Secrets of Calibration Gas- Part 2 of 3

assay weight

 

"So," he tells me.  "Gravimetric standards are the best, right?  I'm looking at this calibration gas bottle and it came wrapped in this "Certificate of Analysis" where it gives all this NIST weight numbers.  It says all the weights they used to make the standard are Traceable to the NIST.  And since it gives their weight numbers, I figured you'd think that was the best I could get."

"You're talking about a calibration gas in a disposable cylinder?"  I asked.

"Well, yeah, it's for a gas detector."

He was looking a little uncomfortable.  We'd had lots of discussions on things like this over the years and he usually didn't like what I had to tell him.

"You believe all that stuff?"  I asked.  "I keep telling you the same thing all the time.  Big companies use weights for big cylinders.  It's normal.  Then they sell those big cylinders- usually the 150 cubic foot aluminum or the 200 cubic foot steel- to other companies who dump that mixture into small cylinders.  Let's say they pay $130 to $200 for one of these big cylinders, then dump into small cylinders and sell them for $100 to $200 each.  They make a lot of money doing that."

"But?"

"But you're paying for a calibration standard.  And the supposedly gravimetric disposable calibration standard you're paying for is not a gravimetric calibration gas standard if it wasn't itself weighed.  It was the source cylinder that was gravimetrically prepared."

He looked confused.

"So," he asked, "are you saying that...?"

"That the disposable calibration standards you're buying aren't really gravimetric standards."

"Isn't it the same gas that's inside the disposable?"

"Not necessarily," I said.  "And it's even worse than that.  I'll tell you more next time."