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Small nickel-iron temp probe needed
Greetings,
I'm working up a demonstration of an antique electronic temperature controller which needs a sensor with a PTC of about 5 to 6 ohms/degree F average (at 20 degrees C); nickel-iron resistance wire fits the bill fairly well, and I would like to find a coil on a low-mass substrate, like ceramic, of roughly 1150 ohms at 20 degrees C, hopefully no more than a few inches long (up to 5 inches is OK). I have some resistance wire salvaged from old soldering iron heater cores, but its gauge is a bit large such that to get 1150 ohms would require quite a lot of it on a rather large form. Perhaps someone has recollections of small assemblies of resistance wire either used for heating, in the 10 to 12 Watt (at 120 VAC) range, or for sensing, that might come in at around 1150 ohms, so that I might avoid a tedious winding of hair-fine wire under magnification. Surprisingly (to me), my cannibalization of some 10 Watt heating tools (like hot melt glue guns) revealed not resistance wire, but some form of semiconductor with a NTC (isn't thermal runaway an issue here?). Michael |
Small nickel-iron temp probe needed
In article t, msg wrote:
Greetings, I'm working up a demonstration of an antique electronic temperature controller which needs a sensor with a PTC of about 5 to 6 ohms/degree F average (at 20 degrees C); nickel-iron resistance wire fits the bill fairly well, and I would like to find a coil on a low-mass substrate, like ceramic, of roughly 1150 ohms at 20 degrees C, hopefully no more than a few inches long (up to 5 inches is OK). I have some resistance wire salvaged from old soldering iron heater cores, but its gauge is a bit large such that to get 1150 ohms would require quite a lot of it on a rather large form. Perhaps someone has recollections of small assemblies of resistance wire either used for heating, in the 10 to 12 Watt (at 120 VAC) range, or for sensing, that might come in at around 1150 ohms, so that I might avoid a tedious winding of hair-fine wire under magnification. Surprisingly (to me), my cannibalization of some 10 Watt heating tools (like hot melt glue guns) revealed not resistance wire, but some form of semiconductor with a NTC (isn't thermal runaway an issue here?). I don't know your linearity requirments. I might suggest using a thermsiter. The only thing I know of, 1 K are hard to find, are some SMT thermisters. I had used one soldeing leads on to it. Typical 1097 ohms @ 25C You can add series resistanc eto calibrate scale. Linearity might be off which is gain of device. http://www.vishay.com/docs/28762/28762.pdf greg |
Small nickel-iron temp probe needed
msg wrote:
Greetings, I'm working up a demonstration of an antique electronic temperature controller which needs a sensor with a PTC of about 5 to 6 ohms/degree F average (at 20 degrees C); nickel-iron resistance wire fits the bill fairly well, and I would like to find a coil on a low-mass substrate, like ceramic, of roughly 1150 ohms at 20 degrees C, hopefully no more than a few inches long (up to 5 inches is OK). I have some resistance wire salvaged from old soldering iron heater cores, but its gauge is a bit large such that to get 1150 ohms would require quite a lot of it on a rather large form. Perhaps someone has recollections of small assemblies of resistance wire either used for heating, in the 10 to 12 Watt (at 120 VAC) range, or for sensing, that might come in at around 1150 ohms, so that I might avoid a tedious winding of hair-fine wire under magnification. Surprisingly (to me), my cannibalization of some 10 Watt heating tools (like hot melt glue guns) revealed not resistance wire, but some form of semiconductor with a NTC (isn't thermal runaway an issue here?). Michael Why not use the filament of a Vacuum Tube ? Try a battery operated one like 1B??? Or one like 12AX?? Might have to break the Glass envelope This would solve all the mounting problems How about a 25 watt 110 Volt Light Bulb? I have often wondered how these might work in a Thermal Conductivity Vacuum Pressure Gage ? Yukio YANO |
Small nickel-iron temp probe needed
On Mar 2, 9:28*am, msg wrote:
I'm working up a demonstration of an antique electronic temperature controller which needs a sensor with a PTC of about 5 to 6 ohms/degree F average (at 20 degrees C); nickel-iron resistance wire fits the bill fairly well, and I would like to find a coil on a low-mass substrate, like ceramic, of roughly 1150 ohms at 20 degrees C What alloy, exactly, of 'nickel-iron' do you have in wire form? 5.5 ohms/F at 1150 ohms is about 0.5% per F, 0.9% per C. That's rather high (0.3% per C at room temperature is 'normal' for pure metals like Cu). Usually, alloys have LOWER temperature coefficient than pure metals. Chrome-nickel (like 80% Ni/20% Cr, "Chromel A") heater wire comes in at 0.007% per F. |
Small nickel-iron temp probe needed
Yukio YANO wrote:
msg wrote: snip Perhaps someone has recollections of small assemblies of resistance wire either used for heating, in the 10 to 12 Watt (at 120 VAC) range, or for sensing, that might come in at around 1150 ohms snip Why not use the filament of a Vacuum Tube ? Try a battery operated one like 1B??? Or one like 12AX?? Might have to break the Glass envelope This would solve all the mounting problems How about a 25 watt 110 Volt Light Bulb? I have often wondered how these might work in a Thermal Conductivity Vacuum Pressure Gage ? Thanks for your reply. I suppose one could work up an R-T chart for various filaments, however for my application, I need more sensitivity than tungsten and other conventional filaments could provide. It would be fun to test a filament and/or heater from a 117V tube and also various low wattage light bulbs in a temp bridge, but I do need to approximate the nickel-iron RTC of about .00518 ohm/ohm/degC. Michael |
