For best results sensors should be hung in areas with good air flow which allow the sensor to more quickly sense changes in temperature and humidity. Additionally, sensors should not be placed near other equipment which may artificially alter readings. For example, placing the sensor too close to lights can cause temperature readings to increase while decreasing humidity. The same goes for placing the sensor too close to A/C and other HVAC hardware.
Generally speaking it's best to place the sensor mid-canopy but not directly underneath a light. With multiple sensors in a room we recommend placing them at different heights (for example mid-canopy on one sensor and closer to the soil/roots on another sensor, etc) to see how parameters may be different across different zones within your space.
If you have multiple different brand/types of sensors you may see some different results. It's important to understand that grow spaces are not like regular rooms in that the environmental parameters in a grow space are constantly being manipulated to maintain ideal grow settings. Due to the hardware required to run a grow space you may often see different readings if multiple sensors are not directly near each other, if one sensor is too close to a heat/humidity source, or if the sensors are simply different (ie. one has a probe while another is embedded within a case like our Air Sensor Lite). To test multiple sensors you can always move the sensors to a 'regular' room like a kitchen or living room and place the sensors side by side to see how results may vary.