Liquid Oxygen & liquid argon level calculation

We have got liquid oxygen storage tank, which is having conversion chart of level (mmWC) to equivalent gaseous oxygen (cubic meter). Now the problem is when we have converted the tank for filling up liquid argon, we don't have the conversion chart. So, exact quantity of liquid argon at certain level cannot be known. So, how can i convert from level in mmWC to cubic meter of liquid argon by simply viewing the level guage.

Regards,

Pankaj