A colleague has just brought to my attention an article where a chap had boiled a cup of water in a microwave, taken it out where it then "blew up" into his face, a phenomenon known as super-heating. We have microwave ovens in our rest-rooms and kitchen used by staff to heat up food etc. Do I need to go through the process of a risk assessment for this equipment? Signage with do's and don'ts?? Where does it end? Grateful for your thoughts.

Dave,

This used to be a problem, but I am not sure that it still is. Do you know the date of the article?

I didn't think it happened with water, but with more viscous liquids such as a soup.

I personally experienced it many years ago (with canned tomato soup), but have not seen or heard of it happening any time recently.

(I do lead a sheltered life though...)

Alan

Thanks everyone - have downloaded a couple of user manuals after having looked at quite a few; several of which don't even mention the superheating phenomenon though all make usual references to over-cooking and other risks/hazards along with safety advice. Going to follow this up though have already had several bright sparks now referring to other pieces of equipment - what if I leave the grill on or touch a hot iron etc etc - can we have instruction in that etc??!! Got to have a GSOH in this job!

The laws of physics haven't changed so I don't see why it might have become no longer possible?

The thing about a hot iron or leaving a grill on is that, as adults, we have prior experience and we should know what is likely to happen.

Superheating water and having it erupting out of a cup is something that many people will not have encountered and therefore they do expect it. Coupled with that, the injuries can be quite severe as it is boiling water that is erupting.

And yes, of course it can still happen. The physics that causes this phenomenon has not changed.

p.s. that's the best reply to any question i've seen in ages!

ahhh like mentos and cola!
Thanks Jane, my Chemistry skills are rusty (or should I say oxidised) these days :)

Digressing just slightly

There are occasions when there is a need to spell out what should and what should not be done when using a microwave oven.

Before I retired I worked in the HE sector and in total we had about 400 microwave ovens in student kitchens

Instructions were posted, as over a number of years there were a number of incidents.
These ranged from:

Trying to dry trainers and other clothing in the oven
Cooking eggs without piecing the shell, this produces a very messy oven
Over heating body wax
Metal objects placed in the oven

And the favourite in the lab was to use a microwave to prepare agar and despite being trained, forgetting to ease the stopper before heating.

I suspect Jane can remember Medicine Flats being used for agar preparations before most Unis banned there use for this purpose

John

I loved the memories that prove the 4th law of opportunity. "If someone can, sooner or later someone will."

I bow to the superior scientific knowledge on this subject but, if this only associated with boiling water, isn't the simplest approach just a note saying
"Remember it is a much safer choice to boil water in the kettle. Why? Ever heard of superheating? Want to know more then call the safety dept for info". Then have a link to one of the many videos showing the effect to refer people to watch.
Stick the note on the microwave for week or so then remove it and replace again in 6 months time.

Risk assessment? Yes of course since there is a hazard. But written and recorded? I wouldn't.

Completely not a H&S response here but - I hate microwaves!!

I have the simplest one that gets primarily used for those 'oh my god I forgot to defrost the meat' moments.

Baked potatoes just go soggy
Soup explodes everywhere
The bowel gets so hot you can't pick it up
And in most workplaces they are the filthiest item in the place

Bring back cookers and kettles!!!!!!

Sounds like you need the instruction sheet we used to give to Students :-)

lol :-) Sorry about that typo!!

I have mentioned this some years ago but don't ask me where I read it, my memory is just so bad. In the early days of microwave cookers a lady, I think in USA, had a leaky micro wave door seal and over a period of time actually cooked her insides. I think including her bowel - so not such a bad typo after all.

By the way have you noticed that the turntable spins one way and if you stop and start it again it goes the other direction?

I'm sure Paul will let us into the secret.

That's a new one on me, but I have heard that microwave cooking was invented after the early experiments with radar were found to be cooking seagulls that flew through the beam. I'm not sure I believe it - based on the fact it takes 2 minutes to heat 450 g of baked beans in a microwave designed for the purpose, how long would a seagull have to stay in the beam? Mind you, I WANT it to be true...



It's because constantly rotating in one direction, within the strong magnetic field used to generate the microwaves, causes an electric charge to accumulate in the rotation mechanism which is discharged during contra-rotation (or more accurately, neutralised by generating an opposing charge). If the charge isn't dissipated, it can build to a level that blows the oven door off.

DISCLAIMER: The previous two sentences were entirely made up and have no basis in fact. I must still be in Friday mode...

Ladies and gentlemen I urge you not to try this at home

Poor bee :-(

"That's a new one on me, but I have heard that microwave cooking was invented after the early experiments with radar were found to be cooking seagulls that flew through the beam. I'm not sure I believe it - based on the fact it takes 2 minutes to heat 450 g of baked beans in a microwave designed for the purpose, how long would a seagull have to stay in the beam? Mind you, I WANT it to be true..."

I don't think it was from flying through the beam once but the fact they sat there for some time so it was accumulation over a long time period that their inside where being cooked. From what i heard it's true this is where microwaves where born

Just to add not putting plates in with metal edging in as well. also don't bring back cookers or toaster we have enough false alarms as it is after people have overcooked their food or just left the oven on all night for no obvious reason.

Phil

Red

Not had chance to contact my microwave comms expert.
However, are we sure that the turn tables are definitely reversed under the control of the oven? This may be possible, but, it may also add a level of cost and complexity that the manufacturer may not want to add.
However, the motors in common use are value engineered (read cheap!).
Some of these do not have a defined direction of rotation and the direction is random at power up. Thus the turntable could turn in either direction depending on the exact stop position and the point in the a.c. voltage wave it is powered up.
Just done a quick experiment with our med price/brand oven and on 4 simultaneous starts it continued in the same direction as the previous start. On the rest it reversed direction. I did about 12 starts (ish). I said a bit of an experiment.
BTW the turn table is not necessary on all designs. It is there to ensure that the food stuff is exposed to as even a radiating as possible.
The same can be achieved with a rotating antennae.
Both are an attempt to minimise hot and cold spots in the food stuff, hence why we are told to rest food for a few minutes after microwave cooking for the temperature in the food to normalise throughout as much as possible.