Measuring microwaves from oven
March 6, 2020 12:56 PM   Subscribe

We have a new microwave oven at the office. One person measured the microwaves with a meter while the oven was running. (photo of meter at rest attached). When standing about 4 feet away with the oven on, the needle goes all the way to the right. I can't imagine, but is this cause for concern?

Oven: Panasonic NN-SN9665

posted by falsedmitri to Technology (14 answers total) 2 users marked this as a favorite
Here are the FDA guidelines about microwave emission safety. I'll quote the important part:
A Federal standard (21 CFR 1030.10) limits the amount of microwaves that can leak from an oven throughout its lifetime to 5 milliwatts (mW) of microwave radiation per square centimeter at approximately 2 inches from the oven surface. This limit is far below the level known to harm people.
Your picture shows 0.03 mW/cm^2, and the level backs off by the square of the distance. You're pefectly safe and your oven door seals are working as designed. You have a much higher chance of being hurt by boiling liquid than the RF emissions from a microwave.

I'd hazard a guess that the 0.03 number is from ambient Wifi signals in the room and not the oven.
posted by JoeZydeco at 1:29 PM on March 6, 2020 [6 favorites]

Microwaves are more energetic than radio waves but less so than infrared radiation (like heat lamps etc). All these are less energetic than visible light. It's on the other end of visible light, UV, X-Rays, etc, that really is a health hazard.

The low-energy end of electromagnetic spectrum generally behaves as a continuous scale. So like radio waves mostly just go through us without really affecting us. Microwaves sorta go through organic matter (food, people) but are sort of absorbed (heating your food). Infrared mostly is absorbed but penetrates deeper than visible light (near infrared photography makes skin look translucent, which btw, makes tattoos look super vibrant). Visible light mostly bounces off your skin.

Wifi is in the microwave range. My biggest concern about the radiation from the microwave oven is that it would disrupt your internet.
posted by aubilenon at 1:30 PM on March 6, 2020

That meter measures electromagnetic fields (instructions and specs here). Some general info on microwave oven radiation from the US FDA, including this bit:
A Federal standard (21 CFR 1030.10) limits the amount of microwaves that can leak from an oven throughout its lifetime to 5 milliwatts (mW) of microwave radiation per square centimeter at approximately 2 inches from the oven surface. This limit is far below the level known to harm people. Microwave energy also decreases dramatically as you move away from the source of radiation. A measurement made 20 inches from an oven would be approximately 1/100th of the value measured at 2 inches from the oven.
I'm no expert, but have had some RF safety training. The main thing I remember about microwave radiation is that you'd notice feeling warmer if it were at levels likely to affect you, much like the food you're heating in the microwave.
posted by asperity at 1:31 PM on March 6, 2020

If the meter is accurate, that does seem rather high.

CDRH standards seem to be 5 mW/cm2 when you're 5 cm away from the microwave. Using the distance-squared law, you'd expect that to be 0.05 mW/cm2 about half a meter away... you're more than a meter away, and according to the meter, are getting about 10x that.

What does this mean? I wouldn't be overly concerned -- it's still not a lot of power, even though it exceeds specs. Rough estimate... if you held a glass of water there... say 50 cm2 exposed surface -- it would take ~100 seconds to absorb a joule if all the energy were absorbed, which is ~1/5 of the amount needed to raise 1 gram of water's temperature by one degree. Assuming the glass had ~300 grams of water, that's 30,000 seconds = 8 hours' exposure to raise the whole glass' of water temperature by one degree. (Someone can check my math.)
posted by cgs06 at 1:35 PM on March 6, 2020

(I assume that when the microwave's going, the meter's near the right end of the dial, which is ~1 mW/cm2 if I'm reading the scale right.)
posted by cgs06 at 1:38 PM on March 6, 2020

(The meter is set to "Radio/Microwave" and using the bottom scale. I read it as 0.03 mw/cm2)
posted by JoeZydeco at 1:56 PM on March 6, 2020

I can't imagine, but is this cause for concern?

I don't know if that particular meter gives trustworthy results. It looks like it's been picked up by ghost hunters. And the wellness/hypochondriac crowd, who have baseless worries about the elementary school's wireless internet router giving everyone cancer.

Poking around, the manufacturer's website has stuff about protecting yourself from RF fields, and (static?!) electric fields, and (static!?) magnetic fields.

For RF fields, they want their victims tacking up space blankets so that the evil spirits don't kill their children or give them cancer.
Metal sheet or mesh can be hung on the inside wall or the outside wall. In some cases,
if the RF is also coming through the ceiling, metal sheeting or mesh may need to be
placed there. The more area that is covered by the metal, the more effective the
shielding. If various transmitters completely surround the house, it may be best to apply
metal sheeting to all walls. You can tell in which direction the transmitters are located
and how strong they are simply by holding the TriField Meter (set to RF) in front of you
and rotating in a 360° circle. Because your body blocks the RF sources that are in back
of you, the meter will only detect RF sources in front of you.

* The very smooth Mylar space blankets that are shiny on both sides, and are very
inexpensive, reflect RF well. However, the more expensive space blankets that are a
tarp on one side (and shiny on the other side) may not reflect RF well, especially if the
shiny side is embossed with shapes or with a weave pattern.

Speaking as a (rusty) physicist, this is a massive, indisputable red flag that the device is being made by kooks and marketed towards kooks. Contributing to inner worlds glistening with fear and worry.

