# [amsat-bb] Re: General Telemetry Question (PITFALL!)

Zach Leffke zleffke at vt.edu
Thu Dec 12 08:15:27 PST 2013

```Perfect Timing!  I'm finishing up details of my Master's Thesis and
using Excel Curve fitting functions for some plotted simulation data

This Pitfall sub-thread was perfectly timed to help me avoid showing
useless data without enough significant figures.

This listserv rocks, Thanks!!

-Zach, KJ4QLP

On 12/11/2013 2:03 PM, Jim White wrote:
> I have been using the free program CurveExpert for years for plotting
> and calculating calibration data. That is, for deriving the cal
> equation from test data.
> http://www.curveexpert.net/
>
> It lets you choose linear or quadratic and if quadratic you can pick
> how many terms you want.  It lets you very quickly experiment with the
> form of the formula so you can see how to get the best fit.  It also
> will suggest the highest exponential value for the formula.
>
> I generally build my table of test data, ADC counts vs measured values
> (V, I, temp, etc.), in Excel. Then I copy and paste that table into
> Curve Expert, click one button to make it fit the curve and another to
> show me the coefficients of the formula.  I copy and past those back
> into Excel next to the table. Then I add another column to the table
> with values calculated using the formula and coefficients. That's the
> double check to be sure the formula is correct.
>
> Of course you can also see the standard deviation and other 'goodness'
> values in the CurveExpert data.  But I find I can tell by just looking
> at the fit plot if I got the data wrong or my measurements weren't
> precise enough.
>
> I find CurveExpert much easier to use and much more flexible than
> Excels' curve fitting functions.
>
> Jim
>
> On 12/11/2013 11:43 AM, Robert Bruninga wrote:
>>> To follow up on Bob's comment.  If you send the raw analog sensor
>> data...
>>> Change calibration values if found to be wrong after launch...
>> We did on PCSAT!
>>
>> Caution to Satellite Builders:  Be careful when using an EXCEL TREND
>> LINE
>> equation for doing Engineering Unit conversion back to original
>> units.  It
>> was a big lesson for us back on PCSAT in 2001.
>>
>> The problem is, generally, EXCEL displays trend line equations in a nice
>> GENERAL human readable form.  For example, for our thermistors, the 3rd
>> order trend line equation to convert from telemetry count back to
>> degrees
>> C was displayed by EXCEL as something like this:   2E-7 X^3  - 2E-4
>> X^2 +
>> 1.804E-1 X + 2.379E2.
>>
>> One would think one is getting a very precise to 4-significant digit
>> equation.  WRONG.  Notice the Cubed and Squared terms (which can be very
>> big at warmer temperatures) are only represented to a single decimal
>> digit(+/- 10%)!!!  (2 and 2)...
>>
>> When this trend  line is used (as displayed), to give back our
>> temperatures from the incoming COUNT, the temperatures were way off!
>>
>> The key is to make sure the trend line equation is displayed in
>> SCIENTIFIC
>> format before you write it down and then try to use it.  Then the first
>> two terms above are properly displayed by EXCEL as 1.544E-7 X^3 and
>> -2.069E-4 X^2.  (instead of 2E-7 and 2E-4).
>>
>> We catch this error in a lot of student's work...
>>
>> Bob, WB4APR
>
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