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

[Brian Dessent]

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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