At some point during a twelve hour, low and slow barbecue, its hard to resist the urge to "peek". Just once. Or to flip the ribs. Or to mop on some sauce.
Opinions vary widely on peeking. Some claim a quick peek makes no difference to the cooking time. Others claim you loose too much heat to compensate for the value of peeking, that is, "if you're lookin', you ain't cookin'". Which viewpoint is right?
Now, on first principles there should be no downside to a brief peek. After all, the cooker probably contains 50 pounds of hot steel, a decent flame, and the meat itself takes time to cool off. When the lid is opened, the air might (and will) cool off, but should just as quickly return to equilibrium.
We tested the effect of opening the lid on a standard, stainless steel three-zone back-yard gas grill. The first time we opened the lid for one minute, and the second time for five minutes:
As you can see, a quick one minute peek (enough time to flip a rib or mop a sauce), returns the grill to its initial temperature in a minute or two. No harm, no foul.
On the other hand, a five minute peek cools off the grill lid and cooking surfaces pretty aggressively, and takes almost 10 minutes to fully reheat (perhaps even remaining a bit under the original setting for another 10 minutes).
The reason it takes so long is the gas grill, unlike a kitchen oven, has no temperature sensor or feedback. Unaware the temperature has dropped, it doesn't increase the gas flame in response.
But many pellet smokers do contain a "negative feedback circuit" to maintain a fixed temperature under all possible conditions. Here is a test on a pellet grill, containing a five pound pork shoulder. We instrumented the grill with three temperature probes- one in the center of the grill in mid-air ("smoker"), one in the top half inch of the pork butt ("surface"), and one in the middle of the butt ("interior"):
Not shown (but similar to the gas grill experiment) a minute's peek was nearly invisible. A five minute peek dropped the air temperature significantly, and the pork butt's surface temperature VERY slightly declined by 3F. However, the interior temperature continued to climb- heated not by the smoker, but via the mass of warmer butt near its surface.
After closing the smoker, the temperature recovered in around two minutes- five times faster than the gas grill. This demonstrates the power of negative feedback cranking up the smoker's fire pot in response to the cooler conditions.
Finally, we tested a Weber kettle cooker fueled with coal- in this case, roasting a whole chicken:
The first thing to note is coal, unlike gas, slowly declines in heating power over time (the GREEN curve labeled "cooker" measures air temperature a few inches above the grill grate). So when you open the lid, you are disproportionately wasting valuable cooking time because a minute now is hotter than a minute later. Still, a one minute peek recovers in about two minutes, while a five minute peek takes about five to ten minutes to recover.
But, does this affect cooking time? The RED curve measures the temperature 1/8" below the surface of the chicken breast, and the BLUE curve the temperature deep in the thigh. Note the first time we peeked for five minutes, the air temperature dropped to about the same level as the skin temperature, so the skin hardly cooled. The second time we peeked, the bird skin was warmer than the air, so it cooled substantially and took almost 20 minutes to return to the temperature it would have been if we hadn't looked.
But that is just the surface temp- deep inside the bird, it's cooking as if nothing had happened. Thermal mass is a great leveler.
So what is our conclusion? Well, you can peek for a minute or two with no adverse consequences. Just don't peek more than a few times an hour.
Any longer than a minute, the answer depends on your grill. Five minutes should be ok if the cooker has a negative feedback capability. Or, if its really big and heavy, like a towed offset smoker. But in general, you ain't cooking if you're lookin' for more than five minutes1.