I figured there'd be a few more comments other than mine...
How long are the Humidipaks lasting? I don't own a hygrometer, that's why I buy Humidipaks. Our summers are hot and humid. I couldn't survive without air conditioning. I recently replaced 3 sets of Humidipaks, so I'm probably good until October. It gets real dry inside during the winter. My Humidipaks need to be changed after about 2 months during our short winters.
You can always try using a different hygrometer (just to be sure), but that might not be practical.
I'm sure some other folks will chime in soon.