Real Quantity with Units Module

[hpohekar]

Developed Real Quantity with Units Module.

Checked all test cases from C++ side units library and working fine.

[jorgepiloto]

Pinging here @RobPasMue. He's feedback may be useful for this, as he implemented a units module on top of pint for an internal project.

[hpohekar]

Pinging here @RobPasMue. He's feedback may be useful for this, as he implemented a units module on top of pint for an internal project.

Thank you, @jorgepiloto

[hpohekar]

Developed Real Quantity with Units Module.

Checked all test cases from C++ side units library and working fine.

Only individual temperature conversion test case is remaining and will handle this after discussion.

Following illustration may help to understand the terminologies used in code.

[h-krishnan]

>>> q
Quantity (5.0, "m")
>>> -q  # should return Quantity(-5.0, "m")
-5.0
>>>

[hpohekar]

Some issues:

>>> a = Quantity (3.0, "m")
>>> c = Quantity(4.0, "")
>>> a + "a" # wrong error message
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "quantity.py", line 588, in __add__
    raise ValueError("Incompatible dimensions.")
ValueError: Incompatible dimensions.
>>> c + 2  # adding float to dimensionless number should be supported
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "quantity.py", line 589, in __add__
    return Quantity(temp_value, self._si_unit)
  File "quantity.py", line 483, in __init__
    self._dimension = Dimension(unit_str)
  File "quantity.py", line 322, in __init__
    self._parser(unit_str)
  File "quantity.py", line 353, in _parser
    unit_dim = self._get_dim(unit_str, term_power)
  File "quantity.py", line 370, in _get_dim
    raise ValueError("Not implemented")
ValueError: Not implemented
>>>

[hpohekar]

>>> q
Quantity (5.0, "m")
>>> -q  # should return Quantity(-5.0, "m")
-5.0
>>>

Done. Thank you.

[h-krishnan]

What is the float or si_value of Quantity(1, 'C')?

[hpohekar]

What is the float or si_value of Quantity(1, 'C')?

@h-krishnan @seanpearsonuk @dnwillia-work @mkundu1

---------------------------------------------

The degree Celsius is itself used in SI units alongside the Kelvin -

https://en.wikipedia.org/wiki/Celsius

"The degree Celsius is the unit of [temperature] on the Celsius scale, one of 2 [temperature scales] used in the [International System of Units] alongside the [Kelvin scale]" (mentioned on Wikipedia)

---------------------------------------------

One Celsius degree is an interval of 1 K, and zero degrees Celsius is 273.15 K.

Above point is mentioned on official website of US government -
https://www.nist.gov/pml/owm/si-units-temperature#:~:text=One%20Celsius%20degree%20is%20an,on%20the%20Fahrenheit%20temperature%20scale.

Hence there are 2 parts now.

1st part - representation of Celsius in terms of Kelvin (includes SI multiplier as 1)

This gives float or si_value and si_unit of Quantity(1, 'C') as (1.0, K)

2nd part - conversion of Celsius to Kelvin (includes addition of an offset 273.15)

This gives conversion Quantity(1, 'C') to Kelvin as (274.15, "K")

It is also defined in units.cfx as follows -

Therefore we have also defined this mapping at 2 places in quantity.py.

1st is in conversion_map as follows -

2nd is in temperature_conversions as follows -

C++ units library does the same thing, so we are consistent with units.cfx as well.

We are also consistent with all conversions mentioned on following website -
https://www.engineeringtoolbox.com/specific-heat-capacity-converter-d_673.html

We have covered and verified conversions from above website, C++ units library and UnitSystems.ccl (including temperatures variance which involves K^2) in test_tempk_48 to test_temp_54 (from test_quantity.py).

Thank you.

[hpohekar]

Pinging here @RobPasMue. He's feedback may be useful for this, as he implemented a units module on top of pint for an internal project.

@jorgepiloto @h-krishnan @seanpearsonuk @dnwillia-work @mkundu1

We have found 1 bug in pint module.

It does not convert temperatures properly.

See following case - Converting (-40000, milli-kelvin) to micro-degree-Celsius

C++ units library gives this -

and this current Real Quantity with Units module is also gives same result in comparison to C++ units library -

Thank you.

[seanpearsonuk]

-40K makes sense only as a temperature difference so the conversion to relative scales should be considered undefined. While pint seems to be inconsistent between C and microC, neither result is worse than the other.

[h-krishnan]

What is the float or si_value of Quantity(1, 'C')?

@h-krishnan @seanpearsonuk @dnwillia-work @mkundu1

---------------------------------------------

The degree Celsius is itself used in SI units alongside the Kelvin -

https://en.wikipedia.org/wiki/Celsius

"The degree Celsius is the unit of [temperature] on the Celsius scale, one of 2 [temperature scales] used in the [International System of Units] alongside the [Kelvin scale]" (mentioned on Wikipedia)

---------------------------------------------

One Celsius degree is an interval of 1 K, and zero degrees Celsius is 273.15 K.

Above point is mentioned on official website of US government - https://www.nist.gov/pml/owm/si-units-temperature#:~:text=One%20Celsius%20degree%20is%20an,on%20the%20Fahrenheit%20temperature%20scale.

Hence there are 2 parts now.

