Multimeter Altitude Limits

[Michael Napier]

While searching for a decent DMM to use during my K2 construction I noticed
several of them have an altitude limit of 2000m (~6500 feet). Since I live
at 7,600 feet and frequent altitudes at 10,000 while camping I am concerned
about this limit. A call to the Extech Tech Support line yielded a service
rep who was as mystified as myself as to this limit.

Any expert opinions out there concerning this issue? Any DMM recommendations
for a unit $60 to $100 or so?

Mike
K0MIS
Cascade, CO


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[W0rw]

Hi Mike,
i have taken my Fluke DMM up into the hills...
No problems.
The only altitude sensitive components might be the LCD and
some DMM's have spark gaps which might short on AC prematurely at high
elevation.
i doubt there is any problem.
i operate out of Cripple Creek frequently.
Paul  w0rw
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[Craig Rairdin]

Plasma displays have a 6500' limit. LCD displays shouldn't have the same
limit, though I've seen specs on meters with LCD displays that say 6500'. So
maybe it's not the display.

Craig
NZ0R

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Michael Napier
Sent: Thursday, August 03, 2006 11:23 AM
To: [hidden email]
Subject: [Elecraft] Multimeter Altitude Limits



While searching for a decent DMM to use during my K2 construction I noticed
several of them have an altitude limit of 2000m (~6500 feet). Since I live
at 7,600 feet and frequent altitudes at 10,000 while camping I am concerned
about this limit. A call to the Extech Tech Support line yielded a service
rep who was as mystified as myself as to this limit.

Any expert opinions out there concerning this issue? Any DMM recommendations
for a unit $60 to $100 or so?

Mike
K0MIS
Cascade, CO


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[N2EY]

In a message dated 8/3/06 12:28:37 PM Eastern Daylight Time,
[hidden email] writes:


> Any expert opinions out there concerning this issue?
>
>


(altitude)

Cooling?

73 de Jim, N2EY
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[Ron D'Eau Claire-2]

What's the upper voltage limit the DMM will handle?

Resistors have a voltage rating as well as resistance and wattage
(dissipation) ratings. We don't normally run into the voltage rating with
leaded parts, especially with low-voltage solid state gear, but it's a very
real concern as the voltage gets "up there". That's why high voltage probes
have big, long resistors or a number of resistors in series -
shorter/smaller ones would arc over.

In a DMM, the voltage divider that determines the voltage range is usually
made in one piece, using tiny resistive elements on a substrate. Compared to
"normal" resistors, each resistive element is tiny, and the voltage required
to produce an arc drops dramatically as you go up in altitude.

Radio equipment intended for use in an unpressurized aircraft or an
unpressurized area in an aircraft is usually pressurized to avoid that
corona discharges and arcing (as any tech will tell you who forgets to
depressurize an aircraft radio before removing the cover bolts to service
it. Me? Naw..... The cover missed me by half an inch, at least...).

If your DMM will measure a few hundred volts, I'd surmise arcing in the
voltage divider is the limiting factor for altitude.

Ron AC7AC

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[n5ib]

In our high altitude ballooning program at LSU (>120,000 feet) it's
necessary to be very careful about sharp edges and corners in circuits of
even modest voltages. Vacuum chamber testing is an essential part of the
payload development. Our student payload had a Geiger-Muller tube, and
that's a couple hundred volts, so they had to be sure to test in the
chamber.

Interestingly... 6500 ft is 1981 meters, so I speculate a conversation
like..

Pointy Haired Boss: We need an altitude limit because of HV arcing, what
do think it should be?
Alice: Oh, couple thousand meters or so should be safe enough.
Pointy Haired Boss: OK, tell marketing.
Alice: Fine, but remember we're selling it in the USA.
Pointy Haired Boss: Not to worry, ad copy writers will take care of
conversion.

I recall a wireless device whose range was specified on the package to be
"up to 328 feet."
Amazing how they knew it would work at 327 but not at 329. Of course what
was meant was that the range was about 100 meters.  :^))

73
Jim N5IB

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[peter gerba]

Hi Jim;

I understand arcing at standard Temp. and pressure.  I thought that the
reduction of media would produce an increase of voltage Vs. distance (vacuum
relays and Caps.)

