UPDATE Re: Re: [Diy_efi] Question relating to old

[Mr Motor]

well that is going beyond my knowledge of the subject....i'm not
looking for it to be supremely accurately, but as long as its
close and i can get a pretty good estimate of the inlet air temp
before the intercooler, that is all i'm looking for.

Also, since the dataq has 4 channels i planned on using another
channel to datalog backpressure. Now since a pressure sensor is
a transducer (i think) and not a thermistor, i shouldn't have to
use any type of resistor correct or am i wrong?

I figured i could just hook up this sensor the same way as the
IAT except without the resistor. Then calibrate the pressure
sensor via the software and then i could datalog the voltage

Am i finally starting to get this?

thanks for all the help and the extra knowledge

jim




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---- On Mon, 09 Jun 2003, Brian Dessent (brian at dessent.net)
wrote:

> Mr Motor wrote:
> > 
> > Wow, this is cool :)
> > 
> > I was reading the dataq 194 manual and i can calibrate
whatever
> > signal i am datalogging and display it as whatever unit i
> > decide
> > 
> > basically you set the voltage level (low/high), the
correlating
> > values, and the unit of measurement
> > 
> > so i'd put in 0 volt for low, 5 volt for high and then use
the
> > multi-meter and a thermometer to determine a few points
along
> > the curve and enter the temp for 5 volt, and then put in
the
> > measurement as F*.
> 
> You can try that, but there are two things to remember: first
the
> thermistor is a NTC type, so the voltage will decrease with
> temperature.  Second, it's not going to be exactly linear. 
How you want
> to handle it depends on how accurate you want to be.
> 
> The thermistor itself has a very nonlinear resistance vs.
temperature
> curve.  It can be modeled as:
> 
> 1/T = A + B*ln(R) + C*ln^3(R)
> 
> where A, B, and C are constants that depend on the particular
> thermistor.  You can see a sample graph in shop manuals. 
There's also
> some data on the www.diy-efi.org server.  I did a curve fit of
the GM
> data, and I get the following values (note that T has units
Kelvin which
> is C+273.15):
> 
> A = 1.476E-03   B = 2.299E-04   C = 1.069E-07
> 
> Anyway, putting the thermistor into a resitive divider tends
to
> counteract the nonlinearity, at least if you stay away from
the extremes
> near 0V and 5V.  If Rp is the pullup resistor to +5V and R is
the
> resistance of the thermistor, the voltage at the divider (the
point
> where the two resistors connect) is:
> 
> V = 5/(1 + Rp/R)
> 
> So, with those two equations you can graph the theoretical
temperature
> vs. volts curve.  (BTW, the first equation solved for R is
here:
> <http://www.betatherm.com/thermistortheory/steinequas.htm>) 
If you look
> at the graph, it's kind of linear at the center area, but it
has sort of
> an S-shape, so that near the outer extremes it becomes more
sloped.
> 
> For kicks I played around with this in Excel, I'll paste the
data below
> in case it's helpful to anyone... Assuming a pullup resistor
of 2.49k,
> and the curve-fit coefficients from the GM sensor:
> 
> temp:   R:      Voltage:
> 250	96	0.186
> 245	104	0.200
> 240	112	0.216
> 235	121	0.232
> 230	131	0.250
> 225	142	0.270
> 220	154	0.292
> 215	168	0.315
> 210	182	0.341
> 205	198	0.369
> 200	216	0.399
> 195	236	0.432
> 190	258	0.469
> 185	282	0.508
> 180	308	0.551
> 175	338	0.598
> 170	371	0.649
> 165	408	0.704
> 160	450	0.765
> 155	496	0.830
> 150	547	0.901
> 145	605	0.978
> 140	670	1.060
> 135	743	1.149
> 130	826	1.245
> 125	919	1.348
> 120	1024	1.457
> 115	1143	1.573
> 110	1278	1.696
> 105	1432	1.825
> 100	1607	1.961
> 95	1806	2.102
> 90	2035	2.248
> 85	2296	2.399
> 80	2596	2.552
> 75	2941	2.707
> 70	3338	2.864
> 65	3797	3.020
> 
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> 


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