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Registered Member #1822
Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300
In my research I have read that peltiers have the ability to cool to 70F below the temperature of the hot side. If the hot side was cooled with dry ice would the cool side still cool by an additional 70F down to -179F?
I forgot to ask about peltiers stacked in series. What is the minimum temperature that can be reached in this way?
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
Just download the datasheet for a random Peltier and it is all in there. A difference of 68 deg C is a common figure. That is measured at zero heat flow, the more heat that needs to be moved the lower the difference will be but you got the general idea correct.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
The efficiency of the device also fall off rapidly with lower temperature, and with a -100f starting point I wouldn't expect more than 20 degrees or so. But it would be a good experiment certainly!
Registered Member #2025
Joined: Fri Mar 13 2009, 05:39PM
Location:
Posts: 16
Yes you can stack them to achive lower temperatures but they will use up a massive amount of electricity and you will have MUCH more heat to get rid of. Also in a stack so that the one ontop has higher "heat pumping ability" then the one below.
They are used for extreme overclocking in computers so getting some information about them on overclocking sites and such should be pretty easy.
There is as much information as you could possibly want about tecs.
Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
For stacking, each one must be successively lower lower then the one below it (bottom one must pump the combined power of all the ones above it). I think it goes that they can still pump heat (cool down) if the cold side is above absolute 0.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Yes but ... the problem is the calibration. Thermistors are non-linear, and to extrapolate a cheapo one from a PC designed to work in the 20-50C region down to the sort of temperatures that you crave would be asking for trouble.
You would be far better off with either a thermocouple or a platinum RTD.
A decent themocouple meter, especially if it goes well below zero, could be fairly expensive, you might be able to borrow one from a lab or school. Just the couple itself is fairly cheap, and an IC to amplify up to 10mV/K and cold junction compensate will set you back around $£euro 20
A basic unpackaged platinum RTD is not desperately expensive (<$£euro 10), and with a bit of care and a multimeter you can make reasonable linear measurements over a huge range. With a Wheatstone bridge and a decent 100ohm low tempco reference resistor you can make very accurate measurements. The dR/dT is published in all sorts of places, try NPL or Kaye and Laby.
Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
There are many good temperature sensors on the market that are easy to use and are already calibrated. I use and recommend devices from Analog Devices! (No I do not work for them! )
Registered Member #2123
Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312
I don't work for Analog Devices either. I highly recommend the AD590. Sinks 1uA/deg K, runs on 4-40 volts, comes in a little 2-lead tab package among other styles.
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