Air Lock Question ?

My fermenter is just a single primary fermentation barrel with a tap at the bottom.
The instructions, and all the books I have read tell me about taking FG readings.
Now I believe that the air lock is there to stop contamenated air and bacteria from
entering the wort.
However when nearing it's final time, and I takeing FG readings , air bubbles travel back into the barrel.
Doesn't this defeat the purpose ?
Is this the norm?
Am I contamenating my beer ?

Fill it with cheap vodka and when you use the spigot to get samples for readings, pull the airlock out so you don't pull the vodka in your bucket.

Once you've got some alcohol in the brew your chances of infection go down considerably. Most infections happen before your primary fermentation takes off. Pull the airlock and get your reading, you'll be fine.

Thanks for your advice and experience. Feel I've learnt something.
Cheers mate's.;; Welcome your knowlage.

Vodka is the way to go.

If you use iodophor and it sucks back into the fermenter, you may get a phenol smell/taste in the beer.

If vodka gets in the beer, it's not a big deal.