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sci.geo.meteorology (Meteorology) (sci.geo.meteorology) For the discussion of meteorology and related topics. |
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#1
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I remember several years ago, on one of the local weather broadcasts,
a reporting station was out of service because of a defective thermocouple. I seem to recall, also, mutterings as to whether a thermocouple could be correctly calibrarted. David Ames |
#2
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On 25 Feb 2007 04:07:26 -0800, "David Ames"
wrote: I remember several years ago, on one of the local weather broadcasts, a reporting station was out of service because of a defective thermocouple. I seem to recall, also, mutterings as to whether a thermocouple could be correctly calibrarted. Thermistors, what about those? They seem accurate enough by using the known properties of expansion/contraction rates relative to known temps. Mercury seems outdated but then again some people considered walking outdated. Personally I find walking to be boring. |
#3
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Bob Brown . writes:
On 25 Feb 2007 04:07:26 -0800, "David Ames" wrote: I remember several years ago, on one of the local weather broadcasts, a reporting station was out of service because of a defective thermocouple. I seem to recall, also, mutterings as to whether a thermocouple could be correctly calibrarted. Thermocouples use the principle that a circuit with two dissimilar metals will create a voltage that is proportional to the difference in temperature between the two junctions. There are many standard combinations of metals; one common one is copper-constantan, which produces a voltage of about 40 micro-volts per Celcius degree of difference. To clarify: by taking one copper wire and one constantan wire and joining them at both ends (this creates two junctions where copper and constantan meet), you create a circuit. If you put the two junctions/ends at different temperatures, a voltage is created. The exact voltage difference is temperature-dependent itself, but the properties of standard thermocouple materials are well known, and manufacturers will guarantee how closely their wire will meet the industry standards. Thermocouple are *great* for measuring temperature *differences*, but knowing the difference between the two junctions tells you nothing about what their actual temperatures are. To use a thermocouple to get an accurate absolute temperature reading, you need to place one junction (the "reference" junction) somewhere where you can measure its temperature can be measured or controlled independently. A common option used as an example is an ice-water bath (0C), and thermocouple voltage specifications are often expressed on this basis. Ice-water baths are not particularly practical for most people, though, so you usually put the reference junction into somthing else and measure the temperature of that by some other means. You need a good volt-meter to read the thermocouple, and a good independent measurement of reference temperature, but I've had no problems measuring temperatures to within a few hundredths of a degree with thermocouples, and better results just cost more money. Thermocouples do not typically degrade over time, and interchangeability is not an issue (due to manufacturing standards). Accuracy is limited by the volt-meter and the reference temperature measurement. Thermistors, what about those? A thermistor is a semi-conductor with a high (and negative) resistance vs. temperature sensitivity. Again, there are a fairly wide variety of standard sensitivities available, as well as a range of "interchangeablities" - which means how closely do any two thermistors in a batch have the same resistance at the same temperature. +/-20% thermistors are pennies each, but aren't good for temperature measurement unless you individually-calibrate them and prepare a temperature-vs-resistance curve for each. For dollars to tens-of-dollars each, you can get thermistors that are interchangeable to the equivalent of 0.1C or 0.2C, which is good enough for many purposes without individual calibration. With calibration, 0.01C is easy. Thermistors may be used to determine reference temperatures for thermocouple measurements. Thermistors can degrade (and thus shift calibration) over time, so periodic checks or calibrations are a Good Idea. They seem accurate enough by using the known properties of expansion/contraction rates relative to known temps. Even better are platinum resistance sensors. Very stable, very accurate. Very common in a lot of the automated meteorological station sensors. Mercury seems outdated but then again some people considered walking outdated. Personally I find walking to be boring. Mercury-in-glass may still be a common type of reference transfer - I worked in a lab that had a mercury thermometer that measured from -5 to 0C, in graduations of 0.01C. It also came with a chart of corrections between what it read and what the true temperature was, so you really could trust it to within 0.01C. You had to make absolutely sure you immersed it to the line engraved on it (and no further), though, as immersing it more or less affected its calibration. |
#4
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On 25 Feb 2007 04:07:26 -0800, "David Ames"
wrote: I remember several years ago, on one of the local weather broadcasts, a reporting station was out of service because of a defective thermocouple. I seem to recall, also, mutterings as to whether a thermocouple could be correctly calibrarted. David Ames To respond to the subject, check froogle, they list many. difficult to automate readings from a mercury thermometer, I would think. |
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