VIDEO :: Carbon Dioxide Instrument
Watch Bruce Daube, HIPPO Scientist and CO2 and QCLS instrument engineer, explain how the CO2 instrument works.
Hi, my name is Bruce Daube, I work in Steve Wofsy’s group at Harvard University, I’m the engineer in the group.
For the HIPPO campaign we have two instruments on board, an older CO2 instrument which we’ll describe in more detail, and the QCLS instrument which is a relatively new design by us. That one also measures CO2, in addition to CO, methane and N2O. But for today I guess we are going to talk about the CO2 instrument. This is something that we built back in 1996 originally for balloon campaigns.
The CO2 instrument draws air in through a rear facing inlet, in one of the the HIMIL which we’ll go over and take a look at, there’s one hanging on the bottom of the plane, it’s easy to see. It’s rear-facing so that we don’t draw in too many water droplets as we fly through clouds, or also avoiding large aerosols. We filter the air as it comes in just in case there’s some things left in there, and then we run it through a permeable tube, a nafion drier which removes the bulk of the water vapor, but we also go through a dry ice cooled trap to get the rest of it so we get the dew point down to -70 C or so, -80, to really dry the air.
The things we really need to worry about is keeping that water trap cold and we have to restock with dry ice at each stop, and we also have to periodically remove the trap and warm up and blow the water that we’ve collected out. And for the CO2 instrument we normally do that about every 3 flights.
One of the things with CO2, in the stratospheric configuration we would measure CO2 with a precision of about 0.03ppm, the background level now is about somewhere around 385ppm, it’s a pretty precise measurement.
One of the really important things for most flight instruments is to do in-flight calibrations, and so we carry 4 cylinders in the instrument that we use during the flight where we zero and we also measure a low concentration - low span, high concentration - high span, and then the fourth bottle we use as a check of the calibration occasionally during the flight and compare what we derive from the calibration what we think is in that tank compared to what we measured in the tank prior to the campaign and that ultimately serves as a real good quality control of how well the instrument is working.
You also asked about the dry ice, we also have to worry about keeping these gases filled and we don’t like to let them run down below about 500 psi, because below that you start to see a small change in concentration that comes out of the tank. So we do have cylinders in Christchurch for refilling tanks, if need be.
Fortunately we’ve managed to get our gas flows low enough now, that actually for everything on board we’re usually able to make the whole trip without refilling, which is nice because we like to cal(ibrate) before the campaign and after the campaign in the lab, we can’t do it on the way during HIPPO and so if we have to change the gases in Christchurch we loose the after-cal(ibration) after the first segment and pre-cal(ibration) for the second segment. We still have the before and after, but it’s nicer to have both.
So we bring the air into the instrument and we have a pump on the front-end that increases the pressure of the flow, from outside we go anywhere from actually over 1 atmosphere because of the way the inlets are configured the pressure is a little bit higher than sea level when we do the low altitude legs and then down to 760 toro at the surface town to 50 toro, not quite 50 torr, at the high altitude leg, and so it’s a large pressure range to operate over. We use a pump to bring in the sample air and deliver it to the instrument at a fixed pressure. And we actually have three different levels of pressure control to make sure things are stable.
We don’t collect anything, we just come through, look at it, measure it and then send it on it’s way, and the nice thing about that is that you have a number already as opposed to collecting samples on board in flasks and whatnot where then you have a lot of lab work after the fact.
If you want we can take a closer look at the HIMIL and maybe the instrument itself on the plane.
So this is a HIMIL - inlet side, outlet side, and you can see that it’s capped at the moment. During flight the caps of course come off. And inside there are three tubes and they can be configured in different ways. For our purposes we have one of the tubes down inside, it’s a quarter inch stainless-steel tube, bent backwards so that it’s rear facing. Air flows through, increases in pressure a little bit on the inside due, to the construction on the outlet, and then continues on it’s way, so we just pick a little bit of sample air off from that and run it up into the instrument.
Other instruments share the HIMIL so you have to careful to make sure if one instrument has a leak and is leaking cabin air back into the HIMIL it creates a problem for the other people, potentially, creates a problem for other people who sample from the same point so we have to be real careful to take care of each other by making sure you don’t have any cabin leaks into your inlet line. Of course at high altitude the pressure outside is quite low, the pressure inside the aircraft is more like equivalent of 10,000’ and you need a lot of pressure to push cabin air into the inlet tube which is a disaster when it happens.
So from the 16th inch stainless tube inside the HIMIL, we have a junction inside under the floor boards, we can open a hatch to get at, and we have a piece of decor tubing, plastic tubing, it comes up a couple of meters up to the instrument in the aircraft and connects right where we have an inlet filter.
Alright, on the forward end of the instruments, relative to the aircraft anyway, we have the dry ice cooled water trap, which gets filled up everyday before the flight. You can see the sample in and out tubes, and a vent as well so the CO2 vapor is vented in a controlled way.
The inlet is difficult to see, but the tubes that we mentioned coming out of the HIMIL, one of these tube actually comes up and goes into the inlet filter. It’s tough to see and a little bit hard to change, fortunately we don’t have to do that too often.
On the upper portion of the rack is RAF’s (NCAR’s Research Aviation Facility) CO instrument, also part of the HIPPO campaign.
One difference of using this instrument on HIPPO is that normally we use a pressure trip to turn on the instrument so when we take off, the pressure decreases to a certain point and the instrument senses that and turns on automatically. In this campaign we want to turn the instrument on manually, so we have some switched here to start the instrument sampling as soon as we start to taxi and then we get a sample, a measurement, right from the ground up. We mentioned the sir coming in the HIMIL, and it comes through the instrument, come s back out of the instruments and finally ends up getting into the exhaust manifold on the plane, and this runs the length of the plane. Many instruments discharge into and it exits out the back of the aircraft.
That’s about it for CO2.