First findout the liquid column of heptane which is equivalent to 1 atm.As we know 1 atm equals to 10,330 mm of water column. If you know the sp.gravity of heptane at the respective temperature (say as 0.730), you can convert the water column in to hepatne column by dividing it by sp. gravity of heptane. For this case 1 atm = (10330/0.730) =14150.68 mm of heptane column.

I think this will fulfil your requirements.

Gunas