[SMCC] can luu y khi dun soi nuoc bang microwave

Vì tầm quan trọng của tin này, Xin quý vị kiểm tra dùm cái tin này có đúng 
không. Rất cám ơn

 

CẦN LƯU Ý KHI ĐUN SÔI NƯỚC BẰNG MICROWAVE OVEN
 

 

Bài này viết bởi một y tá tại Bệnh viện đa khoa khu Laval, Sainte-Foy, Quebec 
Nhiều người không biết!!! 

 

Khoảng năm ngày trước, con trai của tôi 26 tuổi quyết định làm một tách cà phê 
instant. Nó lấy một ly nước và bỏ vào trong microwave để hâm nóng nó lên (cái 
này nó đã làm hoài). Tôi không biết là nó đặt giờ bao nhiêu (phút), nhưng nó 
nói rằng nó muốn hâm cho nước sôi. Khi giờ đã ngưng, nó lấy ly từ lò ra. Lức 
đó, nó có lưu ý rằng nước không sôi. Đột nhiên nước trong ly nổ tung tóe vào 
mặt nó. Cái ly thì còn nguyên vẹn cho đến khi nó liệng đi khỏi tay nó, nhưng 
mặt nó phải đối diện bởi một sự bỏng cháy với cường độ gia tăng. Nó đã bị các 
vết bỏng trên mặt của mình với độ bỏng thứ nhất và thứ hai, mà sẽ có thể còn 
vết thẹo20đáng kể. Hơn thế nữa, nó có thể đã mất đi một phần thị giác của mình 
trong mắt trái. 

 

Khi đến tại bệnh viện, bác sĩ điều trị nó cho rằng, đó là một chuyện thường xảy 
ra và nước (một mình nước) không bao giờ nên được đdun nóng trong lò microwave. 
Nếu nước cần được đun sôi theo cách này, thì phải để thêm cái gì đó vào trong 
ly, như một cây gậy gỗ hoặc một túi trà (không có staples) để giải tỏa năng 
lượng. "Đây là những gì giáo sư khoa học của chúng ta đã phải nói: Tôi đã nhìn 
thấy vụ này trong quá khứ. Điều này được gây ra bởi một hiện tượng được gọi là 
đun nóng quá độ. Nó xảy ra khi nước trong tình trạng sôi, đặc biệt là truờng 
hợp các ly tách mới, chất lỏng nóng lên vượt quá điểm sôi của nó. Khi nó được 
dời đi thì đột nhiên, có một cú sốc đủ để gây ra nhanh chóng tạo ra các bong 
bóng, khiến nước bị trục xuất khỏi ly. Đó cũng là lý do, các bọt hình thành 
bong bóng trong các chất carbonized lỏng (Coke, Champagne) trào ra khi mở nắp. 

 

Xin vui lòng chuyển tiếp thông tin này đến người mà bạn biết để phòng ngừa chấn 
thương và đau khổ cho họ. 

 

Deschesnes Y tá Francine, BSc, trợ lý nghiên cứu lâm sàng ngực Hôpital Laval, 
Sainte-Foy (Quebec)

 

Hope that helps

 

Xin gửi đến các thân hữu để tìm hiểu thêm về vấn đế trên – (Tuấn)

 

A 26-year old man decided to have a cup of coffee. He took a cup of water and 
put it in the microwave to heat it up (something that he had done numerous 
times before). I am not sure how long he set the timer for, but he wanted to 
bring the water to a boil. When the timer shut the oven off, he removed the cup 
from the oven. As he looked into the cup, he noted that the water was not 
boiling, but suddenly the water in the cup "blew up" into his face. The cup 
remained intact until he threw it out of his hand, but all the water had flown 
out into his face due to the build up of energy. His whole face is blistered 
and he has 1st and 2nd degree burns to his face which may leave scarring.

He also may have lost partial sight in his left eye. While at the hospital, the 
doctor who was attending to him stated that this is a fairly common occurrence 
and water (alone) should never be heated in a microwave oven. If water is 
heated in this manner, something should be placed in the cup to diffuse the 
energy such as a wooden stir stick, tea bag, etc., (nothing metal).

