Read RPM's directly from DSO math functions. - Page 1
Forum section: electronics / beginners

author: Emi
author_rank: Contributor
timestamp: September 23, 2019, 05:18:22 pm
content: Hi,I own a Rigol DS2072A including math functions that i totally ignoreHow can i get an RPM converted reading of a frequency?I'know i should grab my calculator and type F(Hz)*60= rpm but it would be cool to get this value right from my DSOis it possible?Thanks !!

author: ledtester
author_rank: Super Contributor
timestamp: September 23, 2019, 09:17:48 pm
content: Does this help?Full manual available at:https://www.batronix.com/pdf/Rigol/UserGuide/DS2000_UserGuide_EN.pdf

author: Emi
author_rank: Contributor
timestamp: September 24, 2019, 06:12:08 am
content: many thanksi already tried this menu without success. .i cant rally understand the "how to"i guess operations can be made in voltage domain only , not in timings/frequencyTell me please if i'm wrong

author: Circlotron
author_rank: Super Contributor
timestamp: September 24, 2019, 06:15:02 am
content: From what I understand, that maths function stuff only outputs to a third trace, not to a numeric reading.

author: Emi
author_rank: Contributor
timestamp: September 24, 2019, 06:19:34 am
content: exaclty what i've sadly supposed. ..stupid scope

author: bitwelder
author_rank: Super Contributor
timestamp: September 24, 2019, 07:12:46 am
content: Would it possible to connect to the DSO via ethernet to a computer/tablet and get (and print numerically) the current value of the math trace via SCPI ?

author: Emi
author_rank: Contributor
timestamp: September 24, 2019, 09:26:52 am
content: uhhh...too much trouble. ..i'll grab the old pocket calculator

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 24, 2019, 08:12:58 pm
content: I'm astonished.By now I would have expected someone to offer to hack the firmware of the scope, to display the numerical result of (trace average frequency) x (constant) in a little window on the scope screen.While someone else comes up with a totally TTL solution, stand-alone, no scope needed, reading pulses and dividing and counting and displaying the result on an array of common-cathode 7-segment numerical LEDs.Then I'm sure someone else will convert their Nixie clock to give a more classic counter readout look, instead of the LEDs.Someone else will of course program a microcontroller to do the whole thing with minimal components and negligible unit cost, using parts so small you need a microscope to see them.But it's only an easy exercise to write a Python app to read the scope over the LAN or USB, dump the data into a spreadsheet, then compute and show the RPM (and duty cycle, phase, maximum dynamic pressure, whatever) on your laptop.Then some old codger will break out the GenRad Strobotac and show us how it used to be done...Or you could just grab the old pocket calculator...

author: Emi
author_rank: Contributor
timestamp: September 25, 2019, 05:24:39 am
content: this is a Pyton for mei can't use a pc or laptop in my garage where i need rpm readings. .a direct reading on the dso woud be convenient. .an old calculator or a simple mind exercise will be enough

author: james_s
author_rank: Super Contributor
timestamp: September 25, 2019, 05:29:42 am
content: Seems like what you want is a microcontroller programmed as a tachometer. This is not really the sort of thing a scope is designed for, I have never found myself wishing I could display RPM directly on my scope.

author: tautech
author_rank: Super Contributor
timestamp: September 26, 2019, 01:48:47 am
content: Quote from: james_s on September 25, 2019, 05:29:42 amI have and any DSO with a frequency counter is perfectly capable of use as an RPM counter with just a RF loop, near field or even a traditional probe with the ground lead clipped into the grabber.Use case:Tune a chainsaw to max permissible factory RPM of 11,000.11000/60 = ~183 Hz reading required on the DSO.

author: Circlotron
author_rank: Super Contributor
timestamp: September 26, 2019, 02:33:29 am
content: ^^ Advantage over a normal frequency counter is you can adjust the trigger level to a stable trace and then be sure you have a valid reading.

author: james_s
author_rank: Super Contributor
timestamp: September 26, 2019, 02:46:31 am
content: Quote from: tautech on September 26, 2019, 01:48:47 amWell yes, it's possible to do, it's also possible to haul a load of groceries in a Ferrari but it would be a bit silly to complain that the Ferrari lacks adequate cargo space to easily carry a load of groceries since that's not really what it was designed to do.For that same use case I would whip out my $15 handheld laser tachometer and get a direct reading.

author: tautech
author_rank: Super Contributor
timestamp: September 26, 2019, 05:33:43 am
content: Quote from: james_s on September 26, 2019, 02:46:31 amAnd you'd have trouble finding somewheresafeto get an optical reading.For single cylinder motors there are a few tacho tools the most basic of which is a Sirometer that works reasonably accurately with engine vibration. I've had one for 40+ years.http://www.treysit.com/english/They are quite accurate at low RPM however when tuning your $1000+ chainsaw running on a 40:1 or 50:1 mix they just don't give the precision required for long engine life at the revs and lean oil mixes the modern chainsaw requires.For decades the most valued tachometer for high performance single cylinder engines has been any number of wireless tachs of which probably the earliest was one from Brigg & Stratton.Lately digital tachos are preferred however there are few that offer fast update rates so to not have a engine screaming at full revs for too long.Several models from PET have been the industry leading tacho's and resold by several of the big chainsaw manufacturers as their preferred maintenance tool.So yes if you have a portable DSO and no other means to tune a small motor use of a RF pickup for the ignition pulse is certainly viable, safe and effective. Done it a few times since I left the small engine shop some 35 yrs ago.

author: james_s
author_rank: Super Contributor
timestamp: September 26, 2019, 03:57:33 pm
content: Ok well I also have a $10 pocket frequency counter I could use with that same RF pickup, again the DSO could do it but would not be the first instrument I drag out to the garage. Or the second.