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QUESTION
Jan 11, 2005, How does a thermocouple work?
ANSWER A ThermoCouple is constructed of 2 wires, each of a different metal, connected together at a measuring junction where the temperature is intended to be measured. By nature, this junction of dissimilar metals will produce a small voltage, which just happens to be relative to temperature, although not linearly.
The main point here is the actual construction of the ThermoCouple: When we connect 2 wires together, and attach them to the battery as you suggest, we have created a short circuit. You can try this with any battery, even a "C" or "D" battery and a piece of wire (or 2 pieces of wire connected in series, if you prefer to more closely duplicate the ThermoCouple concept). The result will be the discharge of the battery through the wire as it's energy is converted into heat (the wire gets hot). The amount of power dissipated will be equal to V²/R where V is the voltage of the battery, and R is the resistance of the wire. The resulting power is measured in watts. For a ThermoCouple, or other piece of wire, R is going to be very low. Depending on the length and diameter of the wire, R is most likely going to be much less than 1 ohm. Therefore, the power generated will be quite high as the wire will get quite hot, possibly red hot. This will also discharge the battery very quickly, and in the case of a wet-cell battery (like a car battery) might cause an explosion.
Regarding the amplifier from SuperLogics, Inc. I believe the unit you are referring to is manufactured by Analog Devices, and is indeed just that, an amplifier. Please go to this link for further specifications:
http://www.analog.com/en/prod/0,2877,5B47,00.html
You will notice that it requires a supply voltage of 5 VDC in order to provide an output of 0-5 VDC linear with Temperature as input from the ThermoCouple.
The bottom line is: You can't get something for nothing, or as the First Law of Thermodynamics tells us: "Energy can be neither created nor destroyed".
Please let me know if we can be of further service.
Best Regards,Dave King - BSME
QUESTION Jan 13, 2005, What is a Thermopile?
ANSWERThe Thermopile (I believe was originally mentioned in a conversation between you and Tom Fishwick) is exactly as you surmised: The same type of array of ThermoCouples wired in Series, just as was used by old Wiley to power that transistor radio. And yes, I suppose he had quite a few of them, and of a type with relatively high output.
Keep in mind that ThermoCouples do not actually produce a voltage based on Temperature, but based on the difference in temperature between a hot junction (on the exhaust manifold) and a cold junction (ambient air probably). So for every hot junction, you will need a cold junction far enough away that it will be at a lower temperature. The greater the temperature differential, the greater the voltage output.
Now, back to diodes. Please check out this link for a quick explanation:
Basic Diode
I think you will want to use 2 rectifier diodes, one on each leg of the circuit, between the battery and the ThermoCouple array (or Thermopile). Each diode should be polarized to block current flowing out of the battery. So that should allow current to flow back into the battery, but should prevent the battery from discharging, correct?
That's just the way I see it, but remember, I'm an ME.
Please let me know if it works.
QUESTION Jan 14, 2005, What type of Diode should I use.
ANSWERI think you need to be careful in your diode selection, so you're sure to have components that: 1. will handle the reverse voltage (at least 14 vdc), 2. will handle the forward current of your ThermoCouple Trickle Charger, and 3. Will introduce only minimal resistance into the system. I think that "Rectifier Diodes" by nature are designed to provide minimal resistance, but that is something that might be best discussed with your mentor, or the Diode manufacturer.
QUESTION Jan 18, 2005, So, I simply can't stick a thermocouple where it's hot and expect voltage when I top it off with a diode?
Actually - You can, because the opposite end of the ThermoCouple wires will actually become something similar to cold junctions. You can just put a Volt-Ohm-Meter across the ThermoCouple and read some voltage. But you will need to connect your ThermoCouples in Series in order to increase the voltage to a level that will be useful to your project. So as you follow your Series circuit of T/C's (ThermoCouples), you need to be sure to alternate the junctions: Hot, Cold, Hot, Cold, etc.
I am thinking of researching the coldest part of the engine and expecting to finding that the coolant area will (obviously) be it.
