A couple of mechanical solutions that may or not be applicable,
depending on your application:

1) put an inverted funnel in the solution, to make a float switch.
when rising vapor bubbles begin to accumulate under the
funnel, the switch is activated

2) in a quiet, vibration free environment, listen.  When boiling
is about to occurr, you will hear high frequency noise ~1000hz
for water.  A rolling boil is louder, and at a lower frequency


----------
> From: Leo van Loon <sbb.simpeltron@TIP.NL>
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: HOW TO MEASURE BOILING POINT
> Date: Saturday, March 21, 1998 11:14 AM
>
> Hi Jon,
>
> In chemistry, the boiling point is measured in the vapour phase. When
there
> is liquid present on a thermometer in the vapour, you measure the boiling
> point of the liquid. When you try to measure the boiling point in the
liquid
> phase you have to cope with problems as 'boiling delay' (i don't know the
> exact english word for it). Clean liquids tend not to boil at
temperatures
> far above boiling point. A little disturbance causis the start of boiling
> with a little explosion.  (try the experiment with a little bit of water
in
> a clean glass in the microwave....)
> Boiling the liquid and measuring the vapour temperature above until it
> stabilises must be easy to automate?!
>
> Leo van Loon
> SBB simpeltronics
> Netherlands
> tel +31 (0481) 450034
> fax+31 (0481) 450051
> mail sbb.simpeltron@tip.nl
> url http://www.sbb-simpeltronics.nl
> SBB simpeltronics ontwikkelt technische projecten voor basisschool en
> basisvorming.
> SBB simpeltronics develops technical projects for children in primary and
> secondary education.
>
>
> -----Oorspronkelijk bericht-----
> Van: PHXSYS <PHXSYS@AOL.COM>
> Aan: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
> Datum: zaterdag 21 maart 1998 17:04
> Onderwerp: HOW TO MEASURE BOILING POINT
>
>
> >Hello everyone,
> >
> >I have a challenge!
> >
> >I need some ideas for measuring the boiling point of various fluids. The
> >specific gravity of one fluid is approx 1.03, very close to water. The
> >accuracy I am looking for is +/- 3 %. I am looking for creative options,
> the
> >simplier the better.
> >
> >The boiling points may be as high as 500 degress F. A very small sample
of
> >fluid will be tested 3-10 ml. I believe I will use a small electric
heating
> >probe (hot wire) to quickly raise the temperature. The tesing time
should
> be
> >between 30-60 seconds. The higher the boiling point the longer the test.
> >
> >Can I look for the presence of vapor is a closed system; measure the
> current
> >of the probe and watch for reaction at boiling point, measure pressure
> changes
> >or pulsations as the fluid boils?
> >
> >I would appreciate any code snippits that help. Thanks in advance.
> >
> >Jon
> >
> >