RTD linearization
in [Design]
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From: Spehro Pefhany on 14 Dec 2009 14:01 On Sun, 13 Dec 2009 11:08:36 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sun, 13 Dec 2009 13:06:36 -0500, Spehro Pefhany ><speffSNIP(a)interlogDOTyou.knowwhat> wrote: > >>On Sun, 13 Dec 2009 09:28:53 -0800, the renowned John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>>On Sun, 13 Dec 2009 12:20:05 GMT, nico(a)puntnl.niks (Nico Coesel) >>>wrote: >>> >>>>John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> >>>>>On Sat, 12 Dec 2009 12:46:37 -0800, Joerg <invalid(a)invalid.invalid> >>>>>wrote: >>>>> >>>>>>John Larkin wrote: >>>>>>> >>>>>>> Does anybody remember the value of negative resistance that linearizes >>>>>>> a 100 ohm platinum RTD? >>>>>>> >>>>>> >>>>>>No uC at hand for this job? Maybe this helps: >>>>>> >>>>>>http://pdfserv.maxim-ic.com/en/an/AN3450.pdf >>>>>> >>>>>>But you don't have to use a Maxim opamp :-) >>>>> >>>>>I'm thinking I'll use 1K RTDs for the automation project, and lay out >>>>>an interface board... easier than hand wiring. The little RS232 widget >>>> >>>>1k RTDs are easier to interface. I used one to control my floor >>>>heating. 2k2 (IIRC) in series from 3.3V and then fed directly into an >>>>ADC. In a limited temperature range, the output is quite linear so >>>>there is not really a need for fancy math. >>> >>>I'm thinking along these lines... >>> >>>ftp://jjlarkin.lmi.net/RTD.jpg >>> >>>All the 1Ks will be 0.1%. >> >>I don't like this concept very much. Typically we'll run a 100R RTD at >>< 1mA for a total Pd of < 100uW (much much less in some applications). >>And usually the RTDs have a lot of added surface area because they're >>glumped into some kind of protection tube. >> >>You have 1K RTDs running at 2.5mA for a power dissipation of around >>6mW at 20C. That alone will result in an error of several degrees C >>with an un-housed thin film sensor in moving air, more in static air, >>natch. >> > >Of course I've considered self-heating. You considered it but didn't scale it to measure wind chill? Sad face. >I was planning to use the largish Minco ceramic-slab parts (we have >1Ks in stock) and stick them in a plastic tube full of epoxy, to >weatherize and reduce theta. The self-heat error should be small. I >could epoxy them to a small strip of aluminum first, if I were >compulsive. And I may as well calibrate the whole thing end-to-end >against a good thermocouple. > >I do have some scope shots that quantify 1206 surface-mount RTD >transient self-heating under different mounting scenarios. I could >post them if there were great popular demand. > >John Sure, be interested. But I imagine the mounting scenario is pretty important.
From: John Larkin on 14 Dec 2009 15:38 On Mon, 14 Dec 2009 14:01:11 -0500, Spehro Pefhany <speffSNIP(a)interlogDOTyou.knowwhat> wrote: >On Sun, 13 Dec 2009 11:08:36 -0800, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Sun, 13 Dec 2009 13:06:36 -0500, Spehro Pefhany >><speffSNIP(a)interlogDOTyou.knowwhat> wrote: >> >>>On Sun, 13 Dec 2009 09:28:53 -0800, the renowned John Larkin >>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>> >>>>On Sun, 13 Dec 2009 12:20:05 GMT, nico(a)puntnl.niks (Nico Coesel) >>>>wrote: >>>> >>>>>John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>>> >>>>>>On Sat, 12 Dec 2009 12:46:37 -0800, Joerg <invalid(a)invalid.invalid> >>>>>>wrote: >>>>>> >>>>>>>John Larkin wrote: >>>>>>>> >>>>>>>> Does anybody remember the value of negative resistance that linearizes >>>>>>>> a 100 ohm platinum RTD? >>>>>>>> >>>>>>> >>>>>>>No uC at hand for this job? Maybe this helps: >>>>>>> >>>>>>>http://pdfserv.maxim-ic.com/en/an/AN3450.pdf >>>>>>> >>>>>>>But you don't have to use a Maxim opamp :-) >>>>>> >>>>>>I'm thinking I'll use 1K RTDs for the automation project, and lay out >>>>>>an interface board... easier than hand wiring. The little RS232 widget >>>>> >>>>>1k RTDs are easier to interface. I used one to control my floor >>>>>heating. 