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[aprssig] FindU Telemetry page - Help needed

Steve Dimse steve at dimse.com
Fri Oct 10 20:02:25 UTC 2008


Let me guess, you learned using the new math... if it doesn't involve  
apples or trains heading in opposite directions, you are lost ;-)

First, this is also a linear system, which means that the a is zero.  
For a given change in the telemetered value, the measured parameter  
changed the same amount, whether the parameter change is from 99 to  
100 or 1 to 2.

Given it is linear, the formula is bx + c

You are right, the problem is easier without converting to F, so let's  
do it in C first.

The way you wrote it is (with an extra set of parens added for clarity)

((tele + 4.7) * 1.9608 ) - 273.15

it is just algebra to put this in the form bx + c

distribute the multiplication

((tele x 1.9608) + (4.7 * 1.9608 )) - 273.15

4.7 * 1.9608 is 9.215, add this to the other constant -273.15

that makes the centigrade formula 1.9608 x + (-263.9)

b = 1.9608 c = -263.9

solving for x = 148 gives 26.3 degrees C, a reasonable room temp

so you want F?

F = (C * (9/5) ) + 32

We know C is 1.9608x - 263.9 and 9/5 is 1.8

F = ((1.9608x - 263.9) * 1.8) + 32

Just like above, you distribute the 1.8, and add the two constants to  
get the b and c. This is left as an exercise for the reader. ;-)

Steve K4HG

On Oct 10, 2008, at 3:25 PM, Mark Cheavens wrote:

> I am trying to work with the telemetry page on FindU. I have a
> telemetry value that requires the attached math to make it display
> the way I want. I haven't figured out how to make the math work to
> get it to display correctly. Any help would be appreciated.
>
> Sample URL:
> http://www1.findu.com/cgi-bin/tele1.cgi?call=PURG&last=96&param=1&b=1.0&label=Purg_Repeater_Cabinet_Temp&units=Temperature&autorange=1
>
> I am reading telemetry parameter 1.
>
> I need to take the value (Currently sitting on my bench in an air
> conditioned room!) with a value of 148
>
> I would LIKE to have the value in Degrees F (I know that C would be
> easier).....
>
> So here is the formula:
>
> ((Telemetry + Correction Factor) X 1.9608 Sensor Multiplier - 273.15)
> * 5 / 9 - 32 = F
>
> For this install the correction factor is 4.7.
> 1.9608 is the sensor multiplier (Constant)
> The rest if just the basic math...
>
> OR ((148-4.7)*1.9608-273.15)*9/5+32=TempF
>
> I don't understand how the a= b= and c= can get me there!
> http://www.findu.com/cgi.html#tele1
>
> Any help would be appreciated!
>
> (I have figured out the easy linear ones in the past - Like this same
> one for voltage -
> http://www1.findu.com/cgi-bin/tele1.cgi?call=PURG&last=96&param=2&b=.072254&label=Purg_Batt_Voltage&units=volts&autorange=1
>
> (FYI - This is the output of an OpenTracker+ being used in an outdoor
> solar only radio site (Emergency Clinic) to record interior
> temperature for the batteries and battery voltage. - I do have it on
> the status line, but I don't like the resolution, and would like to
> plot it like I have done for the voltage) http://map.findu.com/purg
>
>
> Mark
> KC5EVE
>
>
>
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