Sentinel has replaced the CHHC-1 with the CHHC-4 Temperature, Relative Humidity and CO2 Controller. It looks the same so you'll be comfortable making the switch, but inside the improvements are exciting. 

The upgrades include:

More accurate dual beam CO2 sensor
More powerful CPU
Removable remote probe can now be 100' from controller!
Moisture-resistant housing
Plant friendly green LED function lights
ROHS compliant

The CHHC-4 gives you one-touch calibration of the CO2 sensor with simple pushbuttons to enter desired PPM settings and display the current CO2 PPM readings. Can be operated in "Fuzzy Logic" mode for compressed CO2 or On/Off for CO2 generators. There's a CO2 receptacle to connect CO2 generator or 120 volt compressed CO2 solenoid / regulator assembly. The CHHC-4 also has a 12 volt power supply to connect to any temperature controller so that the CO2 devise will be disabled when the fan is operating to prevent CO2 depletion.

Along with complete CO2 control, the Sentinel CHHC-4 can be set for separate day and night settings for both temperature humidity. It records the minimum and maximum temperatures, plus humidity and CO2 levels.

A built-in self-calibration feature allows you to calibrate the controller if you prefer. There are separate outputs to control cooling, heating and humidity device. You can increase or decrease the amount of humidity in the growing area while also controlling carbon dioxide with Part Per Million accuracy.

The CHHC-4 Sensor Probe is housed in a removable remote sensor enclosure that allows the probe be placed up to 100 feet away from the controller. The probe can be placed inside the growing area and the controller outside the area - a nice feature for enclosed grow rooms or chambers.

The Sentinel CHHC-4 has an easy to read LED display. Carries manufacturer's 3 year warranty.