Good one Chris. We had infinitely changing feedstock so I always took a sample of what I was putting in the truck as I was filling up and monitor it as the temps fell. My little bottle out in the wind is gonna cloud well before 30gals in my diesel tank and always gave me enough advance notice to add more diesel. Once I was blending, I'd take a sample from water drain on fuel filter after fresh tank had been in there long enough to get stirred up. (I keep a piece of hose on the discharge for that drain for this very reason - it is long enough to hold a water/soda bottle in the wind and allow me to collect the contents of filter bowl for inspection/disposal).