It is however a much safer choice to boil the water in a tea kettle.

General Electric's Response:

Thanks for contacting us, I will be happy to assist you. The e-mail that you 
received is correct. Microwaved water and other liquids do not always bubble 
when they reach the boiling point. They can actually get superheated and not 
bubble at all. The superheated liquid will bubble up out of the cup when it is 
moved or when something like a spoon or tea bag is put into it.

To! prevent this from happening and causing injury, do not heat any liqui d for 
more than two minutes per cup. After heating, let the cup stand in the 
microwave for thirty seconds! before moving it or adding anything into it.

Here is what our local science teacher had to say on the matter: "Thanks for 
the microwave warning. I have seen this happen before. It is caused by a 
phenomenon known as super heating. It can occur anytime water is heated and 
will particularly occur if the vessel that the water is heated in is new, or 
when heating a small amount of water (less than half a cup).

What happens is that the water heats faster than the vapor bubbles can form. If 
the cup is very new then it is unlikely to have small surface scratches inside 
it that provide a place for the bubbles to form. As the bubbles cannot form and 
release some of the heat has built up, the liquid does not boil, and the liquid 
continues to heat up well past its boiling point.

What then usually happens is that the liquid is bumped or jarred, which is just 
eno! ugh of a shock to cause the bubbles to rapidly form and expel the hot 
liquid. The rapid formation of bubbles is also why a carbonated beverage spews 
when opened after having been shaken."

If you pass this on you could very well save someone from a lot of pain and 
suffering.

Thank You Very Much.

 

 

Mời các thân hữu vào link dưới đây để đọc thêm:

 

http://www.snopes.com/science/microwave.asp

 

Risk of Burns from Eruptions of Hot Water Overheated in Microwave Ovens
The FDA has received reports of serious skin burns or scalding injuries around 
people's hands and faces as a result of hot water erupting out of a cup after 
it had been over-heated in a microwave oven. Over-heating of water in a cup can 
result in superheated water (past its boiling temperature) without appearing to 
boil.

This type of phenomena occurs if water is heated in a clean cup. If foreign 
materials such as instant coffee or sugar are added before heating, the risk is 
greatly reduced. If superheating has occurred, a slight disturbance or movement 
such as picking up the cup, or pouring in a spoon full of instant coffee, may 
result in a violent eruption with the boiling water exploding out of the cup.

What Can Consumers Do to Avoid Super-Heated Water?

  a.. Follow the precautions and recommendations found in the microwave oven 
instruction manuals, specifically the heating time.
  b.. Do not use excessive amounts of time when heating water or liquids in the 
microwave oven.
  c.. Determine the best time setting to heat the water just to the desired 
temperature and use that time setting regularly. 
For Additional Information:

  a.. FDA Radiological Health Program: Microwave Ovens 
 

 

   

Superheating and microwave ovens
Joe Wolfe 
School of Physics, The University of New South Wales, Sydney. 

 

There have been many reports of injury to people using microwave ovens to heat 
water to make hot drinks. Water heated in a microwave oven may be superheated 
and when objects (e.g. a spoon) or granulated materials (e.g. instant coffee) 
are put into it, the water may boil very vigorously or even appear to explode 
out of the container. The vigorously ejected boiling water can cause serious 
burns. Sometimes even the act of taking the container out of the oven and or 
putting it on the bench can cause the boiling.

  a.. Download QuickTime movie (150k) of superheated water boiling on addition 
of coffee powder.
  b.. Same movie in mpg (380k).
 

  When does superheating happen? 

  How to avoid it 

  What is superheating? 

  Why is it dangerous? 

  Why does it occur to a greater degree in microwave ovens than in saucepans 
or kettles? 

  Why is it possible to heat water above its boiling temperature? 

