[Elecraft] [K3] SWR - Numerical Indication

[Don Wilhelm]

All,

There is a big difference between accuracy and precision.
Precision can be accomplished by adding the extra digits, but the 
accuracy of the instrument will tell you how many of those extra digits 
are to be considered.

Most digital voltmeters, even the cheap or free ones from Harbor Freight 
and others are surprisingly accurate (5% or so is common), but typical 
voltage readings of 3 significant digits are reasonable but any digits 
more than that are extraneous unless the instrument is of lab quality.

73,
Don W3FPR

On 11/30/2018 11:12 AM, Charlie T wrote:
> Yep,  ur right Don.  A freebie Harbor Freight digital voltmeter appears to
> have a helluva lot better accuracy than reality.
> Digital calculators on the other hand, ARE accurate.
> This changes our mind-set, simply because we see all those digits past the
> decimal point on a dVOM.
> 
> it was better when we only had slide-rules to calculate things.
> Accuracy to more than three places was at best, a guess.
> Besides, what good does it do to arrive at a required by-pass capacitor's
> value of 0.0110987 µF ?
> 
> 73, Charlie k3ICH
> 
> 
> 
> 
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net <elecraft-bounces at mailman.qth.net> On
> Behalf Of Don Wilhelm
> Sent: Friday, November 30, 2018 9:56 AM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] [K3] SWR - Numerical Indication
> 
> Petr and all,
> 
> Those digital instruments that show 2, 3, or 4 decimal places have given us
> a false sense of accuracy.
> For instance an instrument that is accurate to 5% and has a 4 digit display
> can show us (when measuring a 5 volt source) anywhere between
> 4.750 volts and 5.250 volts and still be within the 5% accuracy window for
> the instrument.
> 
> Review the specs and calibration for whatever meter you are using and do not
> expect those extra digits to be correct - in other words round the numbers
> displayed.
> 
> Many wattmeters are only accurate to 20% of the reading - so if one
> wattmeter at 100 watts shows 120 watts and another shows 80 watts, the
> actual power could be 100 watts.  Take that into consideration.
> 
> The Telepost LP-100 when calibrated to NIST standards is accurate to 5% (it
> can be lower, but Larry will not guarantee it).  So any power it displays
> between 95 and 105 watts can actually be 100 watts.
> 
> In other words, look at the specified accuracy of whatever meter you are
> using and take that into consideration.  Those extra digits on your meter
> may be meaningless.
> 
> 73,
> Don W3FPR
> 
> 
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