I don't get it

[Drug_Phreak]

Why are most Hotplates still analog in this day and age of technology? Most thermometers are still analog too. Aren't analog thermometers kind of hard to read and get an exact temperature from? A lot of chemistry experiments are very exact and having analog devices would make it even harder to be exact... wouldn't they? On most hotplates the dial is numbered 1-10 or 10-100 or something similar. Is there some kind of special way to tell exactly what the temperature is going to be based on where the dial is? Isn't it a pain the ass to use an analog hotplate and thermometer when you are trying to distill something that only comes over between a few degrees? Or at any degrees at all for that matter... it must be easy to turn it up too high and possibly ruin a product or create some dangerous situation etc. Even if you don't ruin your product you will probably have other things besides your product in the flask now too. I just don't get it... why still analog? I guess it's all about saving money. I'm thinking about getting one of those digital stirrer/hotplates that can be programmed. Ony for making breakfast of course.  

[zero_nrg]

Most hotplates are analog because the device responsible for varying the voltage is cheaper to manufacture than its digital counterpart.  What happens to water if you try to heat it beyond its´ boiling point? - it just turns into vapor at a faster rate.  Same thing when distilling chemicals in a flask.  Plug your hotplate into a Variac (varies voltage)if you need more precise temperature control.  Analog thermometers are accurate enough when used correctly and they can even be calibrated(search the web). I sometimes use an RTD (resistance temp. device) probe for distillations. When connected to other circuitry I get a digital readout of the temp.  However, I always like to confirm its´reading by comparing with an analog thermometer when possible (I don´t trust digital as much). IMHO there is no big advantage to digital for the average clandestine chemist.

[superman]

so lets say ur hotplate shows 105c.   put a liter of h2o on it may boil.   put 2 liters on it may not. see?

[zero_nrg]

Two liters H2O on the hotplate takes longer to reach the boiling point than 1 liter.  Perhaps I don't understand your question?

[Drug_Phreak]

Ok... I guess I'll get an analog and digital thermometer and hotplate. Then I'll have something to double check with and it's always good to have a backup.

[r2e3]

with the variation in atmospheric pressure or vacuum (ie they are never constant from what i can tell), either you have to have a high price tag rig or make do (us the home chefs )

[ApprenticeCook]

zero_nrg,

i dont think super'dud'man knows what he is on about...
If your hotplate thermostat is set at 105^oC the hotplate surface (ie the metal) will get to 105^oC and the heater element will click off, then come back on when its below the temp. If there is something on top that your trying to heat, ie a bottle of water, the water will abosrb the heat and therefor the hotplate wont be at 105^oC where it cuts out until the water above it is at 105^oC, this is plain thermodynamics, but if superman wants to disagree with some of the most important foundation theories of chemistry, well.....

If you put 1L of water on the hotplate and let it get to the point where the thermostat clicks off the hotplate at 105, it will be boiling...
If you put 2L of water on the hotplate and let it get to the point where the thermostat clicks off the hotplate at 105, it will be boiling...
If you put 10000L of water on the hotplate and let it get to the point where the thermostat clicks off the hotplate at 105, it will be boiling...

You are right, obviously the more water there is the longer it will take to heat to the thermostat temp of 105^oC therefore the longer it will take to boil.

So wheres super'dud'man's point? i dont think he had one.
-AC

[Snakebyte]

Actually, if you set your digital hotplate to 105 it won't boil.  At least Swim's doesn't.  In fact, depending what liquid Swim's boiling his hotplate will vary dramatically on what temp he needs it to boil.  My guess is the properties of the liquids including their heat conductivity.  For eg, his ketone comes over at roughly 100 but his hotplate will be set to about 180 for the ketone to get to 100.  In my opinion the digital temp gauge is not of any use. And as far as the digital speed control...who cares?  More problems can happen to digital equipment like Swim's hotplate screen is already having problems!

[ApprenticeCook]

My guess is the properties of the liquids including their heat conductivity
Its called heat capacity of the material.

For eg, my ketone comes over at roughly 100 but my hotplate will be set to about 180 for the ketone to get to 100
No what your doing is speeding it up... if you set it to just above the bp of what you have ontop it will heat the material up at a certain speed (depending on the heat capacity of the material) this speed will be initially quick but as the temp increases the rate will slow down... sometimes to a stop when you take into account cooling effects, heat transfers in the glassware (across the disto equip).
If you set the plate at 180 your extending the quick heat up part of the temp graph as described above... so it heats up really quick to where you want.
Fair enough if you set it at 100 it wont get the water inside to 100 unless its at 100% effectivness (which they never are) due to cooling effect of the glass etc... loss of energy other than to the water in the flask.
So by setting it to 180 your heating it quicker and accounting for the far less than 100% effectivness of the hotplate, glass, etc.