I'm missing something here.

pete kn6bi

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]]On Behalf Of [hidden email]
Sent: Thursday, August 03, 2006 5:20 PM
To: [hidden email]
Subject: Re: [Elecraft] Multimeter Altitude Limits


In our high altitude ballooning program at LSU (>120,000 feet) it's
necessary to be very careful about sharp edges and corners in circuits of
even modest voltages. Vacuum chamber testing is an essential part of the
payload development. Our student payload had a Geiger-Muller tube, and
that's a couple hundred volts, so they had to be sure to test in the
chamber.

Interestingly... 6500 ft is 1981 meters, so I speculate a conversation
like..

Pointy Haired Boss: We need an altitude limit because of HV arcing, what
do think it should be?
Alice: Oh, couple thousand meters or so should be safe enough.
Pointy Haired Boss: OK, tell marketing.
Alice: Fine, but remember we're selling it in the USA.
Pointy Haired Boss: Not to worry, ad copy writers will take care of
conversion.

I recall a wireless device whose range was specified on the package to be
"up to 328 feet."
Amazing how they knew it would work at 327 but not at 329. Of course what
was meant was that the range was about 100 meters.  :^))

73
Jim N5IB

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[n5ib]

On Fri, 4 Aug 2006 07:58:37 -0700 Peter writes:
>Hi Jim;
>I understand arcing at standard Temp. and pressure.  I thought that
>the reduction of media would produce an increase of voltage Vs. distance

>(vacuum relays and Caps.)

Air is a dielectric. While its dielectric *constant* is almost identical
to vaccum, its dielectric *strength* (volts per distance) decreases with
density (pressure) until you get into some really hard vacuum situations.
See <http://www.isi-seal.com/searchs/doc/DielectricStrength.htm> for a
chart.
For air at STP it's about 3000 volts per millimeter

In interpreting that chart, remember that standard atmospheric pressure
is 760 torr, and at 2000 meters altitude (the notorious 6500 ft)  the
pressure is about 600 torr. Pressure and density aren't linearly related
because of the temperature drop as you go up, but its a decent
approximation for the first couple thousand meters or so. By the time you
reach 300 torr, at about 7500 meters, the dielctric strength of air is
degraded by about a factor of 2, and by 30,000 meters (10 torr) it's
degraded by about 90%. We routinely fly our balloons above 38,000 meters.

See
<http://weather.uwyo.edu/cgi-bin/sounding?region=naconf&TYPE=TEXT%3ALIST&YEAR=2006&MONTH=08&FROM=0412&TO=0412&STNM=72240>

for a representative chart of pressure, altitude, and other
meteorological parameters.

The vacuum relays and caps incorporate some pretty hard vacuum, I'd
surmise. Pretty bitter pill to swallow... paying premium bucks for a
better grade of nothing :^))

So, if your working around higher voltages than most of our QRP gear, I
guess it's best not to inhale  :^))

73
Jim N5IB



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[dlajr]

Correct.  Pressure and density are not the same thing.  But if we are
talking about arcing, should we be more concerned with pressure altitude, or
density altitude?  A higher than standard temperature can make a huge
difference in the two.

As an old pilot, I had to ask!

Dan Allen
KB4ZVM
K-2 S/N 1757

> Air is a dielectric. While its dielectric *constant* is almost identical
> to vaccum, its dielectric *strength* (volts per distance) decreases with
> density (pressure) until you get into some really hard vacuum situations.
> See <http://www.isi-seal.com/searchs/doc/DielectricStrength.htm> for a
> chart.
> For air at STP it's about 3000 volts per millimeter
>
> In interpreting that chart, remember that standard atmospheric pressure
> is 760 torr, and at 2000 meters altitude (the notorious 6500 ft)  the
> pressure is about 600 torr. Pressure and density aren't linearly related
> because of the temperature drop as you go up, but its a decent
> approximation for the first couple thousand meters or so. By the time you
> reach 300 torr, at about 7500 meters, the dielctric strength of air is
> degraded by about a factor of 2, and by 30,000 meters (10 torr) it's
> degraded by about 90%. We routinely fly our balloons above 38,000 meters.
>

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