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Not all 3.2 L air sampling canisters are 3 liters. Wait… what!?

  • Jason S. Herrington
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  • #Volatile Organic Compound (VOC) Sampling
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The other day a customer contacted me to share the following discrepancy she has observed on several occasions: “when I collect ambient air samples with Restek 3 L canisters and collocated 3.2 L canisters from the competition, the canisters fill in the same amount of time.” Obviously this does not make sense, so I immediately go into trouble-shooting mode asking questions like: what flow controllers, what size critical orifices, what flow rates, are all the canisters evacuated the same, etc…

At no point in time are we able to find anything out of the order. The customer was running an apples-to-apples comparison and by all accounts the competitor’s 3.2 L canisters should have more vacuum left at the end of the sampling duration when compared to our 3 L canisters. At this point in time the only stone left un-turned (you already guessed it from the title of the blog) was the volume of the canisters.

We used the following two-pronged approach to determine the volume of our 3 L canisters and the competitors 3.2 L canisters:

  1. Weighed the canisters empty and weighed them filled with water on our verified shipping scales in the shipping department (yes, the same calibrated scales we used to show you our beefier 6 L canisters only weigh 8 oz. more than the competition). 3 weights were obtained from 3 different scales, like so:                                                                                                  

  2. Measured the volume of water, which came out of the canister (i.e., post weighing). We only had a 1,000 mL graduated cylinder, so we had to take 3 measurements to get the complete volume. This is a picture of the last reading from the competitor's 3.2 L canister:

Here are the values we obtained from the aforementioned investigation:

As you may see in the above table, approaches 1 and 2 indicate the volume of the competitor’s 3.2 L canister is 2.849 and 2.860 L, respectively. These measurements were exactly the same as our 3 L numbers, which for the record are consistent with our internal specifications for min, nominal, and max 3 L canister volumes (i.e., 2821, 2853, and 2885 mL). Both approaches agree reasonably well with one another (99.6% agreement). We speculate the minor discrepancy is the graduated cylinder indicates an accuracy of ±6 mL. With three measurements used, we could have had upwards of an 18 mL swing, so 11 mL is well within limits.

Regardless, both measurements clearly indicate the competitors claimed 3.2 L volume is a far cry away from 3.2 L. Which all makes sense with why our customer saw the competitor canisters filling at the same rate as our 3 L canisters. Now I find myself wondering what other canister dimensions are not as claimed!? I also wonder why is there a 3.2 L canister on the market anyway? Maybe this is the competitor’s approach to excluding others from RFQs (request for quotation). We do not sell a 3.2 L, so we cannot compete on these quotes, but oddly enough neither does the competition.


Thu, Jan 30, 2020

Hi Jason, Wow! Interesting post. I'm not knowledgeable about air sampling - would this discrepancy have any negative impact on your customer's analysis or workflow? Either way, this would be concerning to me if I were a customer of your competitor. -Luke

Sun, May 24, 2020

Great post! Very interesting! (3.2-2.849)/3.2 = 0.11. 11% off the label, that is too much for a monitoring device. As a canister maker, your competitor must know the exact volume, but choose to label it as 3.2 L. Any reason other than excluding others from RFQs?