tempcontroller:sensor
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tempcontroller:sensor [2010/03/03 22:40] ladyada |
tempcontroller:sensor [2016/01/28 18:05] (current) |
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| ===== Thermocouples ===== | ===== Thermocouples ===== | ||
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| + | [[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! | ||
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| + | 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 | ||
| ==== Accuracy & Precision ==== | ==== Accuracy & Precision ==== | ||
| - | K type thermocouples have accuracy of 2°C (±1°C) and precision of about 0.5°C (±0.2°C) | + | 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. | 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. | + | > 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 ==== | ==== Range ==== | ||
| - | K thermocouples can be used from -350 to 1350 - a huge range which is basically the big plus | + | 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 ==== | ==== Amplifiers & Interfaces ==== | ||
| - | The [[http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3149|MAX6675]] has a resolution/precision of 0.25°C | + | The [[http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3149|MAX6675]] has a resolution of 0.25°C. |
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| + | 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 ==== | ==== Pricing ==== | ||
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