[dormae7]

I'm working on a biology project to determine metabolic rate of critters by analyzing their amount of CO2 production. The machine I used measured CO2 in ppm. I am having great difficulty in converting that into another unit, such as mLs/g/sec. I have the room pressure, ambient temperature, total time and mass of the critters. My teacher said something along the lines that the slope of the graph (CO2 ppm vs. time sec) is the metabolic rate, but I don't see how to incorporate that.
Am I supposed to convert all 1045 points into mls from ppm, then use that slope to divide it by g and then sec? Is the ideal gas law incorporated anywhere here?

If anyone could help, I really appreciate this. Thanks!

[Arkcon]

My teacher said something along the lines that the slope of the graph (CO2 ppm vs. time sec) is the metabolic rate, but I don't see how to incorporate that.

Well, you can start with that, graphing it with your favorite spreadsheet program, and determining the slope.  See if that works for you.

I am having great difficulty in converting that into another unit, such as mLs/g/sec.

Often, but not always, ppm is defined as mg per liter, or maybe in this case, ml of CO2 per liter, the instrument's handbook will say exactly what.  You may not have had it handy at the time, or right now, but you can ask your instructor for it later.  If you work out the rest of the conversions, or do it both ways, and show your teacher, they may tell you the answer, having seen that you've done the work yourself.

[dormae7]

In relation to the slope (.7764 ppm/sec), do I use that number to convert to ml/g/sec? But, the main problem is that I'm not sure how to convert ppm to ml. Since 1 ppm = 1 mg/L, .7764ppm/sec = .7764 mg of CO2/L of air/sec? Then I just convert the mg/L to ml/g?

Thank you

[Yggdrasil]

The calculation may depend on the exact specifications of the instrument, but in general, from the slope (ppm/sec) you want to somehow convert this to mL of CO₂ produced per second (the conversion will likely involve the volume of the chamber).

Once you have the metabolic rate of the animal, you want to control for the size of the animal.  To do this, you divide the metabolic rate (mL/sec) by the mass of the animal to give your final measurement in units of (mL/sec)/g.

[dormae7]

I do not have the specifications of the instrument. However, I do have the volume of the container: 295.63mL. But how do I convert that .7764ppm/sec to mL/sec? Do I set it in a ratio to the volume of the container, so .7764 of CO2/1 million = x/295.63mL?
Or does this involve pV=nRT, where n is the moles of the air inside the container, and V (what I want to find), is the volume of CO2 within the container?

Thanks again!

[Borek]

Ahh, never ending ppm problem

I find it hard to believe that nobody around knows what the instrument ppm means - please ask other people in the lab.

If it measures CO₂ (gas) in air (mixture of gases) I would expect the result to be in volume ppm - ie 1 ppm means 1 μL per L.

1 ppm = 1 mg/L

Only if we are talking about diluted water solutions, never in general.

[Arkcon]

Ahh, never ending ppm problem

Ah, yet again.  So many conventions rely on it, and is is often so confusing.

1 ppm = 1 mg/L

Only if we are talking about diluted water solutions, never in general.

Since it is parts per million, could it even mean moles of CO2 per million moles of mixed gases?  I dunno.

[Borek]

Since it is parts per million, could it even mean moles of CO2 per million moles of mixed gases?  I dunno.

Assuming Avogadro's hypothesis is OK, that's just volume ppm

[dormae7]

I set the ppm slope value as a ratio of x/the moles of total air in the container, then used the ideal gas law to determine the volume. Thank you everyone for your help