??? What kind of hygrometer is it - round stick-on dial thing or digital? If it's the dial kind they're not at all accurate so you'd be better off with a digital thermometer/hygrometer anyway. If it's a digital one - can't think of any way in which spraying could lower the humidity (unless the viv's already at 100% humidity and opening the door to spray allows the moist air out and the dry room air in??). So yes, you probably need a new one.
ive got 3 lol 1 dial as a rough guide, and 2 digis. all 3 of them go down, the dials are quite accurate there reading the same as the digis, the humidity is only at 80% so looks like its time for some new ones
3??!! Unlikely that they've all broken at the same time I'd have thought??
Do they drop when you open the viv to spray then slowly go back up? What's the humidity in the room itself?? I suspect the problem has more to do with change of air between the moist viv and the dry room than with the hygrometers.
its a big viv lol thats why ive got 3. it drops as soon as i start spraying. and wont go back up. gonna put one of them outside the viv see what it says