1st part - representation of Celsius in terms of Kelvin (includes SI multiplier as 1)

This gives float or si_value and si_unit of Quantity(1, 'C') as (1.0, K)

2nd part - conversion of Celsius to Kelvin (includes addition of an offset 273.15)

This gives conversion Quantity(1, 'C') to Kelvin as (274.15, "K")

It is also defined in units.cfx as follows -

Therefore we have also defined this mapping at 2 places in quantity.py.

1st is in conversion_map as follows -

2nd is in temperature_conversions as follows -

C++ units library does the same thing, so we are consistent with units.cfx as well.

We are also consistent with all conversions mentioned on following website - https://www.engineeringtoolbox.com/specific-heat-capacity-converter-d_673.html

We have covered and verified conversions from above website, C++ units library and UnitSystems.ccl (including temperatures variance which involves K^2) in test_tempk_48 to test_temp_54 (from test_quantity.py).

Thank you.

My opinion:
If we say Quantity(1, "C") refers to temperature difference always, then the float value should be 1. If we however say that Quantity(1, "C") refers to the absolute temperature, the float value should 274.15.
cc @seanpearsonuk @dnwillia-work

[hpohekar]

What is the float or si_value of Quantity(1, 'C')?

@h-krishnan @seanpearsonuk @dnwillia-work @mkundu1
---------------------------------------------
The degree Celsius is itself used in SI units alongside the Kelvin -
https://en.wikipedia.org/wiki/Celsius
"The degree Celsius is the unit of [temperature] on the Celsius scale, one of 2 [temperature scales] used in the [International System of Units] alongside the [Kelvin scale]" (mentioned on Wikipedia)
---------------------------------------------
One Celsius degree is an interval of 1 K, and zero degrees Celsius is 273.15 K.
Above point is mentioned on official website of US government - https://www.nist.gov/pml/owm/si-units-temperature#:~:text=One%20Celsius%20degree%20is%20an,on%20the%20Fahrenheit%20temperature%20scale.
Hence there are 2 parts now.
1st part - representation of Celsius in terms of Kelvin (includes SI multiplier as 1)
This gives float or si_value and si_unit of Quantity(1, 'C') as (1.0, K)
2nd part - conversion of Celsius to Kelvin (includes addition of an offset 273.15)
This gives conversion Quantity(1, 'C') to Kelvin as (274.15, "K")
It is also defined in units.cfx as follows -

Therefore we have also defined this mapping at 2 places in quantity.py.
1st is in conversion_map as follows -

2nd is in temperature_conversions as follows -

C++ units library does the same thing, so we are consistent with units.cfx as well.
We are also consistent with all conversions mentioned on following website - https://www.engineeringtoolbox.com/specific-heat-capacity-converter-d_673.html
We have covered and verified conversions from above website, C++ units library and UnitSystems.ccl (including temperatures variance which involves K^2) in test_tempk_48 to test_temp_54 (from test_quantity.py).
Thank you.

My opinion: If we say Quantity(1, "C") refers to temperature difference always, then the float value should be 1. If we however say that Quantity(1, "C") refers to the absolute temperature, the float value should 274.15. cc @seanpearsonuk @dnwillia-work

@h-krishnan @seanpearsonuk @dnwillia-work @mkundu1

The main point related to this is, the C++ units library uses same unit for both absolute temperature and temperature difference so we also follows this. (from UnitSystems.ccl)

C++ units library gives this -

Current Quantity module gives this -

pint handles degree_Celsius differently, it uses delta_degree_Celsius unit for temperature difference as follows -

pint does not allow to create degree Celsius quantity as absolute temperature, only allows to treat it as temperature difference

If we try to construct it as absolute temperature, it gives following error.

So we summarize above in following points -

1. Quantity(1, "C") in both C++ units library and Quantity module refers to temperature difference always, so the float value is 1.
2. Quantity(1, "C") in pint also always refers to temperature difference, but represented with delta_degree_Celsius unit.

The conclusion is as follows -

The C++ units library, Quantity module and pint, all of them always refers Quantity(1, "C") as temperature difference only, so the float value is 1.

Thank you.

[h-krishnan]

@hpohekar @seanpearsonuk @dnwillia-work So, what does Quantity(2, "C")*Quantity(3, "m") give? In the absence of any other context, is Quantity(2, "C") a temperature difference or a temperature? I feel it should be "temperature" in which case Quantity(2, "C") should be 275.15.

[dnwillia-work]

I think this test confirms that it works as you suggest:
https://github.com/pyansys/pyfluent/blob/4114599d142afce0f678cc82f3d1d386a2a993f4/tests/test_quantity.py#L430
i.e. it's a temperature.

We could add some additional properties to the quantity object to make it clear what it is. Another one is intensive or extensive, and, if it's the former is it per unit area or pure unit volume.

[hpohekar]

@hpohekar @seanpearsonuk @dnwillia-work So, what does Quantity(2, "C")*Quantity(3, "m") give? In the absence of any other context, is Quantity(2, "C") a temperature difference or a temperature? I feel it should be "temperature" in which case Quantity(2, "C") should be 275.15.

@h-krishnan @dnwillia-work @seanpearsonuk

Right, in absence of any other context, currently we can not differentiate between whether the Quantity(1, "C") is temperature or temperature difference and root cause of this is the use of same unit for both temperature and temperature difference.

We will have to handle this case differently.

Thank you.