!) has the correct values:

degree Fahrenheit hour square foot per British thermal unitIT inch [Â°F Â· h Â· ft2/(BtuIT Â· in)] | meter kelvin per watt (m Â· K/W) | 933 472
| E+00 |

degree Fahrenheit hour square foot per British thermal unitth inch [Â°F Â· h Â· ft2/(Btuth Â· in)] | meter kelvin per watt (m Â· K/W) | 938 112
| E+00 |

Â

None

None