You may need to go into settings and change the sampling rate. If the sampling rate is low - a few seconds, then the battery will be used up more quickly but it will change values more quickly. If you put it to minutes or hours, then it will last longer if you just want an idea of how much it changes over the day.
Some meters have a sampling delay and moving averaging of up to two minutes in response to changes in the air. Can you say if or how much those delays may have made your numbers change so little over the course of your walkthrough?