Not a good assumption. Coolant Temperatures of an internal combustion engine typically run right at the temperature setting of the mechanical thermostat within the engine (typically around 220°F, just above boiling point of water). So your coolant temperature will be around 220°F after the engine has warmed up. There are a lot of points cooler than that, most notably the air coming into the engine compartment in front of the radiator, or where it enters the airbox going into the induction system.
Would multiple diodes be capable of reaching that 14 V resistance, just as a thermopile combines all the voltage output into one complete package.
I don't understand the question. Diodes only direct the current, they don't produce or combine anything.
What really bothers me is how a thermocouple requires a cold junction and that the charts need some sort of difference to actually pump out the most voltage. Even if the cold junction is somewhere relatively lower in temperature in the engine block, it will still produce very little voltage. I calculated the amount of individual thermocouples needed to yield the maximum amount of voltage (21 mV) for the necessary 15 V: around 620!!
Correct! That is what we've been trying to tell you from Day-1.
QUESTION Jan 14, 2005, Even if this is so, this figure assumes that the cold junction is at 0 degrees C. Is that even possible in an engine?
Not after the engine is running, and never when it's not winter.
David J. King - BSME
Sales Engineer
Thermometrics Corporation
Eastern Sales & Engineering Office
802-236-5893
QUESTION Please, Id like to know if is there any chip temperature sensor like LM35 which provide me a not analog output signal, but digital, whatever were the protocol, it's good for me.
ANSWER Thermometrics manufactures Smart Temperature Sensors. These units provide a 9-bit Digital Output over the Temperature Range -55° to +125°C (-67° to +257°F) in increments of 0.5°C (0.9°F). These sensors can be configured in most packages suitable for RTD's with diameters of .250" and larger.
Each unit contains a unique serial number integral to the silicon chip so that multiple units can be wired onto a single 1-wire bus, and each is recognized by its own serial number. This feature suggests applications in HVAC and other areas where multiple temperatures need to be economically monitored by a central station.
QUESTION My business was inquiring in buying a large lot of thermocouples but I was wondering the quality of your products and would like some pictures of your thermocouples.
ANSWER Thermometrics is a Small Disadvantaged Business (SDB) located in Northridge, CA., the corporation was established in 1965 and has been manufacturing temperature sensors since this date. The product line includes Ceramic Encapsulated RTD Sensors, Sanitary Service Resistance Temperature Detectors, Propellant Exhaust Thermocouples, Air Duct Averaging Sensors, Transmitters, Thin Film and Wire Wound Resistance Elements, Flanges and Calibration Services. Pictures of our products are located at Temperature Sensor Pictures
QUESTION I am looking for some thermocouples, and was wondering what you guys can do for me. The application I need them for is pretty simple, which would be logging temperatures anywhere from -70 degrees Celsius to +100 degrees C. I thought that T type would be the best for this range, but in looking at your site it says that T type isn't for temperatures below 0 degrees. Could you recommend another type and give me a quote on various quantities.
ANSWER Type T thermocouples are recommended for temperatures below 0 Celsius.
QUESTION Do you have RTD extension wire 14 AWG solid either individual conductor or three conductor?
ANSWER Thermometrics manufactures a full line of RTD and thermocouple lead wire.
QUESTION I am a final year Physics student at Loughborough University. As part of my final year project I need to connect two length of K-type thermocouple wire together, however the standard and Mini connectors available are to large to fit inside the test equipment. This mean we need to produce our own. To do this we need to know if the material used in the connectors needs to be made out of the same material as the legs of the thermoucouple or can we get away with using any metal?
ANSWER Thermocouple connectors should be manufactured from the same material as each wire type. Type K, positive leg is manufactured from Chromel and the negative leg is manufactured from Alumel.
QUESTION Do you do custom thermocouple probes?
ANSWER Thermometrics is equipped and staffed to manufacture sensors to meet your design requirements. E-mail or fax your idea and our application engineer's will provide a quotation that is below our competitors.
Minnesota and United States Climate Temperature Extremes
NOAA.gov
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