RTD linearization
in [Design]
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From: RogerN on 13 Dec 2009 10:56 "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 > Just ran across this looking for the alpha number for RTD's. http://canteach.candu.org/library/20030701.pdf RogerN
From: John Larkin on 13 Dec 2009 12:28 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
From: John Larkin on 13 Dec 2009 12:39 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 have a little curvature versus temperature, not bad around room temp. But if you excite them from a power suply and a series resistor, avoiding the current source, the resulting voltage has a bunch of curvature. The combination can be corrected very well, either analog (a positive feedback resistor) or in software, with a simple 2nd order polynomial, one line of Basic. As noted in another post, a 100R platinum RTD becomes very linear if you parallel it with -2750 ohms. John
From: Joerg on 13 Dec 2009 12:41 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* > 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. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Spehro Pefhany on 13 Dec 2009 12:46
On Sun, 13 Dec 2009 09:50:22 -0600, the renowned "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. DIN-standard Pt100 sensors change 38.5 ohms between 0�C and 100�C, but it's not linear (although the error would be rather modest over a typical 'comfort heating' temperature range*). Using a taste of positive feedback you can make it very close to linear (the residual error is an S-curve). Typically we would use controlled current sources because it's hard to compensate for leadwire resistance in industrial setups otherwise. The positive feedback could be set to make the reading dead on at a third point (eg. mid-scale) with zero and span getting the first two points, or one could minimize the error according to some cost function. BTW, there is/was also a US standard of 39.2 ohms change from zero to 100 C, but it's about dead now- pretty much everyone uses the European standard. Back the last time I worked with Pt RTD instrumentation design (a few weeks ago now) I simply implemented a binary search to invert the Callendar-Van Dusen equations to give T(Rx) (I needed to convert in both directions-- T(Rx) and Rx(T) for that application). > 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 * Set it 'on' at 0C and 50C, and the error at 25C is less than 0.1C. More of a problem in industrial situations.. set it on at 0C and 400C and the error at 200C is about 6 degees C. Just as you'd expect for a parabolic error curve, it is proportional to the square of the span, eh? 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 |