SAVE MONEY ON THERMOCOUPLE REPLACEMENT
One major advantage of these protection tubes is that you can replace your old thermocouples with the inexpensive standard 8 gauge thermocouples without changing the protection tube. A replacement 8 gauge thermocouple is only $30 vs. $70 for the metallic thermocouples. Over time the protection tubes may have to be replaced if they get contaminated from materials in the kiln; however, it should take a long time for contaminants to leach through the relatively impervious 1/8" thick mullite tube. The tube can be easily replaced independently from the thermocouples. They are not sealed or cemented together.
SIZE AND DESCRIPTION
The protection tubes are ¾” in outside diameter with a ½” inside diameter (which accommodates the standard 8 gauge Type K thermocouple). There is a flange on the back end of the thermocouple to prevent it from going into the kiln.
MULLITE CERAMIC PROTECTION TUBES OVER 8 GAUGE TYPE K THERMOCOUPLES OFFER SUPERIOR LIFE THAN METALLIC THERMOCOUPLES
The industrial 2300mi thermocouples that have been used for several years are no longer available because Hoskins, who made the material, is no longer in business. This is L&L’s careful and thorough response to this issue.
L&L TEST PROGRAM
L&L tested 23 different thermocouples in a kiln by firing them to Cone 10 (2350 Deg F) and soaking the kiln for one hour repeatedly and then measuring the thermocouples with a sophisticated datalogger. The main thermocouple to control the kiln was a platinum Type S. The various test thermocouples were our standard 8 gauge thermocouple with a butt-welded end, one with a twisted end, both in a heavy mullite protection tube and exposed to air, 14 gauge Type K exposed thermocouples, Type N thermocouples of various types, a Hoskins 2300mi, several metallic sheathed thermocouples with inconel and various grades of Pyrocil (a replacement for the 2300mi), etc.
One of our standard Type K 8 gauge thermocouples in the industrial protection tube achieved 139 firings. The standard 8 gauge thermocouple without a protection tube made it to 100 firings. The metal-sheathed Pyrocil was good but not as good as the 8 gauge with the protection tube (it worked for 130 firings). An 8 gauge Type N did slightly better (143 firings) but the potential confusion of using this outweighed the simplicity and backward compatibility of the Type K.
The conclusion from the test is that a standard 8 gauge thermocouple with an industrial mullite ceramic protection tube offered the best combination for cost, cleanliness (there is no contamination of the kiln with metal spalling), resistance to contaminants in the kiln like sulfur, backward compatibility, and long life.
THERMOCOUPLE OFFSET
A thermocouple in a protection tube does have a delayed response and hence an offset from the control type S thermocouple or a metallic sheathed thermocouple. However, our test indicated that every thermocouple measured slightly different temperatures anyway. A detailed analysis of responses after about 130 firings showed the 8-gauge Type K thermocouple in the protection tube to be between the control Type S platinum thermocouple and the metallic sheathed Pyrocil Type K. In any case the DynaTrol control has thermocouple offset and cone offset to allow you to compensate for any such effects. For ceramic work we recommend calibrating the kiln performance with cones and adjusting the control to match the performance of the firing cones. The most important thing is to get a consistent reading from the thermocouples.
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