I think the claim is that the meter is measuring HCN, and the caption writer (and possibly translator) got confused. The more general context is a widespread belief in Hong Kong that CS canisters past their expiration date are somehow more toxic; this is meant to be "proof"
Here's one for a CS munition used in Hong Kong http://www.nonlethaltechnologies.com/pdf/SDS/SDS-MP-4M3-CS.pdf …
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Also a fascinating recent article about the manufacturer if you haven't seen it: https://www.buzzfeednews.com/article/rosalindadams/tear-gas-hong-kong-made-in-usa …
End of conversation
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