  Some quantitative details 

  More about boiling and freezing temperatures

  Other potential dangers associated with microwave ovens

When does it happen?
The following conditions promote these potentially dangerous events: 

    a.. Using a container with a very smooth surface, such as an unscratched 
glass or glazed container.
    b.. Heating for too long.
    c.. Quickly adding a powder, such as instant coffee (or sometimes even an 
object to stir it).
    d.. Standing with one's face above the container makes injury more likely.
How it could be dangerous: You put water in a new mug (one that has no cracks 
in the internal glazing and which has never been scoured). You put it in the 
oven with a setting that is a little too long for the amount of water. While it 
is heating the phone rings. You return some time later, decide to reheat it, so 
you restart the oven. You take out the cup and immediately add a spoonful of 
instant coffee. The water boils vigorously, throwing boiling water over your 
arm and face.

How to avoid it
    a.. Before putting the water into the oven, insert a non-metal object with 
a surface that is not smooth. (e.g. a wooden stirrer. A wooden skewer or 
icecream stick will do.)
    b.. Use a container whose surface is at least a little scratched.
    c.. Do not heat for longer than the recommended time for the quantity of 
water used.
    d.. Tap the outside of the container a few times with a solid object while 
it is still in the oven. Use a long object so that your hand remains outside 
the oven. Alternatively, and still keeping your hand outside the oven, insert a 
stirrer while the container is still in the oven. (Thus, if vigorous boiling 
occurs, most of the boiling water will strike the inside of the oven.)
    e.. Keep your face well away from the open oven door and from the container.
All these precautions should reduce the chance or extent of superheating and 
resultant injury. Nevertheless, very hot water is always dangerous and one 
should always treat it with caution.

What is superheating?
In this context superheating means the heating of a liquid to a temperature 
above its normal boiling point. The superheated state is unstable, and it can 
very rapidly turn into liquid at the boiling point, plus a substantial quantity 
of vapour.

Why is it dangerous?
If one litre of water is superheated by only 1 °C (ie if it is heated to 101 °C 
without boiling), it is in an unstable state, and it can suddenly produce about 
3 litres of steam. The rapid production of a substantial quantity of steam 
within the bulk of the water will cause it to boil vigorously and possibly to 
appear to explode. The result is boiling water flying at speed out of the 
container.

Why does it occur to a greater degree in microwave ovens than in saucepans or 
kettles?
In a microwave oven, the water is usually hotter than the container, whereas 
parts of the kettle or saucepan are usually hotter than the water. Further, the 
surfaces of some containers used in microwave ovens may be very smooth, almost 
at a molecular scale, whereas this is not true for kettles or saucepans.

Microwave ovens heat the water directly: the microwaves pass through the 
container and the water, and the water itself absorbs energy from them. In a 
kettle or saucepan, the container itself (saucepan) or a heating element (some 
kettles) is hotter than the water. The hottest points cause a small amount of 
local superheating, boiling is initiated here, and this then stirs the water.

Why is it possible to heat water above its boiling temperature?
Let's talk only about pure water, and only water at or close to atmospheric 
pressure.

At the surface between air and water, or between steam and water, water boils 
at 100 °C. Water boils at 100 °C if there is already a bubble of steam (or air) 
present. But in the absence of bubbles, water can be heated above 100 °C. There 
are two reasons. First, to make a stable bubble, a lot of water molecules in 
the same small area must form steam. This is improbable. Second, it takes extra 
energy to form the bubble itself: energy to push the water out of the way, and 
energy to make the surface between water and steam. Once a bubble forms (a 
process called nucleation), it is easy to increase its size. So the superheated 
water nearby evaporates very quickly, producing a large volume of steam.

Smooth containers do not have bubbles of air clinging to their sides. Rough 
walled or scratched containers may hold microscopic bubbles in their cracks. 
These become nucleii for boiling. Even a crack that is fully filled with water 
can be a boiling nucleus because it reduces the required area of the 
water-vapour surface.

Some quantitative details
The latent heat of vapourisation of water is L = 2.23 MJ/kg. This means that it 
takes 2,230,000 Joules of heat to evaporate 1 kg of water at 100 °C and at 
normal atmospheric pressure. (One kilogramme of water is about one litre.)

