Waters ACQUITY UPLC M-Class System Manual page 105

Solvent viscosity
A liquid is classified in the M-number system by testing for miscibility with a
sequence of standard solvents. A correction term of 15 units is then either
added or subtracted from the cutoff point for miscibility.
Solvent viscosity
Generally, viscosity is not a consideration when you operate with a single
solvent or under low pressure. However, with gradient chromatography, the
viscosity changes that occur as the solvents are mixed in different proportions
can effect pressure changes during the run. For example, a 1:1
water/methanol mixture produces twice the pressure of either water or
methanol alone.
If you do not know the extent to which pressure changes affect the analysis,
monitor the pressure during the run.
March 10, 2014, 715003588 Rev. A 105