John Baymore on mon 22 jul 96
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From: Spehro Pefhany
Subject: Re: Kiln controller question
PRICE@MASTER.PS.UCI.EDU wrote:
: I am getting ready to (finally) buy a kiln controller. .......... I want
one that uses a platinum (type R or S) thermocouple. Do any of the commercially
available controllers take such a thermocouple?
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Yes, the Perfect Fire controllers (made by CIC) are available .....
..................... There is a only a slight price premium for the controller,
but of course the thermocouple is much more expensive .....
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I am not familiar with the CIC controllers, and they may be designed internally
to accept multiple input types, but I would make sure that the resolution of the
electronics (accuracy) is up to the job of the platnium thermocouples.
If a controller is designed with a resolution of say, +/- 20C, then it is
possible that it will not be any more accurate than that built in resolution
with the other (more accurate) thermocouple input. You will get slightly more
accuracy, because the thermocouple is giving a more "predictable" signal,
however there may still be a problem. The software was not designed for more
accuracy. The resolution of the A/D (Analog -voltage to Digital- on/off pulses)
Converter is also a stumbling block in this issue....... higher resolution
generally costs more money.
For a change of 1 degree C on a type R thermocouple at 1000C, (from 1000C to
1001C), the signal change is only 13.23 microvolts. That's a VERY small voltage
change! To measure to one degree accuracy requires a very, very good digital
voltage meter (DVM). The A/D converter quality sets the stage for the accuracy
of the entire unit. If it can only resolve a 50 microvolt change, then it is
only accurate to about +/- 4 to 5 degrees C.
If the controller electronics is made so that the accuracy is there to start
with, then you'll be getting what you are looking for. If the electronics is
NOT, then you'll just be wasting your money on the platnium thermocouples hooked
on a unit that was designed for a broader margin for error..
This is similar to the many differnt "qualitiy" issues in analog pyrometer meter
movements. Some are pretty accurate, some are effectively worthless. You need
to know the resolution of the meter....... in this case the controller. Just
being able to hook on Platnium / Platnium-Rhodium thermocouples is not really
the issue (although it is one issue).
..........................john
John Baymore
River Bend Pottery
Wilton, NH
76506.3102@Compuserve.com
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