2k2 (IIRC) in series from 3.3V and then fed directly into an >>>>>ADC. In a limited temperature range, the output is quite linear so >>>>>there is not really a need for fancy math. >>>> >>>>I'm thinking along these lines... >>>> >>>>ftp://jjlarkin.lmi.net/RTD.jpg >>>> >>>>All the 1Ks will be 0.1%. >>> >>>I don't like this concept very much. Typically we'll run a 100R RTD at >>>< 1mA for a total Pd of < 100uW (much much less in some applications). >>>And usually the RTDs have a lot of added surface area because they're >>>glumped into some kind of protection tube. >>> >>>You have 1K RTDs running at 2.5mA for a power dissipation of around >>>6mW at 20C. That alone will result in an error of several degrees C >>>with an un-housed thin film sensor in moving air, more in static air, >>>natch. >>> >> >>Of course I've considered self-heating. > >You considered it but didn't scale it to measure wind chill? Sad face. > >>I was planning to use the largish Minco ceramic-slab parts (we have >>1Ks in stock) and stick them in a plastic tube full of epoxy, to >>weatherize and reduce theta. The self-heat error should be small. I >>could epoxy them to a small strip of aluminum first, if I were >>compulsive. And I may as well calibrate the whole thing end-to-end >>against a good thermocouple. >> >>I do have some scope shots that quantify 1206 surface-mount RTD >>transient self-heating under different mounting scenarios. I could >>post them if there were great popular demand. >> >>John > >Sure, be interested. But I imagine the mounting scenario is pretty >important. ftp://jjlarkin.lmi.net/RTD_in_air.JPG ftp://jjlarkin.lmi.net/RTD_on_board.JPG ftp://jjlarkin.lmi.net/RTD_lotsa_copper.JPG This confirms my observation that tiny surface-mount resistors can dissipate a lot of power if they are soldered to big chunks of copper. John
From: Jan Panteltje on 14 Dec 2009 15:52 On a sunny day (Mon, 14 Dec 2009 12:38:00 -0800) it happened John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in <na8di5tp5s8sdtfclqmrhf5erfuvhod3ur(a)4ax.com>: >ftp://jjlarkin.lmi.net/RTD_in_air.JPG > >ftp://jjlarkin.lmi.net/RTD_on_board.JPG > >ftp://jjlarkin.lmi.net/RTD_lotsa_copper.JPG > > >This confirms my observation that tiny surface-mount resistors can >dissipate a lot of power if they are soldered to big chunks of copper. > >John All good and well, but when measuring temperature I would think you wanted a low thermal mass to get fast response? I have LM135 in an old SMD plastic tube filled with silicone kit, outside, 1 meter from the ground, about 10 cm from the wall, hanging from the wire. Been working now OK for 10 years or so. Nice defined output voltage versus temperature. You can calibrate those too. But you really do not need better accuracy then 1C... ever.
From: Jan Panteltje on 14 Dec 2009 17:10 I wrote: All good and well, but when measuring temperature I would think you wanted a low thermal mass >to get fast response? I have LM135 in an old SMD plastic tube filled with silicone kit, outside, 1 meter from the ground, about 10 cm from the wall, hanging from the wire. Been working now OK for 10 years or so. Nice defined output voltage versus temperature. You can calibrate those too. But you really do not need better accuracy then 1C... ever. And I want to add something to that. All that talk about measuring to .x digits temperature (substitute for 'x' what you had in mind), will really look funny when: The sun shines on your outside sensor, and the temp is all of the sudden 20 degrees to high. Hail falls on it, and starts melting, and it insists it is zero C. Snow, rain, so: You need a very good place, shielded from wind, sun, away from hot walls, other radiating objects, at the correct height. I have been measuring outside temp now for many years, and seen some very funny readings.
From: John Larkin on 14 Dec 2009 17:40
On Mon, 14 Dec 2009 22:10:22 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >I wrote: > All good and well, but when measuring temperature I would think you wanted a low thermal mass > >to get fast response? > > I have LM135 in an old SMD plastic tube filled with silicone kit, outside, > 1 meter from the ground, about 10 cm from the wall, hanging from the wire. > Been working now OK for 10 years or so. > Nice defined output voltage versus temperature. > You can calibrate those too. > But you really do not need better accuracy then 1C... ever. But I am an artiste! > > >And I want to add something to that. >All that talk about measuring to .x digits temperature (substitute for 'x' >what you had in mind), will really look funny when: > The sun shines on your outside sensor, and the temp is all of the sudden 20 degrees to high. > Hail falls on it, and starts melting, and it insists it is zero C. >Snow, rain, so: > You need a very good place, shielded from wind, sun, away from hot walls, > other radiating objects, at the correct height. > >I have been measuring outside temp now for many years, and seen some very funny readings. I plan to put the outdoor sensor (1K RTD in a plastic tube, epoxy filled) on the north side of the cabin, in the niche under the stairs where the gas meter (and all the old skis) is stashed, exposed to the world but protected from rain and snow. The adjacent wall is concrete blocks, the wall of the unheated garage. Response speed isn't much of an issue here. John |