RTD linearization
in [Design]
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From: JosephKK on 19 Dec 2009 20:34 On Sun, 13 Dec 2009 09:50:22 -0600, "RogerN" <regor(a)midwest.net> wrote: > >"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message >news:p1u7i5thbjmtjvqcj63b291l19rf7ktllp(a)4ax.com... >> >> >> Does anybody remember the value of negative resistance that linearizes >> a 100 ohm platinum RTD? >> >> John > >I thought RTD's were supposed to be linear. The 100 ohm resistance being at >0 Degrees C and a change of .385 ohms (for a 100 Ohm RTD) per Degree C. I >read a description of instrumentation for RTD's once. They said they used a >1ma current source to the RTD and compared it to the voltage drop with 1ma >in a 100 Ohm resistor. Low current, 1ma, was used to minimize the RTD >heating up from power. I know there is a 3rd and sometimes 4th sense wire >used to compensate for the lead resistance. I don't exactly remember the >source but I read the information when looking up info for my Allen Bradley >RTD input card for my PLC 5 rack. > >RogerN > Platinum RTDs are about total repeatability, a real mantra in the measurement community. And 393 ppm/K is an exponential, like most all resistance tempcos.
From: John Larkin on 19 Dec 2009 20:39 On Sat, 19 Dec 2009 17:34:01 -0800, "JosephKK"<quiettechblue(a)yahoo.com> wrote: >On Sun, 13 Dec 2009 09:50:22 -0600, "RogerN" <regor(a)midwest.net> wrote: > >> >>"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message >>news:p1u7i5thbjmtjvqcj63b291l19rf7ktllp(a)4ax.com... >>> >>> >>> Does anybody remember the value of negative resistance that linearizes >>> a 100 ohm platinum RTD? >>> >>> John >> >>I thought RTD's were supposed to be linear. The 100 ohm resistance being at >>0 Degrees C and a change of .385 ohms (for a 100 Ohm RTD) per Degree C. I >>read a description of instrumentation for RTD's once. They said they used a >>1ma current source to the RTD and compared it to the voltage drop with 1ma >>in a 100 Ohm resistor. Low current, 1ma, was used to minimize the RTD >>heating up from power. I know there is a 3rd and sometimes 4th sense wire >>used to compensate for the lead resistance. I don't exactly remember the >>source but I read the information when looking up info for my Allen Bradley >>RTD input card for my PLC 5 rack. >> >>RogerN >> >Platinum RTDs are about total repeatability, a real mantra in the measurement >community. And 393 ppm/K is an exponential, like most all resistance tempcos. Exponential? How so? John
From: JosephKK on 22 Dec 2009 02:17 On Sat, 19 Dec 2009 16:38:01 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >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 Thranx. I learned.
From: JosephKK on 22 Dec 2009 02:30 On Sat, 19 Dec 2009 16:51:57 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >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 Basically it was 1000 K/W that triggered me. The fine leads in free air is not always representative. Your other measurements show that. Part of my job task for years has been check for possible absurdities and request or require backup. It shows, eh?
From: JosephKK on 22 Dec 2009 02:38
On Sat, 19 Dec 2009 17:39:19 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sat, 19 Dec 2009 17:34:01 -0800, >"JosephKK"<quiettechblue(a)yahoo.com> wrote: > >>On Sun, 13 Dec 2009 09:50:22 -0600, "RogerN" <regor(a)midwest.net> wrote: >> >>> >>>"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message >>>news:p1u7i5thbjmtjvqcj63b291l19rf7ktllp(a)4ax.com... >>>> >>>> >>>> Does anybody remember the value of negative resistance that linearizes >>>> a 100 ohm platinum RTD? >>>> >>>> John >>> >>>I thought RTD's were supposed to be linear. The 100 ohm resistance being at >>>0 Degrees C and a change of .385 ohms (for a 100 Ohm RTD) per Degree C. I >>>read a description of instrumentation for RTD's once. They said they used a >>>1ma current source to the RTD and compared it to the voltage drop with 1ma >>>in a 100 Ohm resistor. Low current, 1ma, was used to minimize the RTD >>>heating up from power. I know there is a 3rd and sometimes 4th sense wire >>>used to compensate for the lead resistance. I don't exactly remember the >>>source but I read the information when looking up info for my Allen Bradley >>>RTD input card for my PLC 5 rack. >>> >>>RogerN >>> >>Platinum RTDs are about total repeatability, a real mantra in the measurement >>community. And 393 ppm/K is an exponential, like most all resistance tempcos. > >Exponential? How so? > >John Each degree of temperature change is a multiplier on the degree base before it. so the recurrence relation results in an exponential. Thus it can be modeled as r' = r(25) * k(1)e^[k(2)*t], an exponential. BTW just look at a R vs T plot. |