Altitude and instrument specifications/accuracy - Page 1
Forum section: electronics / projects

author: Lightages
author_rank: Unknown
timestamp: January 14, 2013, 11:31:41 pm
content: I have been comparing many multimeters over the past few months and one thing I have noticed is that some manufacturers specify a maximum altitude for the guaranteed specifications. Most list 2000m as the maximum. Some don't list anything for altitude.My problem? I live at 2500m altitude. I have seen little ill effect on reputable brands of test equipment but I wonder what the real, if any, effect would be. In hindsight, this might be , MIGHT, a cause of my UT71E being so far out of calibration when it arrived. It has been the only thing that has been so blatantly out of spec so I doubt it.The only real effect I can say I experience is more frequent failure of hard drives than I have ever experienced in my life. I understand the mechanism of the failures for hard drives here but I really wonder if there is any reason for manufacturers to limit their specifications to 2000m. It seems arbitrary to me.Any ideas for the 2000m spec?

author: ftransform
author_rank: Frequent Contributor
timestamp: January 14, 2013, 11:42:25 pm
content: I wonder if the harddrives would live longer if they were sped up slightly for higher altitudes?

author: Wytnucls
author_rank: Super Contributor
timestamp: January 15, 2013, 03:10:13 am
content: DMMs are not hermetically sealed, so air becomes a component of their circuitry. The electrical characteristics of air are affected by density (altitude) and humidity, so designers set boundaries on these parameters.http://www.newark.com/pdfs/techarticles/fluke/UnderstandingPrecisionMeasurements.pdfAlso, bear in mind that, in summer, the density altitude at your place of abode will be quite a bit higher than 2500 meters. 3300 meters at 20 deg C, for instance.I seriously doubt that your 71E would be affected by altitude to such an extend.I suspect yours came from an old batch of meters and probably just needs a new calibration. The meter should have come with a factory card stating the date of manufacture.

author: Skimask
author_rank: Super Contributor
timestamp: January 15, 2013, 03:55:44 am
content: Quote from: Lightages on January 14, 2013, 11:31:41 pmThe heads ride on an air cushion. No air, no cushion, crash!Past that, maybe the air isn't dense enough to provide enough convection cooling for the components?

author: Lightages
author_rank: Unknown
timestamp: January 15, 2013, 04:17:47 am
content: If anything, for general electronic devices, less atmosphere should be better for its diminished ability to interact with things. There ARE some parts that are hermetically sealed though. ICs and other components are built at closer to sea level pressures and there for might some trapped air in the component. As long as the component stays at the same altitude the outside pressure balances the inside pressure and there is no effect. Perhaps some components might be sensitive to the pressure difference inside and outside and the stresses might change a component value or two. I am not saying this as a fact but just a possibility.I have a vacuum chamber so maybe this would be a good experiment!Yes, hard drive heads fly on a cushion of air and are tuned for the "normal" air density. I am sure they are not flying correctly below a certain air density.

author: Skimask
author_rank: Super Contributor
timestamp: January 15, 2013, 04:22:31 am
content: Quote from: Lightages on January 15, 2013, 04:17:47 amThat would be interesting...but...at the risk of damaging your own equipment, I would bet the big H/D makers probably have data on that subject they would send out.Great...now I've got something else to surf on...and I thought I'd keep my mind fairly free tonight...

author: Lightages
author_rank: Unknown
timestamp: January 15, 2013, 04:31:56 am
content: Oh I wasn't going to put a hard drive in the chamber. I was thinking of wiring up my UT71E on a feedthrough and see what happens to the readings as the pressure is dropped down slowly to a "vacuum".

author: qno
author_rank: Frequent Contributor
timestamp: January 15, 2013, 08:01:08 am
content: HiFrom my days as an Aerospace engineer I know that the air pressure at 2500 m is about 3/4 of the pressure at see level.This is also the pressure maintained in the cabin in airliners.Concequense is that the cooling capacity of the air is 3/4 compared with sea level.This is why harddisks probbably malfuncion quickly.In power electronics requiring cooling you have to take tis in accound.If you place a motor and frequency inverter on top of a mountaine for a skilift you need to take a model lager motor and inverter to keep the temperature within safe limits.

author: penfold
author_rank: Frequent Contributor
timestamp: January 15, 2013, 10:03:53 am
content: For a multimeter could it not be that the safety creepage and clearance is specified only for lower altitudes, I seem to remember that the difference between above and below 3000 metres is almost an order of magnitude different in the required distances.I remember an experiment conducted by some colleagues a while ago on high pressure testing of hermetic devices for submarine apps and basically the pressure differential required to kill anything was the kind only experienced in movements from deep sea to land (both positive pressure and vacuum)In regards to cooling, I'd imagine that for the relatively short durations meters specify measurement at appreciable currents and the general sealed and insulating nature of multimeter cases that they are specified based on the radiant cooling of the shunt resistors

author: Lightages
author_rank: Unknown
timestamp: January 15, 2013, 07:07:41 pm
content: I emailed Brymen asking for clarification on this matter and this is the reply:"Altitude up to 2 000 m is the normal environmental condition (clause 1.4.1) in IEC61010-1 safety regulation. Altitude above 2 000 m is specified as extended environmental condition (clause 1.4.2), and all CLEARANCES shall be multiplied by an applicable factor. They are:1.14 at 2001 to 3000 m1.29 at 3001 to 4000 m1.48 at 4001 to 5000 mPer altitude <2000 m, BM869 is a CAT IV 1000V design using clearance of 14 mm, which is, in turn, 1.75 times of 8 mm as in the one step lower CAT III 1000V & CAT IV 600V requirements. It is then a CAT III 1000V & CAT IV 600V design up to 5000 m, the highest altitude as specified in the regulation.Best Regards,Gary Wang/BrymenProduct Marketing Manager"So not only does Brymen make their meters to the proper standards, they actually under rate them. I had forgotten that air actually becomes more conductive at lower pressures. I am supposed to know this from my high vacuum work. Funny how you forget things.

author: penfold
author_rank: Frequent Contributor
timestamp: January 16, 2013, 09:07:49 am
content: Okay so my multiplication factors were way off! Funny how you forget things ain't it

author: qno
author_rank: Frequent Contributor
timestamp: January 16, 2013, 08:19:09 pm
content: Google for "Paschen curve"This gives the flashover voltage @ air pressure / hight