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--- tempcontroller:sensor [2010/03/03 22:09]
ladyada
+++ tempcontroller:sensor [2016/01/28 18:05] (current)
@@ Line 37: / Line 37: @@
 ===== Thermocouples =====
+[[http://en.wikipedia.org/wiki/Thermocouple|Thermocouples]] are extraordinarily common for use as temperature measuring devices. They have a lot of 'pros' to them - including huge range (more than 1000°C!) and interchangability (all K types are the same and can be swapped out). They're common and cheap and if you don't care too much about accuracy they're fine. If you need to measure hot things, they're your only choice!
-K type thermocouples have accuracy of 2°C (±1°C) and precision of about 0.1-0.5°C
+In particular here I'll refer to K type. J type is also common and T is more accurate but the off-the-shelf chips are for K (and J) and so that's what hobbyists like
-Because the voltages across the wires are so small (in the uV range!) the precision has more to do with your amplifier and any hash (noise) pickup than with the thermocouple itself.
+==== Accuracy & Precision ====
+K type thermocouples have accuracy of 2.2°C (±1.1°C) or 0.75% (whichever is higher) and precision of about 0.5°C (±0.2°C)
+Because the voltages across the wires are so small (in the uV range!) the precision probably has more to do with your amplifier and any hash (noise) pickup than with the thermocouple itself.
+> I find it confusing that products like the [[http://www.thermoworks.com/products/thermapen/splashproof_thermapen.html|Thermapen]] have 0.1°F precision displays when the product accuracy & precision is almost certainly much lower (that is, less precise). If anyone understands this, would appreciate an explanation.
+==== Range ====
+K thermocouples can be used from -350°C to 1350°C - a huge range which is basically the big plus. J type can go up to 750°C
+==== Amplifiers & Interfaces ====
+The [[http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3149|MAX6675]] has a resolution of 0.25°C.
+However, the error (inaccuracy?) is about +-8 LSB. 2*8*0.25°C = 4°C (±2°C) that should be added on TOP of the ±1.1°C / 0.75% sensor probe inaccuracy
+==== Pricing ====
+Thermocouples themselves can be had for cheap - we've seen K-type for $3/each. They're also incredibly common, available anywhere you can get a multimeter (multimeters often have them as probes)
+The expensive part is the amplifier - unless you're going to build your own (which is hard, you need extremely low bias and low offset input amplifiers) the chips will cost you about $13

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