You should take your sensor back to the manufacturer. A change
of 5mbar in 15 minutes is well outside the bounds of experience,
at least in non-artificial conditions, at a fixed site in the UK ...
save, perhaps, for a tornado passing overhead. Although gravity
waves may, exceedingly rarely, approach it.

Pressure changes are normally smooth enough for an hourly value,
recorded on the hour, to provide more than enough information
about them. Shorter fluctuations associated with mesoscale
phenomena (e.g. thunderstorms) often merit closer inspection,
but then an hourly rate-of-change would not be relevant for them.

Philip

It was a gravity wave that produced the fastest pressure change I
have ever seen.. In January 1977 I was working for BP in Sunbury and
happened during a slack moment (there were many) to look at the lab
barometer. The mercury surface was visibly moving and the pressure
fell about 8 mb in 5 minutes. Not being aware of gravity waves I
thought the instrument had sprung a leak. There was no extraordinary
weather, a dull, drizzly breezy morning becoming for a few minutes
quite windy (gusts to force 7). The pressure stopped falling and
recovered at a more decorous rate. The time was about 10 a.m. and the
date (I think) the 19th. This must have been written up somewhere but
I forget where.

Tudor Hughes, Warlingham, Surrey.