Old man winter is gone and the temperatures are rising… but why are my TO-15 air canister pressures rising too?29 Apr 2015
In case my colleagues have not gone on record with the following, I shall: “customer inquiries make for THE best blogs!”
So… as you can imagine I have been receiving some customer inquiries as of late. And it should come as no surprise that these questions are related to air sampling during the recent winter season. A couple of them have gone a little like this:
“They sampled ambient air into 6L cans back in February in Illinois at an outdoor ambient temperature of -3 F. The vacuum gauge readings for four of their cans read around 5 to 7 "Hg in the field. The lab report is showing the lab took canister vacuum/pressure readings upon receipt at the lab at ambient indoor temperature, most likely around 70 F and recorded gauge readings that were quite a bit different. Some were recorded at 1"Hg, some at 0, and one or two at 1psi.”
The rest of the inquiry goes on to ask if the aforementioned observations are reasonable or do we think there was a canister leak, etc... Short answer – YES, the aforementioned observation is reasonable. Long answer – continue reading.
Without making this blog too long, we can use the combined gas law in the following fashion:
Where we know the following:
Pi = 5” Hg (12.25 psia at sea level)*
Vi = 5 L
Ti = -3 F = 252 K
Vf = 5 L
Tf = 70 F = 294 K
This means we are solving for Pf. If we do so with the aforementioned information we get Pf = 14.29 psia (0.83” Hg). So everything checks out and life is good. *You will notice the slight of hand, as it is much easier to work with absolute units like psia, so consider that if you find yourself getting twisted up with gauge units like "Hg. Hopefully the Illinois field technician has thawed out by now! Just remember… if the temperature goes up, so does the pressure.