Plant Science

So, my little sister wants to do a science experiment involving the CO₂ uptake of various plants. She's asked for my help getting things setup, and will be using some of my equipment (most notably, my LabJack) to collect data.

Our original plan was to use a ppmCO₂ sensor (the extraordinarily crappy datasheet can be found here). This sensor is basically an electrolytic cell whose voltage (not current) is proportional to the logarithm of CO₂ concentration.

Unfortunately, this thing produces a signal that ranges only between 0 to 50 mV, and can source no more than one pico-amp (pico means "* 10^-12", or one trillionth). This meant that the LM324 quad opamp I was planning on using would be inadequate- it has an input bias current of a couple nano-amps, which is of course three orders of magnitude too much.

My original plan was to use one op-amp in a non-inverting configuration to boost the sensor voltage into the range of the LabJack's single-ended ADC range, which is about 2.44 V.

Unfortunately, we've run into several problems. First, at the minuscule currents we're dealign with, noise is a huge issue so I can't really effectively use my breadboard. Second, my soldering skills still need a lot of work. Third, the TL082 op-amp that I found at radioshack is not a rail-to-rail amplifier, meaning that I'll have to use two power supplies to get any maningful data from it.

I have managed to get the heater control circuit working. This isn't much of a victory, since it's just a transistor switch and some carefully arranged power resistors, but seeing as to how I can't even get a non-inverting amplifier to work I guess I need to take what I can get. I was originally hesitant to even build this circuit, but because the heater for the sensor produces about 1200 mW of heat, which can add up quickly inside an insulated glass jar, I decided to implement it to allow the labjack to control the heater and activate it only when it is about to take a reading.

I'm super tired at the moment so I'm going to hit the sack. I plan on working on this more tomorrow before I have to head back to Bozeman to see my lovely lady, and if I make any headway I'll update this entry. If things don't work out and Shannon ends up needing to choose an alternative science experiment, I'm still going to take the CO₂ sensor back home with so that I can prove to myself that I can get it working. Once I have that, I'll probably couple it with my ethanol sensor to built a "fermentation telemetry" rig.