Further to the comment about the rotameter: there are "Purge rotameters" with constant flow controllers that modulate to hod a pretty constant flowrate even if the purge pressure varies.  Example: Brooks model 1350-8900.
If the purge line causes 1" Wc head developed at the flowrate for which ball valve is set, and the instrument range of the monitoring device is 200" WC., then the error introduced by the purge flow can be ignored.    Earlier comment about zeroing the device in situ is alo quite valid. 
I won't really take exception to DanW2's remark but to say that purges have been used since your granpaw was a little boy and they ameliorate a problem.  Diaphragm seals on a filled purge line sometimes work better and sometimes they initiate different problems.  I remember a power plant that indicated a flooding raw water intake  on a frequent basis.  A little documentation identified  that it was about the same time every day.  Somebody took a look, and the sunlight was hitting one (but not the other) leg of a sealed system.

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