Sciencemadness Discussion Board

Method to "calibrate" a dimmer for heating mantles?

Hilski - 18-1-2007 at 09:10

I recently picked up a couple different sizes and brand names of heating mantles. They didn't come with heat controllers, so I am using dimmer switches to control them. I was just wondering if anyone knew a reasonable method for "calibrating" a typical dimmer switch. I have been measuring voltage from the dimmer while operating the mantles, and I seem to be able to roughly reproduce a given temp based on a given voltage. (i.e. 23v - 24v = 125C - 130C) The only problems I have seen is that 20v is near the bottom of the dimmer switches capabilities, and when I need to regulate to much lower temps, I won't be able to do so with the dimmer. The only solution I can think of is to put a rectifier diode in one of the lines between the dimmer and the mantle.
Also, as is typical with triac based controllers, the voltage will creep higher as resistance drops and more current is drawn by the mantle as it heats up.

Anyone else have any similar experiences?

[Edited on 18-1-2007 by Hilski]

Sauron - 18-1-2007 at 09:37

There's no need.

You should measure the temperature in the reaction mix directly, working up to where you ought to be slowly.

Typical inexpensive GlasCol mantle controllers have proportional controls, and are available very cheaply second hand on LabX and eBay. Numvered dials so you can note setting you settle on for next run.

As the pigtail leads for the mantles will not be standard you will have grief trying to connect them to anything else. Yes you could rewire them but you'd best avoid that. Note that there are several different GlasCol connectors, two prong, two prong plus center post etc. So you may need to politely ask a distributor for the factory to help you obtain a cord if the mantles didn't come with one.

The other usual suspect is Electrothermal but their mantles have controllers built into the blue plastic housing these days.

I hope this is helpful.

bio2 - 18-1-2007 at 09:53

There has been discussion by me and others on this so do
a search.

The dial can be calibrated in percent power using the voltage/RMS relationship and the low end and drop out can be
solved by modifying the dimmer to a symmetrical firing circuit.

MargaretThatcher - 18-1-2007 at 16:34

Have a look for a PID temperature controller instead of a dimmer. You can often pick these up cheap from ebay and surplus sites. With a cheap thermocouple, you can control temperature automatically.

Baphomet - 18-1-2007 at 18:43

PID is a great idea especially since a lot of Glas-Col units come with a built-in thermocouple.

I've got a question.. my mantle is a Glas-Col TM110, designed for use with 2Lt RBFs. Is it OK to use 1Lt or 500mL flasks in a unit like this? Is there a good way to adapt such a mantle so that there is not a huge space around the flask and hence it may be heated more efficiently?

Sauron - 18-1-2007 at 19:36

I would advise strongly against using a smaller RB flask in a 2L TM110, you would be reducing the contact area between mantle and flask to a small fraction of normal.

That's a nice unit, a table mantle. I have a 3L and a 5L in that series plus a 250 ml. For 1L I use an Electrothermal with built in controller. The Electrothermal is 220V while all the others are 100V purchased used in USA. So I run them on a stepdown xformer. Interposed between the transformer and the mantle in those setups is a Glas Col PowerTrol controller. Previously I have used Variacs, other GlasCol controllers and also Ace controllers for thermocouple inserted into the rxn mixture or stillpot. All of these work well.

Also have a couple of huge 72L TMs brand new, paid a small fortune for them and have never used them. Two 220V 11A circuits on each id I recall.

Magpie - 18-1-2007 at 19:40

I'm having a little trouble seeing the need to "calibrate" any kind of a voltage regulator for a heating mantle. I say this just based on my experience. Once you work with your mantle/controller a few times you will get a feel for where you should start and how to trim it as your vessel heats up. Variables that will affect this are: the size of the vessel, how much liquid is in the vessel, liquid characteristics, desired reflux rate (if any), etc. Eg, where I set my controller initially and how I trim it when the solvent is water will differ from where I would do same if the solvent were dichloromethane.

Sauron - 18-1-2007 at 19:43

@Maggie is correct, but don't skimp on your thermocouple. You will want an inconel jacketed one. and get an extension connector cord so as not to strain the setup.

A short thermocouple is fine for immersing in vapor at top of a column, usually 76mm immersion length so 150mm OAL ok. If you need to measure and control pot temperature you will need a longer thermocouple. Mine is 12" and fits in standard thermometer adapters, works for 2N/3N flasks up to 5L. I don't have any 12L setups so can't comment.

bio2 - 18-1-2007 at 20:30

...... the need to "calibrate" any kind of a voltage regulator for a heating mantle.........

All that it really does is give a percent of wattage marked on the dial which is helpful as the RMS wattage is not linear to the
voltage.

Still, as you say, the "feel" for it is what is really needed although it's nice to set it to say 15% and get 150watts output on a 1KW phase control circuit without quessing.

Hilski - 18-1-2007 at 20:50

Quote:
I'm having a little trouble seeing the need to "calibrate" any kind of a voltage regulator for a heating mantle.

Yes I agree with the point you (and others) have made about 'feel' and adjustment throughout the reaction. All I wanted to do was get a few settings that I could use to consistantly get me in the ball park of where I need to be for a given reaction or distillation or whatever.
Quote:
All that it really does is give a percent of wattage marked on the dial which is helpful as the RMS wattage is not linear to the
voltage.

Still, as you say, the "feel" for it is what is really needed although it's nice to set it to say 15% and get 150watts output on a 1KW phase control circuit without quessing


Those are my thoughts exactly, and just what I would like to accomplish.

Thanks for all the replies and suggestions.

-Hilski

not_important - 18-1-2007 at 23:53

Note that most dimmers are simple triac designs, they switch on partway through the halves of the AC sine wave. This gives a waveform for low settings that does not measure well with normal meters. While the best way to do things is to measure the mantle and pot temperatures, you could calibrate a dimmer by filling the mantle with clean sand, stick a thermometer or thermocouple in the sand, setting the dimmer to a position, and measuring the temperature repeatedly until it stablizes. Repeat for as many setting as you want.

Putting a diode in series will drop the power by half, but puts a DC offset on the power draw and thus power factor, which sometimes is a Bad Thing. It might be better to hook up some resistive load in series, like a space heater or several high wattage lightbulbs in parallel, to divide the power between that load and the mantle.

The proportional controls switch on and off for some number of cycles, good designs make sure they count full cycles while cheaper ones do half cycles which can result in DC offset on the power mains and odd power factors.

Hilski - 19-1-2007 at 10:54

Quote:
The dial can be calibrated in percent power using the voltage/RMS relationship and the low end and drop out can be solved by modifying the dimmer to a symmetrical firing circuit.

Thanks for the tip. I added the four diodes and 3 extra resistors to a standard cheap dimmer, and it works perfect now. No more hysteresis, and it will turn all the way down to about 5% power with no problems whatsoever. I also removed the inductor and capacitor that were in there for RF noise filtering purposes, because they were basically just in the way.

-Hilski