Understanding dial gauge's calibration certificate - Page 1
Forum section: electronics / beginners

author: terryt
author_rank: Contributor
timestamp: May 10, 2016, 12:37:38 pm
content: I recently purchased a Dasqua dial gauge 31085101 (0-10mm x 0.01mm) for basic DIY home workshop duties.I would appreciate some help understanding the supplied calibration certificate (copy attached).I am used to seeing (and understanding) tolerance/accuracy for gauges expressed as "+/- 0.XX mm". and I do not understand the supplied "Test results" or Error Curve shown in the certificate.Would appreciate it if someone could advise what this gauge's "+/-" tolerance is, even if it's some sort of average for the supplied data. Just trying to find out how accurate this gauge is supposed to be.Before posting here, I asked the supplier (he couldn't help); asked the UK Dasqua distributor, Chronos (they couldn't help and suggested I contactwww.dasqua.com...which is a dead link!).

author: rstofer
author_rank: Super Contributor
timestamp: May 10, 2016, 02:24:55 pm
content: Just think of the graph as pointing out non-linearities in the gear rack. The gauge isn't perfect but it is pretty darn close. The X axis is 0..10mm and the range of error is +- 5 microns (millionth of a meter). Or +- 0.005 mm.The rest of the stuff is just bin sorting. But the 'within ...' means from the lowest value to the highest value within any possible two measurements. So they scan the graph and find a low point, move over some distance and see what the total error is between the two points. You can see where some readings are low and some are high. It is the total difference that they are measuring on a sliding window.

author: babysitter
author_rank: Frequent Contributor
timestamp: May 10, 2016, 03:16:57 pm
content: Its just the error they have read, X axis is length, Y axis is the error you can give or take to fix it. Look at say X=7.3 mm, where the graphs are about crossing zero - this length will be measured correctly. The further from the zero line, the bigger the error - which you can nicely read from the Y axis. You see that there is a bright and a dark graph - due to hysteresis, you will get a different reading depending if you approach a given X point with the tip going either out or in.

author: woodchips
author_rank: Frequent Contributor
timestamp: May 11, 2016, 01:02:00 pm
content: Well, first question is, does the serial number match the one on the DTI?Second question, have you read DIN878?Just look at the calibration certificate (CC), does it actually seem reasonable, possible, to you? The DTI has graduation makrks at 10um intervals, the CC is indicating errors of sometimes less than 1um. Does this seem reasonable? If it does, then please tell me how the error was actually measured. It is easy to move the DTI by 100um, it is also easy to move it by 101um, just use slip gauges. But how do you measure the pointer movement to 1um? Do you actually think they did measure the pointer movement?The CC is worthless.The DTI works by a rack turning a pinion which drives the pointer. There is no obvious cyclic error due to the pinion, or pinions. It would be interesting to do an FFT on the error plot to pick out the frequencies contained within it, this will tell you the pinion sizes. You will also note that the return trace is to all intents and purposes identical to the forwrd trace. This is impossible, there will be slop, hysteresis, poor bearings etc etc in such a cheap DTI that you would, in reality, be amazed if the return trace matched within 20um. Try it, set the DTI up and put slip gauges under it and record the reading, up and down. Alternatively use a micrometer barrel.If you want to measure to 10um then you need a piece of measuring equipment that will accurately measure to 2 or 3 um. A pointer moving over a straight graduation mark is just about the most inaccurate method of doing it, you need something with a V shape so you can judge the exact centre position by the equal gaps in the V.

author: Cerebus
author_rank: Super Contributor
timestamp: May 11, 2016, 01:43:17 pm
content: Quote from: woodchips on May 11, 2016, 01:02:00 pmWhy do you feel the need to take such a combative tone with someone who's just asked "I would appreciate some help understanding the supplied calibration certificate"? Or do you not realize that your tone is combative? If you don't get this, look at how many questions you've asked of the OP, and how many he's likely to be able to answer given that he's effectively said 'I don't understand this'.The frustrating thing is that I suspect you have the knowledge to provide a truly helpful and educational reply to the OP and, by extension, to the rest of us. I don't or I would. By the sounds of it you've got grounds for suspecting the veracity of the calibration certificate. Please, rather than making lots of demands for information that the OP clearly doesn't have - he's already expressed his ignorance quite humbly - explain to him what the calibration certificate says and why you think it is erroneous.

author: woodchips
author_rank: Frequent Contributor
timestamp: May 12, 2016, 03:11:05 pm
content: You are absolutely correct, it is a totally inappropriate answer for which I apologise.

author: terryt
author_rank: Contributor
timestamp: May 13, 2016, 12:31:55 am
content: To RSTOFER and BABYSITTER: Thank you for the replies, apologies for not replying sooner as I got tied up with other stuff. After reading your posts and again looking at the calibration certificate some of the cloud has lifted.To put things into context, I have two other dial gauges 0-10mm x 0.01mm: a Mitutoyo with calibration certificate +/- 13um and a Starrett with c.c. +/- 15um (both through the range). I posted here simply to try to find out where the Dasqua gauge fitted in with my other two gauges, accuracy-wise.IF the Dasqua accuracy is really +/- 5um I would be pleasantly surprised. But, going by price and woodchips' post, I suspect the Dasqua is No.3 on the list...and that's ok because I will use it for quick & dirty jobs where close enough is good enough and save my better gauges for the finer work.And woodchips' reply (especially "the cc is absolutely worthless") has given me an answer of sorts to my issue.TO CEREBUS: Thank you for the support, you get where I'm at.TO WOODCHIPS: Apology accepted. No harm, no foul. For what it's worth, the serial number printed on the gauge and on the calibration certificate are the same but I know that doesn't necessarily prove anything. I'm a 71-y-o male with a brain that often struggles to grasp technical stuff, courtesy of stepping off a motorbike at speed one too many times in my youth. I envy those like yourself and others on this forum who can absorb and use the "technical" without problems.

author: Cerebus
author_rank: Super Contributor
timestamp: May 13, 2016, 12:56:20 am
content: Quote from: terryt on May 13, 2016, 12:31:55 amWe'd be much more impressed if that said: "stepping off a motorbike at speed one too many times in my 60's".

author: terryt
author_rank: Contributor
timestamp: May 13, 2016, 01:15:51 am
content: Quote from: Cerebus on May 13, 2016, 12:56:20 amLOL, good one. Almost made it, last crashed off in my 50's (but limped away from that one, not carried away). Stopped riding some years later...I could see the writing on the wall and it didn't have a happy ending.

author: Cerebus
author_rank: Super Contributor
timestamp: May 13, 2016, 01:22:41 am
content: Quote from: terryt on May 13, 2016, 01:15:51 amYou know what they say. There are old bikers and bold bikers, but no old bold bikers.

author: terryt
author_rank: Contributor
timestamp: May 13, 2016, 03:02:05 am
content: Quote from: Cerebus on May 13, 2016, 01:22:41 amYep, know it well. And certainly no able, good looking ones.