@DefyTheNorms When you buy a hygrometer be sure to thoroughly investigate the product. I say that as the analog dial ones are straightforward (placed directly in the habitat), but the digital ones are often misleading or flat-out lied about by uneducated or (greedy) sellers.
For example the basic digital model style
without probe you would have to place the whole model in your greenhouse container, or mantid habitat to measure humidity. Then there are the styles
with a probe, or
with a probe a cover, but those probes are
only for temperature (no matter what it says). With those models you will still have to place the entire product in your greenhouse container, or mantid habitat to measure humidity.
A
real digital humidity probe itself looks like that, identifiable by all the holes in the plastic cover. I personally have
this version in 110v (wires to a wall outlet) with a probe, but it requires a bit of easy DIY.
Interestingly enough the last model can be hooked-up to a device if humidity is over a set limit, or falls under a set limit. That is if I set it for 60% humidity and it reach 65% it would turn on a fan to circulate the air. Or I could set it for 60% humidity and if it fell to 55% it could turn on a ultrasonic humidifier. The model will allow setting up only one device, be it the fan or humidifier, depending which would be more useful. There are other models that offer both options, the fan if too high, or a humidifier if humidity is too low.
Hygrometers come in many other more models and styles too, but these are the basic ones you will see for sale online at eBay, Amazon, pet stores, etc.