The specific heat capacity of water is c = 4.2 kJ/kg. This means that it takes 
4,200 Joules of heat to raise the temperature of 1 kg of water by 1 °C.

Suppose that we heat one kilogram of water from 100 °C (its normal boiling 
temperature) to 101 °C, i.e. it is now superheated by 1 °C. When it begins to 
boil, it will very quickly cool to 100 °C, and the heat liberated turns water 
into steam. Cooling this kg of water by 1 °C gives 4.2 kJ, which is enough to 
evaporate c/L = 4200/2230000 kg of water. This is only 1.9 millilitres of 
water, which does not sound very much, but it turns into 3 litres of steam. 
Those three litres of steam are created insidethe hot water, quite suddenly, so 
the water is ejected violently from the container.

More about boiling and freezing temperatures
The temperature for equilibrium boiling and freezing (ie boiling and freezing 
in the absence of superheating or supercooling) is affected by pressure and 
composition. Add a solute to the liquid phase, and you usually depress the 
freezing point and raise the boiling point. Increase the pressure and the 
boiling temperature rises, while the freezing temperature changes by a small 
amount: downwards for water and upwards for almost everything else. A 
non-technical explanation is given on Boiling and freezing: the effects of 
solutes and of pressure. 


--------------------------------------------------------------------------------

Other potential dangers associated with microwave ovens
Sparks and fires
Electrical conductors, such as aluminium foil, cutlery, even gold leaf on 
plates, should not be used in microwave ovens. These conductors concentrate the 
electric field and so can produce sparks. The sparks in turn may cause a fire. 

    a.. Don't put metals in the microwave oven
Uneven heating
Partly because the electromagnetic field in a loaded oven is not uniform, and 
partly because they heat quickly, microwave ovens can give rise to uneven 
heating. When we take food out of a normal oven, we expect the outside to be at 
least as hot as the inside. With a microwave oven, the reverse is often true: 
the liquid inside is hotter than its container. Users may underestimate the 
temperature and thus cause burns. 

    a.. Be aware that high temperatures may be present, even if the container 
is only warm
Steam in sealed continers
A gram of steam at 100°C contains more heat than a gram of boiling water, and 
so may produce more severe burns. If food is heated in a sealed container, this 
can produce steam inside the container. When released, the steam, perhaps under 
a small pressure excess, may burn the user. 

    a.. Don't heat things in sealed containers 
    b.. If the instructions say 'leave to cool', do so.
Microwave leakage
Microwave ovens are usually made of metal, and there is a metal screen in the 
door. An automatic switch turns the oven off when the door is opened. Thus, 
when microwaves are radiated into the oven, the volume is almost completely 
surrounded by electrical conductor, which means that the microwaves are 
reflected rather than being transmitted to thespace outside. ('Almost' is there 
because the screen in the door usually has small holes to allow the user to 
look in. These holes are much smaller in size than the wavelengths used and so 
very little energy is radiated through these holes.)

If it were possible to turn the oven on with the door open, a beam of 
microwaves would radiate through the open door. This beam would be dangerous: 
it would rapidly heat up human tissue (just like cooking meat) and could have 
other health effects as well. 

    a.. Never disconnect the automatic safety switch 
    b.. If the door or the case of the oven are damaged, either discard the 
device or have it checked for microwave leakage. 
 

Opinions expressed in these notes are mine and do not necessarily reflect the 
policy of the University of New South Wales or of the School of Physics. 


--------------------------------------------------------------------------------

Some sites with related material

  a.. How Things Work: Microwave Ovens by Prof Louis A. Bloomfield, University 
of Virginia
  b.. What is 'unfreezable water'? Supercooling, rather than superheating, is 
sometimes involved.
  c.. Despite its ubiquity, water is a remarkable and unusual chemical. See 
Martin Chaplin's page: Water structure and behaviour. 
  d.. Boiling and freezing: the effects of solutes and of pressure. 
 

Tran Ba Thien
tranbathien@xxxxxxxxx

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