I just used a $35 sniffer that I bought new on Ebay this morning to pinpoint an AC leak from a compressor. It worked great and there seemed to be a contact point to get additional tips.

With other gas sniffers, I *think* their 'nose' get desensitized the more of the target chemical it sniffs. I don't think they have a 'shelf life' if not in use (and not exposed to the chemical).

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car