This humidity probe is compatible with the SpyderWeb series units and standard models that have firmware version 5.0 or greater.
All Herpstat 1 SpyderWeb
All Herpstat 2 SpyderWeb
All Herpstat 4 SpyderWeb
Herpstat 1 (firmware 5.2 or greater)
Herpstat 2 (firmware 5.1 output #1 only, 5.2 or greater works on both outputs.)
Note: Redline 1/2 models flashed to firmware 5.2 or greater to work with this new humidity probe will deactivate the red led glow system.
Herpstat 4 (firmware 5.0 or greater)
Herpstat 6 (firmware 5.0 or greater)
Note:  To check firmware version on unit unplug it for ten seconds and plug it back in.  The unit will display the firmware version on startup.  
Each output on a herpstat has a single function.  This means you cannot set the output for temperature control AND humidity at the same time.  So if you purchase this for a Herpstat 1 or Herpstat 1 SpyderWeb you will only be able to monitor/control humidity and not have control of temperature with the herpstat since it only has one output and one probe jack.