How long for medium rare 2 pound roast beef and evaporation?

Hi, I would like to know how long I need to cook a 2-pound (1kg) tenderloin roast beef. I see from recipes 2-48 hours, but I would need a bit more precision… I am also concerned about evaporation while leaving the Anova on throughout the night…

@Gary1976 You’ll want to insulate the vessel you’re cooking in to avoid losing too much water - some of our community members have come up with some great solutions in this thread:

The sous vide cooking method is much more about the precision of temp rather than time, which is why you’re seeing a lot of variance in the recipe cook times. What is the thickness of the tenderloin you’re cooking?

If you take a look at Kenji’s recipe on Serious Eats, you’ll see a little more background on the technique for sous vide tenderloin:

I have found that if I am planning that long of a cook, I will place some saran wrap over the top to help hold in the evaporation. Also, the evaporation is slow enough that you can simply add a couple of cups of water and it will only drop a few degrees before coming back up to temp.

@psiu_glen True, great point! Haven’t tried saran wrap yet but will give it a go for longer cooks. Thanks!