Calibrating a Thermapen in Boiling Water

[Shel_B]

After a few months of hesitancy, I splurged and purchased a Thermapen which arrived yesterday.  The calibration instructions say not to test the boiling point of water using water heated in a microwave oven, but to only calibrate using water heated on a stove burner.  Why is that?

 

I did check the boiling point using microwaved water, and, SURPRISE! the temp of the boiling water was greater than 212-degrees.  I recall 217-degrees. Assuming the new Thermapen is accurate (and other tests suggest that it is) how can the boiling point of water be greater than 212-degrees?  And why when heated in a microwave oven?

[dcarch]

After a few months of hesitancy, I splurged and purchased a Thermapen which arrived yesterday.  The calibration instructions say not to test the boiling point of water using water heated in a microwave oven, but to only calibrate using water heated on a stove burner.  Why is that?

 

I did check the boiling point using microwaved water, and, SURPRISE! the temp of the boiling water was greater than 212-degrees.  I recall 217-degrees. Assuming the new Thermapen is accurate (and other tests suggest that it is) how can the boiling point of water be greater than 212-degrees?  And why when heated in a microwave oven?

 

 

Super heated water (more than 212F) can be dangerous. Super heated water is not easy to make. You need pure water in a very smooth vessel and no vibration.

 

dcarch

Edited December 9, 2014 by dcarch (log)

[Tjex]

A microwave can superheat water and this is the reason it's not suggested as a calibrating method.

 

Side note, boiling water temp can very depending on altitude. For example, on Everest it's 160F, at the dead sea shore it's 214F.

[KennethT]

I don't know if this is actually the reason why, but one thing I have found is that you can get water very hot in a glass vessel (say a measuring cup) in the microwave since (my theory is) the glass is very smooth and doesn't provide nucleation sites for vapor bubbles. So, you can get the water hotter than 212, and then, if you put something in the vessel that is rough (say a toothpick or something), the water will boil EXTREMELY vigorously (think volcano) until the temp drops to normal boiling point. When boiling on a stove burner, typically, your pan will not be as smooth as a glass container and will have no problems with lack of nucleation sites.

Conversely, I think about an explanation the TSA (I think it was them - maybe an international body?) gave regarding the downing of the Air France flight over the Atlantic enroute to France from Brazil. They showed that extremely pure water can stay liquid at much colder temperatures than 0C if it is kept in a very smooth vessel. As soon as 1 ice crystal is introduced, the whole container spontaneously froze and turned solid. Really cool to watch - but terrible for that plane and all the people in it.

[gfweb]

The other way to start the ice in supercooled water is to tap the vessel lightly... ice spreads from the tap in a moment. Doesn't even have to be pure water. I've had it happen about a hundred times when I was in the lab.

[Shel_B]

OK, super heated water ... thanks.

Edited December 9, 2014 by Shel_B (log)

[Shel_B]

Super heated water (more than 212F) can be dangerous. Super heated water is not easy to make. You need pure water in a very smooth vessel and no vibration.

 

dcarch

 

Hmm ... I attained 217-deg with basically just tap water (not pure water, if by pure water you mean absent any impurities), and there was plenty of vibration as the microwave table rotated.  I'm assuming the Pyrex bowl in which the water was heated is a "smooth" vessel.

[Martin Fisher]

You must account for barometric pressure and altitude to get a reasonably accurate calibration.

 

http://www.thermoworks.com/software/bpcalc.html

[dcarch]

Hmm ... I attained 217-deg with basically just tap water (not pure water, if by pure water you mean absent any impurities), and there was plenty of vibration as the microwave table rotated.  I'm assuming the Pyrex bowl in which the water was heated is a "smooth" vessel.

 

That is a very significant deviance.

 

It is possible if the water is not pure. Heavily salted water can be that high in boiling point.

 

Try again. If you read over 212F again then the thermometer should be returned.

 

dcarch

[Shel_B]

That is a very significant deviance.

 

It is possible if the water is not pure. Heavily salted water can be that high in boiling point.

 

Try again. If you read over 212F again then the thermometer should be returned.

 

dcarch

 

No salt in the water - just plain, filtered tap water.  I will try again later with water boiled in a pot on the stove, as recommended by ThermoWorks.

[Shel_B]

You must account for barometric pressure and altitude to get a reasonably accurate calibration.

