If this is a regular occurrence, you might do as well with a simple analog meter. It is really hard to do much with a digital meter unless it will save the low point. Even then you don't know if that was just a spike or a real sag. The analog meter is far more visual.
I agree Greg,
I have an elderly analouge meter here that has the facility to set the needle at centre scale and it has +/- values printed on the scale.
In some cases, I actually prefer the analogue meter to a digital one, especially if the readings are all over the place or changing to any great degree.
Having said all that I also own a data logger, it's handy to be able to leave the thing behind (behind the panel dead-front, of course) and go and get it say two days later and see what it has recorded.
It's told me a few interesting stories to date.