我的代码以某种方式乘以 0.5 倍，我不知道为什么

Question

I wrote a unit converter for one of my plotting scripts. This converter works by determining the unit of the provided data, converting it to SI-units and then converting it into a target unit.

It works without issues for all unit conversions, except for one: The target unit 'mmol/g'.

Here's the code for the converter:

import scipy.constants as units
import numpy as np

example_data = np.arange(10,20)

def unit_converter(unit,target_unit,numerical_data):

    unit_converter_dict = {'kPa':units.kilo,
                           'atm':units.atm,
                           'bar':units.bar,
                           'mmHg':units.mmHg,
                           'psi':units.psi,
                           'cc/g':units.centi**3/units.gram,
                           'cm/g':1/units.gram,
                           'cm/kg':1,
                           'mmol/g':1/(22.41396954)}

    converted_data = numerical_data*(unit_converter_dict[unit]/unit_converter_dict[target_unit])
    return converted_data

print(unit_converter('cc/g','mmol/g',example_data))

For some reason the converted data is exactly half of what it should be and I can't figure out why. If you don't know why mmol/g is supposed to be 22.41..., that's because one mole of an ideal gas at 1.0325 bar pressure and 0 °C takes up a volume of 22.41... liters.

So when I start with cc/g and convert that to cm/kg, I should only need to multiply by 22.41 mmol/g to get the correct conversion. But for some reason, all my values are only half as big as they should be after the conversion.

I get the correct result if I use 2*22.41... btw.

Does anyone know why this happens? Where does the factor 2 (or conversely, 1/2) come from?

I have already established that this issue is caused by this part of the code and not by some other operation further downstream.

Answer 1

It's not off by 1/2, though it's close. Also, since you're in mmol, I think you want 1000/22.41396954. Also, since you're doubly dividing, I think the units need to be flipped when you convert.

import scipy.constants as units
import numpy as np

example_data = np.arange(10,20)

def unit_converter(unit,target_unit,numerical_data):

    unit_converter_dict = {'kPa':units.kilo,
                            'atm':units.atm,
                            'bar':units.bar,
                            'mmHg':units.mmHg,
                            'psi':units.psi,
                            'cc/g':(units.centi**3 /units.gram),
                            'cm/g':1/units.gram,
                            'cm/kg':1,
                            'mmol/g': (22.41396954/1000)}

    converted_data = numerical_data*(unit_converter_dict[unit] / unit_converter_dict[target_unit])
    return converted_data

print(unit_converter('cc/g','mmol/g',example_data))

# [0.44615033 0.49076537 0.5353804  0.57999543 0.62461047 0.6692255 0.71384053 0.75845557 0.8030706  0.84768563]