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CX2SA > SATDIG 11.12.13 21:04l 167 Lines 6438 Bytes #999 (0) @ WW
BID : AMSATBB8454
Read: GUEST
Subj: AMSAT-BB-digest V8 454
Path: IZ3LSV<IW0QNL<JH4XSY<JE7YGF<7M3TJZ<CX2SA
Sent: 131211/2003Z @:CX2SA.SAL.URY.SA #:1365 [Salto] FBB7.00e $:AMSATBB8454
From: CX2SA@CX2SA.SAL.URY.SA
To : SATDIG@WW
Today's Topics:
1. Re: why not to buy an M2 antenna Sealing Coax (Gordon JC Pearce)
2. Re: General Telemetry Question (PITFALL!) (Robert Bruninga)
3. Re: General Telemetry Question (PITFALL!) (Jim White)
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Message: 1
Date: Wed, 11 Dec 2013 12:39:17 +0000
From: Gordon JC Pearce <gordonjcp@xxxx.xxx>
To: amsat-bb@xxxxx.xxx
Subject: [amsat-bb] Re: why not to buy an M2 antenna Sealing Coax
Message-ID: <20131211123917.GA28514@xxxx.xxx>
Content-Type: text/plain; charset=iso-8859-1
On Tue, Dec 10, 2013 at 04:40:26PM -0800, Glen Zook wrote:
>
> The "cheap stuff" congeals into a waterproof mass after a few days in the
sun. ?More expensive tape often comes loose. ?One does have to cut through
the mass to remove the tape. ?I have removed this tape from antennas that
have been several hundred feet in the air, for decades, and, when removed,
the connector looked just like it did when new!
> ?
> Glen, K9STH
What I think is fun about the rubber self-amalgamating tape, is when you cut
it off if you've put it on right you can see all the manufacturer's
engraving on the connectors in relief in the rubber ;-)
--
Gordonjcp MM0YEQ
------------------------------
Message: 2
Date: Wed, 11 Dec 2013 13:43:48 -0500
From: Robert Bruninga <bruninga@xxxx.xxx>
To: Jim White <jim@xxxxxxxxxxxxxxxxx.xxx>, amsat-bb@xxxxx.xxx
Subject: [amsat-bb] Re: General Telemetry Question (PITFALL!)
Message-ID: <5036cb6100a595388663345c4135f00f@xxxx.xxxxx.xxx>
Content-Type: text/plain; charset=ISO-8859-1
> 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
------------------------------
Message: 3
Date: Wed, 11 Dec 2013 12:03:52 -0700
From: Jim White <jim@xxxxxxxxxxxxxxxxx.xxx>
To: Robert Bruninga <bruninga@xxxx.xxx>, amsat-bb@xxxxx.xxx
Subject: [amsat-bb] Re: General Telemetry Question (PITFALL!)
Message-ID: <52A8B718.5030705@xxxxxxxxxxxxxxxxx.xxx>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
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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End of AMSAT-BB Digest, Vol 8, Issue 454
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