RTD linearization
in [Design]
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From: Jan Panteltje on 14 Dec 2009 17:54 On a sunny day (Mon, 14 Dec 2009 14:40:42 -0800) it happened John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in <ocfdi55vqof40ba34eu8efe47ntn5vgrqp(a)4ax.com>: >But I am an artiste! > Are not we all? Designing is an artistic action. >>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,\ Well, at least you can exchange it anytime for a LM135 in that same circuit, without needing linearisation, only a software update. >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. Sounds good, hope the cat does not sleep on it there :-) >Response speed isn't much of an issue here. > >John Yes that is true, weather does not change that fast.
From: langwadt on 14 Dec 2009 18:07 On 13 Dec., 18:54, Jan Panteltje <pNaonStpealm...(a)yahoo.com> wrote: > On a sunny day (Sun, 13 Dec 2009 09:28:53 -0800) it happened John Larkin > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote in > <8k7ai51i2m2us5a8nl49cb0llls0lq4...(a)4ax.com>: > > > > >On Sun, 13 Dec 2009 12:20:05 GMT, n...(a)puntnl.niks (Nico Coesel) > >wrote: > > >>John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > > >>>On Sat, 12 Dec 2009 12:46:37 -0800, Joerg <inva...(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. Here we normally use pt1000 so copper resistance is not such a big issue, and driving it with a current source has caused lots of issues, because the common mode noise rejection sucks when one wire sees ground the other a high impedance current source it is possible to get around it with a much more complex circuit but why bother if you can correct in software -Lasse
From: Jan Panteltje on 14 Dec 2009 18:18 On a sunny day (Mon, 14 Dec 2009 15:07:57 -0800 (PST)) it happened "langwadt(a)fonz.dk" <langwadt(a)fonz.dk> wrote in <58449058-bc64-4418-8392-8ed79dd5023d(a)m25g2000yqc.googlegroups.com>: >On 13 Dec., 18:54, Jan Panteltje <pNaonStpealm...(a)yahoo.com> wrote: >> On a sunny day (Sun, 13 Dec 2009 09:28:53 -0800) it happened John Larkin >> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote in >> <8k7ai51i2m2us5a8nl49cb0llls0lq4...(a)4ax.com>: >> >> >> >> >On Sun, 13 Dec 2009 12:20:05 GMT, n...(a)puntnl.niks (Nico Coesel) >> >wrote: >> >> >>John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >> >>>On Sat, 12 Dec 2009 12:46:37 -0800, Joerg <inva...(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. > >Here we normally use pt1000 so copper resistance is not such a big >issue, and driving it with >a current source has caused lots of issues, because the common mode >noise rejection sucks >when one wire sees ground the other a high impedance current source >it is possible to get around it with a much more complex circuit but >why bother if you can >correct in software > >-Lasse Why not use LM135 or similar?
From: langwadt on 14 Dec 2009 19:01 On 15 Dec., 00:18, Jan Panteltje <pNaonStpealm...(a)yahoo.com> wrote: > On a sunny day (Mon, 14 Dec 2009 15:07:57 -0800 (PST)) it happened > "langw...(a)fonz.dk" <langw...(a)fonz.dk> wrote in > <58449058-bc64-4418-8392-8ed79dd50...(a)m25g2000yqc.googlegroups.com>: > > > > >On 13 Dec., 18:54, Jan Panteltje <pNaonStpealm...(a)yahoo.com> wrote: > >> On a sunny day (Sun, 13 Dec 2009 09:28:53 -0800) it happened John Larkin > >> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote in > >> <8k7ai51i2m2us5a8nl49cb0llls0lq4...(a)4ax.com>: > > >> >On Sun, 13 Dec 2009 12:20:05 GMT, n...(a)puntnl.niks (Nico Coesel) > >> >wrote: > > >> >>John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > > >> >>>On Sat, 12 Dec 2009 12:46:37 -0800, Joerg <inva...(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. > > >Here we normally use pt1000 so copper resistance is not such a big > >issue, and driving it with > >a current source has caused lots of issues, because the common mode > >noise rejection sucks > >when one wire sees ground the other a high impedance current source > >it is possible to get around it with a much more complex circuit but > >why bother if you can > >correct in software > > >-Lasse > > Why not use LM135 or similar? might be simpler electrically, but generally the inductrial type temperature sensors in stainless steel meant to screw into e.g. pressuretanks pipes etc. are pt1000/pt100 types -Lasse
From: Nico Coesel on 14 Dec 2009 19:18
Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >On a sunny day (Mon, 14 Dec 2009 15:07:57 -0800 (PST)) it happened >"langwadt(a)fonz.dk" <langwadt(a)fonz.dk> wrote in ><58449058-bc64-4418-8392-8ed79dd5023d(a)m25g2000yqc.googlegroups.com>: > >>> that wil lgenerate, and how to handle that. >>> I find this a bit primitive. >> >>Here we normally use pt1000 so copper resistance is not such a big >>issue, and driving it with >>a current source has caused lots of issues, because the common mode >>noise rejection sucks >>when one wire sees ground the other a high impedance current source >>it is possible to get around it with a much more complex circuit but >>why bother if you can >>correct in software >> >>-Lasse > >Why not use LM135 or similar? A pt1000 element is available in industrial bolt-on versions and no polarity issues. IOW any fool can install a pt1000 element. -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... "If it doesn't fit, use a bigger hammer!" -------------------------------------------------------------- |