Ive noticed that on a lot of microwave meals, they say to remove it halfway, stir, and then replace after giving the meal a 1/4-1/2 turn. Is this turning of the meal actually supposed to accomplish anything? Cuz Ive tried this and ignored it with microwaves of various wattage and it doesnt make squat of a difference to me. The bottom glass rotates anyways. Whats a measly 1/4 turn between nukes gonna do?
Those directions are for platters that don’t rotate. Are they still being made?
I dont think Ive ever seen one that didnt rotate. But if theyre still printing it on the box for that reason, I guess they still exist somewhere…
Well, even if the microwave has a rotating platter, unless you put the item in the exact center, one side of the item is always going to be closer to the middle of the microwave, and the other side will always be closer to the outside. Presumably, rotating the item 180 degrees halfway through will prevent uneven heating if your microwave doesn’t evenly heat in the middle and outer locations.
Yeah, turning the plate doesn’t matter if your microwave rotates already, but it still might be a good idea to stir, since the microwaves will not penetrate more than about 1 inch into food.
Heh. My old microwave finally gave up the ghost last year, and it didn’t have a rotating platter. Some appliances can last a long, long time if they’re properly cared for.
So the manufacturers are just assuming that someone, somewhere, still has a working microwave that doesn’t have a rotating platter.
Some new Japanese microwaves don’t have a platter. I think they use variable frequency to heat evenly?
The geometry of the magnetron sets the frequency of the microwave photons that are heating your food; that can’t be changed without installing a different magnetron.
The exact center in particular is the main concern. It might receive higher or lower energy than other parts. So I’d recommend not so much rotating the dish, but moving it so that a different part is at the center of the platter. I.e., don’t put your dish exactly on the center to begin with, then partway thru move it so it’s askew on the other side.
I never quite got this. So food within/behind that 1 inch heats by conduction?
Pretty much. Most of the microwave energy is absorbed by the first inch of depth; very little reaches the interior. Absorption can be expressed in “percent per unit distance.” So a beam of microwave photons hits a food item, and after the first 1/4-inch of depth, x percent of that amount is absorbed, after the second 1/4-inch of depth, x percent of the remaining amount (not the original amount) is absorbed, and so on. It’s a bit like compount interest in reverse. 
The microwave beam never achieves zero intensity; it just loses another X percent of itself for every fraction of an inch it advances. But practically speaking, after the first inch of depth, the remaining beam intensity is a small fraction of what it was at the surface, meaning the majority of it was absorbed in that first inch.
If you have a food item that’s less than an inch thick, it’ll cook pretty evenly via microwave absorption, and you don’t have to allow time for conduction.
A big cup or bowl of low-viscosity liquid, e.g. water or coffee, can circulate freely, heating the interior region of the liquid body via convection, so again you don’t have to worry about allowing time for conduction.
For a big cup or bowl of high-viscosity liquid, e.g. spaghetti sauce, you can manually stir it at regular intervals to mix heated sauce with cold sauce.
If you’re trying to cook a larger item such as a pot roast that’s several inches across whichever dimension you choose to measure it, the microwaves won’t reach the center with any significant intensity. You will have to allow time for heat in the outer inch of meat to be conducted into the interior. IOW, for large food items, a microwave oven isn’t really any better than a conventional oven.
that is why rotating, stirring or heat a minute then rest for two minutes works.
you can have food burnt or too hot to eat on the surface and still frozen an inch away.
some units will have a defrost mode where it is on a few seconds then off a few more so that no food gets hot enough to cook and the heat can transfer by conduction into the food mass.
Thanks, I checked, they use a spinning fan/waveguide, like commercial ovens. You can search for Panasonic microwave no turntable for models.
Another problem causing uneven heating is water absorbs microwaves better than ice, leading to thermal runaway. Actually this property means you can melt glass in a microwave.
Indeed. My over-the-stove microwave was made in 1994, still works, and doesn’t have a rotating platter. I think my FIL’s microwave is like 30 years old!
Zyada does melt glass in the microwave, though not with water. She makes jewellery pieces in a small kiln. Search for “glass fusing microwave kiln” to see more.
ETA: No spinning platter in that one.
Heh, yup, you just perfectly described my microwave methodology to ensure even heating. It became necessary when I got a new microwave that is amazingly powerful, particularly in the center. So, everything (that takes a while) goes as far off-center as possible, with an adjustment halfway through to give both sides equal time closer to the center.
I just realised if you can see microwaves, the spinning thing must look like a disco ball.