I'm seeking those clips that hold TO-92 cases. They look sorta like the number 9, with the TO-92 going in the donut hole, and the tail having a hole for a mounting screw. They hold the device parallel to the mounting surface.
But I am having no luck finding the right term to describe such to Mr. Digikey or Ms Mouser.....Suggestions?
I'm open to alternatives; our task is to fasten a temp sensor to a 0.25" OD copper line to sense the fluid temp. within... I planned to solder the clips to the line...
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I found a few attached to transistors, but nothing sold individually. Try your luck and see if you can do better:
Ummm... TO92 heat sink?
If that's part of a temperature control system, it's not going to work very well. The problem is that you're measuring the temperature of a big copper tubular heat sink. It will take a while for the fluid to heat or cool the copper tubing resulting in a rather slow response time. If there's any air flow over this thing, it will cool the temp sensor and copper, resulting in what might be a substantial temperature error. Basically, you want to measure the temperature of the fluid and not the temperature of the tubing or the environment.
I suggest you drill a small hole in the copper tubing, shove in a small glass thermistor probe that contacts the fluid and *NOT* the copper tubing, and seal with epoxy glue. You'll have much faster response, better accuracy, and less time wasted trying to find strange heat sinks.
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Moving fluids, especially water, have enormous equivalent thermal conductivities, and copper is pretty good, too. Air is a terrible thermal conductor. So the surface temp of a copper tube that carries a flowing liquid is very close to the liquid temp. The time constants will be very fast.
Add a little external insulation to do even better.
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A more intrusive way is to adapt the 0.25" tube to a larger (maybe
3/8") tube with a T-fitting so the fluid makes a 90° turn, and insert a sealed probe deep into the larger tubing (say 10 diameters of the probe OD plus the sensitive end length). The hard right turn also causes turbulence if you have enough flow, and prevents laminar flow which will screw up your readings. Read about Reynolds numbers and such if you're interested- it can be important. A 1/8" OD probe would be better than something a TO-92 will fit into.
More Mickey Mouse (tm) would be to put a dollop of heatsink compount on the TO-92 (with attached and electrically insulated leads, obviously), wrap wide adhesive copper tape around the two, and finish with some adhesive-lined shrink tubing and outer (thermal) insulation.
What's worse, TO-92 packages are excellent insulators--all that sensor will do is measure the temperature of its leads. TO220 would be a much better idea.
Thermal diffusion in organic materials is amazingly slow--like 5000 times slower than in aluminum or copper. A half-mil layer of Kapton tape is slower than an inch of aluminum.
A small copper tube's temperature won't be a bad proxy for the temperature of the fluid _in_the_tube_, not necessarily in some reservoir. Certainly the error will be smaller than the die temperature to tube temperature error, if you use a TO-92.
My vote would be for a PT1000 RTD attached to the tube with Arctic Silver epoxy or something equivalent, or else a TO220 transistor.
It would be interesting to think about using the gate-cathode junction of a TO220 sensitive-gate SCR instead of a transistor. Since the tab is normally the anode, you'd have junction isolation between the tubing and the signal ground, which should help a lot with ground loops without requiring a slow and/or messy thermal junction.
Cheers
Phil Hobbs
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Dr Philip C D Hobbs
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Whoops, diffusion speeds up quadratically as you go thinner. The half-mil of Kapton is about as fast as sqrt(2500) mils of aluminum.
The thermal _conductivity_ is thousands of times less.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
Realistically given the thermal TCs of all the rest of this stuff, that would probably be fine. I'd want to do a bit better job of thermally grounding the leads than they seem to be doing.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
Agreed, and quite true where the mass of the fluid is much larger than the mass of the pipe, such as in a household steel 1/2" or 3/4" water pipe. Not so true for the OP's 1/4" copper tubing where: OD = 1/4" ID = 3/16" Cross sectional area = Pi * r^2 OD area = Pi * 0.125^2 = 0.0491 sq in ID area = Pi * 0.188^2 = 0.0276 sq in Cross sectional area of copper = 0.0491 - 0.0276 = 0.0215 sq in. Cross sectional area of fluid = 0.0276 sq in.
In the OP's case, the volume of the fluid per unit length is only about 20% more than that of the copper. For a slow moving fluid, with slow temperature changes, that's probably ok. For a fast moving fluid, with abrupt temperature changes, the doubling of the thermal mass by the copper tubing is going to slow things down.
Also, because the fluid mass is about equal to the mass of the copper heat sink, air currents over the 1/4" copper tubing is going to have an effect. Thermal insulation of the sensor is useful, but with high air flow, such as under the hood of an automobile, one would need to insulate the entire length of tubing.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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