Hi,

Just got an echometer off a mate to try out.

Has anyone used these before?

How accurate are they?

I've just calibrated a cable to measure a 100 metre reel, and the results seem a bit strange.

I cut a meter off the reel and the total length only goes down by 300mm. Hmmm....

I've never seen a unit like you are using so I'm probably talking rubbish but ....

300mm sounds suspiciously like one foot.

Sanity check: are you sure the units are consistent? Cal in feet / readout in metres or vice versa?

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Andy Fierman

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http://signality.co.uk
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I take it from the description that you are talking about a Time Domain Reflectometer (TDR), also sometimes know as a Pulse Echo Tester (PET).

The uncertainty of these instruments is usually around few percent of reading and often the resolution to which they can be read is only one metre. If you are having to position a cursor on a feature on the trace to determine the distance to that feature then there is also the uncertainty associated with determining the start of the feature.

Unless the instrument has a variable velocity of propagation (VoP) setting, there will be further uncertainties introduced if the instrument is used on cable types other than that for which the VoP has been set by the manufacturer.

The result which you have obtained (excepting any uncertainties in the measurement of the cable and the piece which you cut off) is less than 1% which is a reasonable uncertainty for the type of instrument in question.

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Ross Currie TMIET