-AC

[placebo]

Why not just get something like this it has a temperature probe so the hotplate is thermostatically controlled to whatever temp you set. They work fantastic. Set and forget.

[starlight]

i dont think super'dud'man knows what he is on about...
If you put 10000L of water on the hotplate and let it get to the point where the thermostat clicks off the hotplate at 105, it will be boiling...

Oh dear, I do wish people would not spout off about stuff so knowledgeably when they are mistaken.

Practical experience should tell you that the temperature attained by a container of water on a hot plate will, to some extent, depend upon its size. Containers are not perfect insulators, and in addition heat is lost to the environment from the top of the liquid being heated. Heat transfer between the hotplate and the container is also less than perfect. As the surface area of the container/liquid being heated increases, the amount of heat lost increases. At some water volume, the hotplate at 105C is unable to transfer as much heat into a container of water at 100C as the container loses. This point is way less than 10,000L - try it and see. It varies greatly upon the container used. A 3L metal water bath on my hotplate will be at about 55C when the hotplate dial is set to 100C.

[moo]

The temperature of the water with the hotplate at a certain temperature is dictated by two factors. The heat energy the water receives from the hotplate and the heat enegry that leaves the water. With a larger amount of water the surface area of the water with surroundings that are colder is larger and more heat leaves the water compared to smaller amount of water. But supposedly the amount of heat energy that comes from the hotplate stays the same, so the temperature of the water is lower with more water.

There are hotplates with temperature controllers and immersible temperature probes. These things can be set to maintain liquid at a certain temperature, but if you haven't got one you have to do the temperature control manually. What's the problem with that? It's what you do when you boil eggs on the stove anyways.

Want to learn more? Read thermodynamics.

[ApprenticeCook]

If you read the rest of the thread i actually expanded and corrected myself in my previous post....
I mentioned the thermodynamic inefficiency of the hotplate, container, material and heat intertransitions...
I was creating an example of 100% efficiency which of course in the real world never occurs so thats why i corrected myself....
As for my massive over exageration of 10KL boiling i was just demonstrating at 100% efficiency it would occur, however... the water would lose the heat as it aquires it in the real world and would only get to and maintain a certain temperature...

Dear oh dear read the rest of the thread and THEN comment...
-AC

[Pots]

Good on ya AC.....it's good to read threads like this...a good refresher to some of the basics.

[java]

As someone suggested  read thermodynamics......

http://online.cctt.org/physicslab/content/PhyAPB/lessonnotes/thermodynamics/lessonthermodynamics.asp

[starlight]

I was commenting on the fact that you spouted off calling another member names when what you said was incorrect. Whether you corrected yourself later (without apology for the name calling) does not change this fact. The original poster (superman) was entirely correct. You cannot change this fact either.

[Snakebyte]

Actually Swim's ketone will not reach 100 unless his hotplate is set at 180.  But, it is his verbal mistake because he forgot to say that the hotplate heats up an oil first in which his RB flask sits in.  Either way, heat transfer is not even close to 100%.

[ranxerox]

A hotplate with a temperature readout is measuring the temperature of the hotplate surface.  This is different from the temperature of something in a container on that surface.

[XrLeap]

For thermometers still being analog, probably is due to the ease of reading the direction of the temperature going, and the rate of the temperature going.

I might be wrong

[armageddon]

First thing: no matter how small or big the amount/vessel to be heated is - there are always losses (there simply is NO perfect isolation!) due to energy being given off by the liquid/vessel...

Second thing: A well-branded hotplate/stirrer combo can usually be fitted with a digital thermocouple - meaning you can adjust the desired temperature digitally, and the thermocouple will control the hotplate in a way that insures the liquid reaches the desired temp. - because it measures with its own thermometer immersed into the liquid to be heated...

And you can also use this piece of equipment together with heating mantles, given that they are eqipped with a variac having a possibility for external heat adjustment...

And there are also glass adapters with joints for those thermocouple thermometers; they allow for measurements inside closed flasks, so you can even perform a vacuum distillation with automated, digital temp. control/adjustment if you use such an adaptor/said thermocouple and place it on your still head - instead of an ordinary dist. thermometer (liquid is automatically heated until steam temp. reaches desired temp.   )...

Greetz A

[p2e3r4f5e6c7t8]

[Drug_Phreak]

Wow... that's a nice hotplate/stirrer. I bet it costs around 1,500 though. I found a nice hotplate/stirrer that goes for 300-400 at most places I've seen.