I use the British-made EVOLUTION co2 controller, which has a co2 measurement probe and will automatically keep co2 levels at whatever you want. Cost ? I paid 800 british pounds for it. Expensive, yeah if you have a half-dozen plants. If you have a roomfull and you get faster growth and earlier finishing times, pays for itself quickly.Might have to up co2 on my priority list. How much did the co2 meter set you back ?
Mouse