RTD linearization
in [Design]
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From: JosephKK on 19 Dec 2009 17:44 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. > I think you slipped some decimal points there. That would require a theta(device-air) on the order of 1000 K/W. I don't think even a bare 01005 could be that bad. > >>The opamp will have a gain of about 8, input and output centered on >>+2.5. This should be accurate to a fraction of a degree C, so we'll >>know how to dress. The Z-wave home automation systems have a combined >>motion detector and temperature sensor (no temp-only nodes I know of), >>but I don't trust their accuracy and they only go down to 40F, not >>good for outdoors. >> >>I can run RG174 to the RTDs. No EMI/ESD hazards as with semiconductor >>sensors. >> >>John >> >> >> >> > > >Best regards, >Spehro Pefhany
From: JosephKK on 19 Dec 2009 17:48 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. > >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 I would like to see them just as a matter of curiosity.
From: JosephKK on 19 Dec 2009 18:00 On Mon, 14 Dec 2009 20:52:00 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >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. > For household use you are probably right. In some other cases that is no where near good enough. For example some sensitive chemical processes.
From: John Larkin on 19 Dec 2009 19:38 On Sat, 19 Dec 2009 14:48:16 -0800, "JosephKK"<quiettechblue(a)yahoo.com> 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. >> >>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 > >I would like to see them just as a matter of curiosity. ftp://jjlarkin.lmi.net/RTD_in_air.JPG ftp://jjlarkin.lmi.net/RTD_on_board.JPG ftp://jjlarkin.lmi.net/RTD_lotsa_copper.JPG John
From: John Larkin on 19 Dec 2009 19:51
On Sat, 19 Dec 2009 14:44:45 -0800, "JosephKK"<quiettechblue(a)yahoo.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. >> >I think you slipped some decimal points there. That would require a >theta(device-air) on the order of 1000 K/W. I don't think even a bare 01005 >could be that bad. His numbers are about right. My 1206 RTD, in free air with skinny leadwires, was roughly 300 K/W, so 6 mW make about a 2 degC error. On an actual pcb with deliberately fat traces, theta is maybe 5x better. I intend to use a bigger ceramic-slab RTD and epoxy it into a plastic tube... and check it against a good thermocouple anyhow. I expect a self-heating error well below 1C. I think I can use the coax as sort of a thermal antenna, too. You might check your numbers before you suggest that Sphero has "slipped some decimal points." John |