Static E fields don't hurt you unless you're getting electric shocks from them. Static M fields don't hurt you unless they're pulling your wheelchair or oxygen tank into the MRI scanner.

So probably, this is a highly sensitive meter that detects stuff below a threshold where it can hurt you. And possibly, it gives readings that are 10x-100x higher than what real calibrated lab/test equipment would show.

So you probably need the stuff real engineers/technicians use, if you want meter reading numbers that aren't utter bullshit. Stuff that's calibrated, and where people double-check things and periodically send meters back to be re-calibrated. Stuff where the manufacturer and user care about real-world numbers being accurate.

Randomly, I remember we used Fluke tools in my undergrad physics labs, so:

That one's USD600, and they don't have it, but I'm sure you can dig up some something real technicians use that is cheaper.

If you want to have fun, wave your friend's meter near the office's wireless router, or near your phone, and it will probably falsely report that those things are trying to kill you.

Because the bozos that made that meter are, in some parallel reality, the sort of people who'd be accidentally or maliciously making home bathroom weight scales or blood pressure cuffs that gave results that are two to ten times too high. Assuming we didn't have a way of quickly double-checking these things.
posted by sebastienbailard at 2:00 PM on March 6, 2020 [26 favorites]

It's 0.03 in the photo, but the OP says that when measuring the microwave, the needle is all the way to the right. OP, please clarify?
posted by cgs06 at 2:01 PM on March 6, 2020

Oh, I missed that part. Yeah can we get a clarification here.

And as far as the meter goes, it looks legit to me. The parent company has a website here: Just because ghost hunters buy it doesn't mean the original product is wacky. It's looks like a standard EMF measurement tool.
posted by JoeZydeco at 2:04 PM on March 6, 2020

Probably not a problem. If you can check readings on another microwave, just to see if it behaves the same way, that would be reasonable, just in case you have a machine where the door is sealed improperly or something. But most likely it's a perfectly normal microwave and a misleading meter result.

As others have mentioned, for microwaves, allowed leakage (in US, and parts of Europe though I don't know which countries exactly) is 5 mW/cm2 at a distance of 5 cm (2 inches). Radiation drops off with the square of distance, so that would only equate to about .2 mW/cm2 at 4 feet, which would be midway on your meter.

However, the first question is what does the meter read when the microwave is off? That particular meter considers everything from 0.1 MHz – 2500 MHz (2.56 GHz) to fall into the radio/microwave range. So it's going to pick up wifi signals from cells phones and modems, and potentially from outside towers if you have a window in the room with the microwave, and possibly other things in that range. So make sure you are considering the differential reading between microwave being on and off.

For the frequency at which most microwaves run (2450 mH), the recommended FCC exposure limit is 1 mW/cm^2 averaged over 30 minutes. Your meter maxes out at 1, and starts indicating high at a quarter of that. So possibly your microwave is way off the scale, but also possibly your meter is designed to make the result look scary.

This is actually pretty common for RF/EMF meters. Looking up radiation exposure on the internet it rather like looking up carcinogens. You can find a certain amount of scientifically valid info, and then whole range of scaremongering data (some of it superficially very technical and valid sounding) designed to convince you that you will die if you don't move immediately to the middle of the woods. A fair number of EMF/RF meters are put out for people who are looking for very low levels of readings either because they are trying to eliminate all sources, or because they are ghost hunting (seriously), and thus they max out and look scary at low levels, or simply because they are mostly used to check EMF with the RF added in with a less useful range.

They can also be rather fiddlier to read than you'd think. They tend to fluctuate a lot if you move them during the reading, or change the angle, etc.

And finally, even if your meter is reading accurate at 1 mW/cm^2 from just the microwave, you are unlikely to be hanging out right in front of it for extended periods of time. If you are worried, just wait a few feet further away while it's running, and you'll be fine. (This is also a good way of figuring out if you are actually over range- how far away do you have to go before the needled drops away from the right of the meter?)

(I hope this is helpful, and not just long. I have done some RMF/EMF surveys for work, so figured I might as well take the opportunity to dump some of the background info.)
posted by Dorothea Ladislaw at 2:07 PM on March 6, 2020 [2 favorites]

Most Panasonic microwaves use "inverter" technology (basically their trade name for a high voltage switch mode power supply to drive the microwave generating magnetron.) The inverter runs at high frequency, allowing the use of a relatively small transformer and the inverter itself can create all kinds of EMF noise which might be confusing the meter. (Conventional microwaves use an "old fashioned" transformer running at 60hz to generate the high voltage for the magnetron and do not generate as much high frequency EMF.)
I would examine the mating surface of the microwave cavity (i.e. where the door touches.) If it's nice and straight & the door closes tightly/ is aligned properly I wouldn't be too concerned. Microwaves do sometimes get bent up in shipping... that is the primary cause of microwave leakage.
posted by Larry David Syndrome at 2:24 PM on March 6, 2020

(The static magnetic field meter they sell is to check compliance with air shipment regulations.)
posted by Huffy Puffy at 2:44 PM on March 6, 2020

Response by poster: Yes when measuring the microwave, the needle is all the way to the right.
posted by falsedmitri at 4:25 PM on March 6, 2020

Try the same test on other microwave ovens and see how they compare. Does this oven's results stand out as different?

Also, what is the ambient reading when the microwave oven is off?
posted by JackFlash at 6:20 PM on March 6, 2020 [1 favorite]

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