Monday, April 28, 2014

Opps! How we learn from Surprises – Part 1

I am a modeler.  I just love to model to predict exposures but that does not mean I do not understand and appreciate the value of getting real data.  Actually measuring what is happening in the real world is very often a serious eye opener.  Just when you think you know what the situation is – Mother Nature throws you a real curve ball.

One of the biggest surprises of my career happened when my friend and colleague, Bill Shade, and I were constructing an exposure laboratory.    We were the only exposure assessors in a Toxicology Laboratory and convinced our management to give us a room large enough to construct our lab.  The idea was to build a wood-framed room 8’ x 8’ x 8’ with gypsum board walls, ceiling, a small window and a door.   We also put a duct into the room with adjustable exhaust air-flow so that we could vary the ventilation rate within our little room.   We almost always used fans to assure good mixing of this small volume.  This space was large enough to conduct emissions studies that included having a person in the room performing tasks.  I will go into the details and value of this setup in a future blog; however, I only mention it here to set up what we found when we were doing a “shake down” and calibration of this facility.

I covered the details of using a tracer gas technique to evaluate ventilation rate in a previous blog so I will not go over them here.   We decided to use carbon dioxide as a trace gas.   It is cheap, relatively non-toxic and readily available in a large tank which worked out well since we did not require it to be portable.  At the time there was only about 340 ppmv ambient CO2 in the atmosphere (Now because of our constant emission of this gas it is about 400 ppmv).    We had an IR detector that would measure CO2 in real time and our plan was to put in a few thousand ppmv, measure the fall off with time and calculate the mixing air changes/hour.

Our first test was to see just how “tight” our room was when it was closed up and all ventilation from the duct turned off.    We pumped the CO2 into the room to about 5000 ppmv and measured the fall off in concentration.   To  our surprise the CO2 disappeared quite fast!   Indeed, we calculated about 5 air changes per hour in a closed room! 
We went inside the room, turned off the lights within the room and caulked all the cracks we could see.  When we got done with it,  it was like a tomb!    You could sense that the room was very well-sealed.   We re-ran the test and got essentially the same result!

At that point we were guessing that there was a CO2 “sink” in the room such that the gas was being absorded into the walls surface rather than being exhausted from it.   

In our next test, we turned on the CO2 gas and let the concentration rise to about 2000 ppmv while we left the C02 ON for 8 hour while monitoring the resulting CO2 levels in the room.    If my memory serves me correctly, the room concentration of CO2 essentially stayed the same for 8 hours.   Thus, there was definitely a sink within the room and it was a fairly deep sink in that we did not exhaust it in the 8 hours we ran our tests.

Our next step was to prime and paint the interior of the room and re-run the tracer gas test.   We got about 0.1 mixing air changes/hr which is what one would expect from a highly sealed room.    Clearly the unpainted wallboard was a sink for CO2 gas and painting it sealed it enough that it was no longer functioning as a sink.

A year or so later I was at a meeting with a colleague who knew a lot about gypsum board and I mentioned our experience.   He told me that there often is a fair amount of residual calcium oxide (CaO) in the gypsum board.  As such, the thesis is that calcium oxide (CaO) is fairly reactive with the CO2 to form calcium carbonate (CaCO3), thus we believe that we found the deep, strong and one-way sink of fresh wallboard for C02. 

This certainly explains what we measured.   It also is a good reason to test your models in the real world.

Years later another friend and colleague, Bob Morrison, did his Master Degree project measuring the sink effect using various tracer gases and chamber wall material.  Someday I hope to convince him to publish these very interesting data.

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