Using the RCTIME with low values of resistance....question?

[deno]

Hello, I seem to be having a problem with measuring low values of resistance on a water tank sender using RCTIME.· This sender will vary between 33 ohms and 240 ohms, depending if full or empty.· Sample time can be up to 1 minute, so there is plenty of time to charge the capacitor.·

Have any of you fellow "Stampers" been sucessful in measuring low values of resistance using the RCTIME command.· I have tried several values of capacitance and I seem to get:

········· 1. wandering readings that slowly increase or decrease.
··········2. a reading for the 33 ohm value but a value of 1 for the 240 ohm.
·········
When I do the math on paper...the formulas used in the "HELP" section of RCTIME, it works out to the proper units of time (depending on which stamp I use).· I am thinking it would be better to use a BS2 stamp, to keep the units of time below the value of a WORD varaible (65536).

Any thoughts on this?

Deno

[Bruce Bates]

Deno -

I have a suspicion that you will find the applications notes on RCTIME very helpful. Thanks go to Dr. Tracy Allen:
http://www.emesystems.com/BS2rct.htm

Regards,

Bruce Bates

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[PJ Allen]

This was discussed a few months back -- http://forums.parallax.com/showthread.php?p=575371 (http://forums.parallax.com/showthread.php?p=575371)
[ The low-ohms situation came up in the posts about 2/3 of the way down. ]
·

[deno]

Thanks to all who replyed.· I found Dr. Allen's article/paper on the RCTIME very helpful.·I would have never found it on the·internet without your direction. ·I also had to play around by trial and error to get a value of capacitance that would not "bottom" out the time units, either with a "0" or a "1".

Thanks again...deno