You are talking about very small changes of pressure here. The order of magnitude is
1 atmosphere = 760mm Hg = about 100 kPa
So 0.1mm Hg is about 13 Pa.
That will produce a very small strain unless you have a flexible pipe, but a flexible pipe could give calibration problems by not staying circular (i.e. the strain gauge is stiff compared with the pipe).
You might be able to use a laser vibrometer (LDV) to measure the change in shape of the pipe directly (to an accuracy of nm, or even pm with special image processing software). They can measure steady-state as well as dynamic effects.