Small nickel-iron temp probe needed
GregS wrote:
msg wrote: snip Perhaps someone has recollections of small assemblies of resistance wire either used for heating, in the 10 to 12 Watt (at 120 VAC) range, or for sensing, that might come in at around 1150 ohms snip I don't know your linearity requirments. I might suggest using a thermsiter. The only thing I know of, 1 K are hard to find, are some SMT thermisters. I had used one soldeing leads on to it. Typical 1097 ohms @ 25C You can add series resistanc eto calibrate scale. Linearity might be off which is gain of device. Thanks for your reply. I had originally hoped to find a compatible PTC thermistor but decided that I probably would find it easier to scrounge enough nickel-iron wire to do the job, as that was what the controller originally expected for its sensor and is matched to its RTC. Michael |
Small nickel-iron temp probe needed
whit3rd wrote:
On Mar 2, 9:28 am, msg wrote: I'm working up a demonstration of an antique electronic temperature controller which needs a sensor with a PTC of about 5 to 6 ohms/degree F average (at 20 degrees C); nickel-iron resistance wire fits the bill fairly well, and I would like to find a coil on a low-mass substrate, like ceramic, of roughly 1150 ohms at 20 degrees C What alloy, exactly, of 'nickel-iron' do you have in wire form? 5.5 ohms/F at 1150 ohms is about 0.5% per F, 0.9% per C. That's rather high (0.3% per C at room temperature is 'normal' for pure metals like Cu). Usually, alloys have LOWER temperature coefficient than pure metals. Chrome-nickel (like 80% Ni/20% Cr, "Chromel A") heater wire comes in at 0.007% per F. I was hoping that folks could recollect some consumer or industrial items to cannibalize for various nickel-iron alloy wires to try, such as curling irons (of a certain vintage), kitchen appliances, etc. I have some nichrome wire in different gauges but nothing fine enough to make 1k to 2k ohms in a small space yet. The published data for the RTC of (unspecified alloy) nickel-iron I took from the white paper by Minco: "Resistance Thermometry: Principles and Applications" http://www.minco.com/download-media.aspx?id=2284 as .00527 ohm/ohm/deg C. The control range is about 17 deg. C and delta R is 175 ohms over that range; recalculating for nickel-iron shows I should be using a sensor of about 1950 ohm at 20 deg. C; my previous value of 1150 ohms was derived by measurement using a variable resistor to balance the bridge. Evidently component aging (dates from 1946) are to blame and I will need to recalibrate the bridge to use a 2k ohm sensor. Michael |
Small nickel-iron temp probe needed
msg wrote:
Greetings, I'm working up a demonstration of an antique electronic temperature controller which needs a sensor with a PTC of about 5 to 6 ohms/degree F average (at 20 degrees C); nickel-iron resistance wire fits the bill fairly well, and I would like to find a coil on a low-mass substrate, like ceramic, of roughly 1150 ohms at 20 degrees C, hopefully no more than a few inches long (up to 5 inches is OK). I have some resistance wire salvaged from old soldering iron heater cores, but its gauge is a bit large such that to get 1150 ohms would require quite a lot of it on a rather large form. Perhaps someone has recollections of small assemblies of resistance wire either used for heating, in the 10 to 12 Watt (at 120 VAC) range... snip I disassembled a wooden-handled curling iron (unknown mfg, made in USA perhaps in the 1940s or 50s) and discovered only an eight inch length of heater cable doubled up and crimped to twin-lead line cord. I would very much appreciate some recollections of appliances which contain lengths of fine-gauge resistance wire. Regards, Michael |
Small nickel-iron temp probe needed
In article t,
msg wrote: Perhaps someone has recollections of small assemblies of resistance wire either used for heating, in the 10 to 12 Watt (at 120 VAC) range, or for sensing, that might come in at around 1150 ohms, so that I might avoid a tedious winding of hair-fine wire under magnification. Michael- Look for a 5 or 10 watt, 1200 Ohm wire wound resistor at Radio Shack. Perhaps its temperature coefficient would not be what you need, but it would give you a point of reference. I wonder if a very old wire wound resistor from someone's junk box, would be more like what you need? Fred |
Small nickel-iron temp probe needed
Fred McKenzie wrote:
In article t, msg wrote: Perhaps someone has recollections of small assemblies of resistance wire either used for heating, in the 10 to 12 Watt (at 120 VAC) range, or for sensing, that might come in at around 1150 ohms, so that I might avoid a tedious winding of hair-fine wire under magnification. Michael- Look for a 5 or 10 watt, 1200 Ohm wire wound resistor at Radio Shack. Perhaps its temperature coefficient would not be what you need, but it would give you a point of reference. I wonder if a very old wire wound resistor from someone's junk box, would be more like what you need? Perhaps some 'antique' wirewound resistors would have a useful R-T characteristic; I tried some post-war types and they seem to be somehow temp compensated and not very sensitive, but I have not by any means exhausted the subset of potential candidates that deserve to be tested. I assumed that the alloys used for (modern) wirewound resistors would also be chosen to reduce temperature sensitivity. Michael |
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