Handheld CO2 meters let you conveniently monitor gas levels anywhere. They come in 2 basic variations:
Diffused Air - let you measure or monitor the air around you, indoors or outdoors. Because the internal batter only has to power the sensor, diffused air devices last much longer between charges. However, they typically take about a minute to register changes in CO2 levels.
Air Sampling - contain a micro pump that pulls air into a port, sends it across the sensor inside, then expels it through a second port. The ports can be connected to tubing so that the sampled gas is returned to a closed container. Since sampling meters react much more quickly to changes in CO2 levels, with a length of tubing, a sampling meter can be used to check for leaks. They also register changes in CO2 levels in seconds.