RTD linearization
in [Design]
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From: Jan Panteltje on 13 Dec 2009 12:54 On a sunny day (Sun, 13 Dec 2009 09:28:53 -0800) it happened John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in <8k7ai51i2m2us5a8nl49cb0llls0lq4jdr(a)4ax.com>: >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%. > >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 For a reasonable long cable to the sensor, a 4 wire system with current source would be better. Else you will have to include the variance of the resistance of the copper, plus the voltage drop in the cable. You will also have to take into account cut or shorted cables, and the sort of signals that wil lgenerate, and how to handle that. I find this a bit primitive.
From: Spehro Pefhany on 13 Dec 2009 13:06 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. >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 -- "it's the network..." "The Journey is the reward" speff(a)interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
From: John Larkin on 13 Dec 2009 14:08 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
From: John Larkin on 13 Dec 2009 14:19 On Sun, 13 Dec 2009 17:54:56 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >On a sunny day (Sun, 13 Dec 2009 09:28:53 -0800) it happened John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in ><8k7ai51i2m2us5a8nl49cb0llls0lq4jdr(a)4ax.com>: > >>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%. >> >>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 > >For a reasonable long cable to the sensor, a 4 wire system with current source would be better. Too complex. My engineering budget (design, pcb layout) is one day. I can do the software up in the cabin during the holidays. >Else you will have to include the variance of the resistance of the copper, >plus the voltage drop in the cable. RG174 is about 40 milliOhms per foot, and I doubt I'll need to run over 25 feet to the outdoor sensor (I get to use my Makita hammer drill again!) That's about a 0.25C error, which I can mostly correct for anyhow. >You will also have to take into account cut or shorted cables, and the sort of signals >that wil lgenerate, and how to handle that. Why would my cables get cut or shorted? And it's not as if that would go undetected. >I find this a bit primitive. Simple is not primitive, it's good engineering. Go to a store and look at electronic or mechanical thermometers on display. You may see a 5 degree C spread. I'll be happy with 1 degree true accuracy. John
From: John Larkin on 13 Dec 2009 14:24
On Sun, 13 Dec 2009 09:41:02 -0800, Joerg <invalid(a)invalid.invalid> wrote: >John Larkin 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%. >> >> 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. >> > >Add a BAV99 to +5V and GND at C1 through C4, and 100ohms or so each from >there to the inputs of the 4051. Otherwise, if you have a thunderstorm >in Truckee like we had one yesterday (flash, followed by loud bang in a >couple hundred milliseconds) your 4051 might go *PHUT* The RCs (like R1, C1) ahead of the mux should take care of that. Using grounded coax helps, too. > > >> I can run RG174 to the RTDs. No EMI/ESD hazards as with semiconductor >> sensors. >> > >Ain't that bad. Most semi sensors have a resistive output and you can >bypass the dickens out of it. Accuracy is usually bad, 3C for many parts, and I'd need a 3-wire run. The RTDs are nice. John |