Hi, I'm working on a project and need a constant heat source (no open flame) so an electric burner would be best, but from what I've read a vast majority of them cycle on and off, is there a way to modify them to make them run constant? like remove a resistor or something? this is very similar to what I'm talking about
I'd expect any such source to use a thermostat to provide the "cycling"
- removing power from the element when the set temperature is reached, then restoring power when the temperature drops below that temperature. Often an embedded bi-metallic strip type device. If the device has a variable temperature setting, this can be confirmed to a high degree of confidence by adjusting the temperature setting with the device off, if you get get the temperature setting down to room temp, and hear/feel the contact open then you can be pretty sure it's a bi-metallic strip.
To bypass this type of device, simply short the terminals across the device.
Of course there could be other devices at play here, especially as the cost of electronic controllers has dropped.
Daft question?, why do you want to circumvent the control system that should be there to maintain a constant temperature?, what you propose is a device that's going to heat *something* until the point at which the heat provided by the pad is matched by the heat being dissipated by the
*something*. Or *something* catches fire, melts, or goes bang.
Maybe because a cyclic control system has pretty crap short term temperature control. The long term mean can be quite good but the high and low excursions whne the power is on/off can be huge.
As to the OPs problem get a hot plate, bypass the built in thermostat and feed the thing via a suitably rated variac or electronic dimmer (power controller). There would have to be some trial and error to calibrate the variac/dimmer to temperature and that calibration would vary with what ever is on the plate as well but for the same thing I should imagine fairly repeatable.
There maybe electronic power controllers that have an external temp sensor that one could bung in the pan but I've not looked.
Then it won't be a constant heat source. It will be influenced by surrounding temperature and load, etc. What you apparently need is a device which maintains an exact temperature. If you remove the thermostat from most such devices it will self destruct.
If you'd give the application it might help.
Are you trying to hospitalise the OP?
Do not wire over the stat. Use a series impedance of some kind in series with the heating element, keeping the fan on full mains voltage, and set the stat to above the temp you want to get out. Use a thermometer to measure what youre getting and adjust the impedance to get the desired temp. This will give a stable temp once you've run it for 5 minutes or so to stabiilse the temp of all the parts.
Suitable series impedances: _very_ big resistor, light bulbs, capacitor(s), or suitably rated inductors. If tht heating elemnt is low power (
lots of replies! I'm trying to distill alcohol without an open flame, ethanol boils off at 78.4C (174F) and I don't want a source that is going to constantly turn on and off because it will lower the temp and then have to get back to where it was and cycle again and again... I was hoping there was some sort of heating device that you just turn to whatever temp you want and it stays there... maybe one of those $800 lab hot plates I don't know.
it doesn't seem like it would be all that hard to have an electric hot plate that stayed at a certain temp, but then again I'm no electrician.
why didn't you say so !
allegedly, your local Customs & Excise office - look under HMCR in the yellow pages can supply all the advise you'll need.
One of the phenomena of _all_ control systems is called hysterisis: What you've described as "going to (constantly) turn on and off because it will lower the temp and then have to get back to where it was and cycle again and again"
A system will ramp up to a control value, perhaps maintain that value then fall off to a lower value, perhaps maintain this lower value before ramping back up to the top value. Any temperature maintaining system will exhibit hysterisis. 'Tight' control loop systems _might_ achieve 78.4 deg but it'll be 78.4 +/- a 'bit'. Roughly; the size, and direction, of 'a bit' is going to become more expensive the smaller 'a 'bit' is.
78.4 (0 / -2) would be cheaper than 78.4 (+0.001/-0.001). You seem to desire a 'perfect' system with no hysterisis that will heat a fluid to 78.4 +0.000 /-0.000. But as the old saying runs; anything is possible given enough time and money.BTW; why?
then all these replies have been a waste of time.
NT
flame,
electrician.
Well when I was doing that some 30 years ago (me being a process control engineer at the time, and my flatmate an industrial chemist) I designed and built a pulse width modulated driver for a normal kettle element which was part of a control loop based on a type 'k' thermocouple located at the bend point of our still. Worked splendidly - the heater element maintaining a steady state of light boiling. Discard the first few drops distilled - they contain the ketones that give you a hangover !!!!
AWEM
Aha, I see you failed to quote my post that detailed the likely result of what I suggested ("catches fire, melts, or goes bang"), despite having clearly answered the OP's *specific* question as to how to modify them to make them run constant.
Now that the OP has provided the proper statement of the problem, we can progress. To be honest, at the temperature that he's citing I'd actually recommend a variable-temperature immersion coil, into an oil bath, into which the distillation vessel is submerged. That should provide a reasonably controllable system, which, once up to temperature, the *slow
The message from "Brian Sharrock" contains these words:
And anyway, 78.4°C is only the standard boiling point. If the air pressure isn't 1Bar that day then it'll wander anyway.
I'd make a bain-marie and warm that gently - the fractions will come off one by one anyway provided the temperature's raised gradually enough. A large mass of water in the outer jacket will make this easier.
On 8 Aug 2006 11:26:06 -0700 someone who may be "vairox" wrote this:-
The still you can see at
The still can also be run on gas and "traditional" fuels like peat, but electric is less messy and avoids the potential problems of an open flame.
Such as smoke rising through the trees and attracting the Revenue men?
John
The message from John White contains these words:
The revenue don't mind you distilling - provided you pay the duty.
On Wed, 09 Aug 2006 13:02:08 +0100 someone who may be John White wrote this:-
One of them did once see the still and ask if there was a licence for it. When we said, "yes", she did not ask to see it. The licence was in a filing cabinet near the still.
On Wed, 9 Aug 2006 14:56:43 +0100 someone who may be Guy King wrote this:-
Only if the still is above a certain size. Below that they are not keen.
They probably don't care either way so long as you don't sell it! ;-)
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