Sugar thermometers

[Richard Tobin]

What is the liquid used in a traditional-style sugar thermometer?
It's not mercury, and the temperatures are well above the boiling
point of alcohol.

-- Richard

[Bertie Doe]

Post by Richard Tobin
What is the liquid used in a traditional-style sugar thermometer?
It's not mercury, and the temperatures are well above the boiling
point of alcohol.

Sometimes referred to as a 'spirit' thermometer. Most common indicators
(with their boiling points) are :-

Toluene (BP 111 C)
Iso-amyl acetate (142 C)
Kerosene (200 C)

https://chemistry.stackexchange.com/questions/91619/what-is-the-blue-spirit-or-red-spirit-in-mercury-free-thermometers/91626

If swallowed, I'd guess the glass would cause the most damage, rather than
the chemicals.

I've got a glass jam thermometer. Awkward to use, even when wearing oven
gloves. I prefer to use the digital thermometer (£12) with a 2 mm thick
stainless steel K type thermocouple probe (£6 eBay).

I have a heavy s/s pan with a glass lid. I poke the probe thru' the steam
vent in the glass lid. The trick to making jam quickly is NOT to bulk boil.
Heat about 1lb / 500gr at a time.

[Richard Tobin]

Post by Bertie Doe
Sometimes referred to as a 'spirit' thermometer. Most common indicators
(with their boiling points) are :-
Toluene (BP 111 C)
Iso-amyl acetate (142 C)
Kerosene (200 C)

Thanks.

The sugar thermometer I have is labelled up to 220 C.

Perhaps it relies on the pressure in the sealed column to keep it
liquid?

-- Richard

[Bertie Doe]

Post by Richard Tobin

Post by Bertie Doe
Sometimes referred to as a 'spirit' thermometer. Most common indicators
(with their boiling points) are :-
Toluene (BP 111 C)
Iso-amyl acetate (142 C)
Kerosene (200 C)

Thanks.
The sugar thermometer I have is labelled up to 220 C.
Perhaps it relies on the pressure in the sealed column to keep it
liquid?

There's also a gas inside a glass thermometer but I can't remember what type
or it's purpose.

[Brian Reay]

Post by Richard Tobin

Post by Bertie Doe
Sometimes referred to as a 'spirit' thermometer. Most common indicators
(with their boiling points) are :-
Toluene (BP 111 C)
Iso-amyl acetate (142 C)
Kerosene (200 C)

Thanks.
The sugar thermometer I have is labelled up to 220 C.
Perhaps it relies on the pressure in the sealed column to keep it
liquid?

There are a couple of factors which determine when a liquid boils. One
is the Vapour Pressure and, in simple terms, the 'smoothness' of the
vessel.

My memory is a bit rusty but the Vapour Pressure factor is (mainly) a
surface effect. As the temp rises and more vapour is formed (it is a
two way process, vapour is formed and it also condenses back to liquid)
than is condensing, if you confine the vapour, it increases the Vapour
pressure which, in simple terms, makes it more difficult for vapour to
be formed.

Also, normally when a liquid boils in a container, you get bubbles. The
pressure in a bubble is related to the inverse of the radius (possibly
r^3- I can't recall). That means, in theory, a 'new' bubble has an
infinite pressure inside. In practice, bubbles actually form around
'rough spots' in the container and the initial r isn't zero. A very
smooth bulb in the thermometer would therefore help prevent bubbles
forming.

[graham]

Post by Richard Tobin
What is the liquid used in a traditional-style sugar thermometer?
It's not mercury, and the temperatures are well above the boiling
point of alcohol.
-- Richard

Mine has mercury in it. I don't use it any more as it's too difficult to
read. I use a Thermapen instead. I often make Italian meringue and it's
critical to start whipping the egg whites when the syrup reaches a
certain temperature, just before the softball stage.