Airguide humitity

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How can I set an Airguide 1 - 2272 model percent relative humidity deceive with no standard reference instrument to check this against? The only other numbers are etched in and I think they are P – 623 – 723 – 766 – 721 with looks like MD below this (poorly etched in)…. By hand as best as I can tell.

Unit was made at Aiguide Instrument Company in Chicago ILL... my return email is: [email protected]

Thank you for any help you can provide...

- Bill

I remember I answered this question once before. You may want to search Airguide in the AMHD and see what my answer was then. I remember it was rather simple.