 

http://www.thermoworks.com/software/bpcalc.html

 

I used their software ... my elevation is about 55 feet above sea level.  Essentially a non issue.

[dcarch]

I am sure this was not what happened, just in case:

 

When you measure, do not let the tip of the probe touch the bottom of the metal pot. You can be measuring the metal temperature.

 

The very tip is where the temperature senor is located.

 

dcarch

[scubadoo97]

I'm at sea level and every Thermapen I own reads 212 with boiling water and 32 in a glass full of ice and water. It's rare that one would be so off

[Shel_B]

I'm at sea level and every Thermapen I own reads 212 with boiling water and 32 in a glass full of ice and water. It's rare that one would be so off

 

When I did the boiling water test by boiling the water in a saucepan on the stove, instead of in the microwave, the Thermapen registered within 2/10ths of a degree +/- depending on where in the pot the probe was inserted.  It registered 212-deg in some spots as well.

 

The ice water test, once I got the technique down, was accurate also.

Edited December 10, 2014 by Shel_B (log)

[AlaMoi]

Shel -

 

yuppers.  a boiling pot of water has more than one temperature.

 

.....as does a pot of oil for deep frying, etc etc etc.

 

the ThermaPens are extremely good gear.  I picked up a Chef Alarm on sale - it's another superior piece of gear.

tastes like xrap; works great!

[daveb]

Thermoworks (strongly) suggests that if the Thermapen is "on" with an ice bath it will be accurate through the rest of the spectrum.  I like to check the boiling point as well and make sure I'm moving the probe slowly through the water column while doing so.  I've never had one be off at either end.

[rotuts]

"""  Super heated water (more than 212F) can be dangerous. Super heated water is not easy to make. You need pure water in a very smooth vessel and no vibration. ""

 

its very easy to make ( and hurt yourself if you are not aware of it )

 

I make it every morning w tap water and a pyrex glass measuring item

 

for my coffee.  Im very careful to tough the water w a looooooooong spoon and stir before I add the coffee grounds

 

the water looks a bit cloudy before it explodes.

 

PS  this is done in the microwave, forgot that important part !

Edited December 11, 2014 by rotuts (log)

[Shalmanese]

"""  Super heated water (more than 212F) can be dangerous. Super heated water is not easy to make. You need pure water in a very smooth vessel and no vibration. ""

 

its very easy to make ( and hurt yourself if you are not aware of it )

 

I make it every morning w tap water and a pyrex glass measuring item

 

for my coffee.  Im very careful to tough the water w a looooooooong spoon and stir before I add the coffee grounds

 

the water looks a bit cloudy before it explodes.

 

PS  this is done in the microwave, forgot that important part !

Instead of potentially scalding yourself every morning, just put a toothpick in with the water when you microwave. You can even reuse the same toothpick!

[gfweb]

Kind of takes the romance out of it.

[rotuts]

Alton Brown on the Coffee show a while back used a chop stick

 

Im very very careful

 

still, id like to know how this happens and why the water looks a bit murky and 'thick' before it explodes

[paulraphael]

We'll need another thread on how to calibrate your altimeter / barometer.

 

Maybe with a thermapen?

[btbyrd]

Alton Brown on the Coffee show a while back used a chop stick

 

Im very very careful

 

still, id like to know how this happens and why the water looks a bit murky and 'thick' before it explodes

It happens because there are no nucleation sites in the container for bubbles to form and the water to convert to gas. When you stick something in the water, all of a sudden it's possible for bubbles to form (and they do en masse). The glass probably looks murky because there are thousands of tiny bubbles suspended in it.

[rotuts]

interesting

 

however, with respect, 

 

""    there are no nucleation sites in the container for bubbles to form ""

 

and

 

""  looks murky because there are thousands of tiny bubbles suspended in it. ""

 

dont seem to go hand-in-hand

Edited December 12, 2014 by rotuts (log)

[btbyrd]

I misunderstood the setup... I was thinking that clear water turned murky for a moment after you put the spoon in, but before it exploded.

[rotuts]

fair enough.  it looks different before it gets spooned.  and some times it does have a rare lazy bubble in there too

 

then   .................... w the spoon which Im very careful with it 'explodes'

 

it would be a poorly informed Coffee Aficionado to dump the grinds in pre-spoon-tap !