How can I correlate mbars, as in :

Ultimate vacuum mbar 1.10-4

with inches of mercury so as to make a comparison between various makes
of vacuum pumps. I'm not in the market today but I'm reasonably
confident that I eventually will be ... so I'm trying to understand the
lingo.

Just asking wikipedia about millibars (mbars) brought me to a nice chart
comparing them to hectoPascals. On this side of the pond, that's still
apples & oranges.

Bill


--
When the rich wage war it's the poor who die.
Jean-Paul Sartre, The Devil and the Good Lord (1951) act 1


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if you recall that one atmosphere is 14.7 PSI, this chart will help:
http://www.britishmetrics.com/html/pis-bar.htm
and if you further reacall that one atmosphere is 30 inches of mercury
(aprox), then it should all be obvious. You don't need a vacuum less than
..07 bar or so, that's 70 milibar, or 7X10E5 greater than the value you have
below. You REALLY REALLY REALLY do NOT NOT NOT want a high vaccuum pump.

you may find my article on vacuum pumps and chucks helpful - you can
download it from my web page (www.wbnoble.com, click on vacuum pumps) or you
can download it from the tips section of www.woodturners.org - it's the same
article both places.

bill (not in Detroit)



"Bill in Detroit" wrote in message
...
How can I correlate mbars, as in :

Ultimate vacuum mbar 1.10-4

with inches of mercury so as to make a comparison between various makes of
vacuum pumps. I'm not in the market today but I'm reasonably confident
that I eventually will be ... so I'm trying to understand the lingo.

Just asking wikipedia about millibars (mbars) brought me to a nice chart
comparing them to hectoPascals. On this side of the pond, that's still
apples & oranges.

Bill


--
When the rich wage war it's the poor who die.
Jean-Paul Sartre, The Devil and the Good Lord (1951) act 1


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On Wed, 20 Dec 2006 01:00:17 -0500, Bill in Detroit
wrote:

How can I correlate mbars, as in :

Ultimate vacuum mbar 1.10-4

with inches of mercury so as to make a comparison between various makes
of vacuum pumps. I'm not in the market today but I'm reasonably
confident that I eventually will be ... so I'm trying to understand the
lingo.

Just asking wikipedia about millibars (mbars) brought me to a nice chart
comparing them to hectoPascals. On this side of the pond, that's still
apples & oranges.

Not sure I understand you: a hectopascal IS a millibar. Shipping
forecasts and reports here (in France) now use the hectopascal as
standard for all atmospheric pressures.
For medical purposes (the only area where it is still used in Europe,
apparently), 1 mm mercury = 133.322 pascal, or 1.33322 hectopascal.

As 1"=25.4 mm, my calculation makes that 1" mercury = 33.864
hectopascal.

Unless I've screwed up somewhere...


only one p in my real address / un seul p dans ma véritable adresse

A pressure of 1 Atmosphere = 30" mercury = 760mm mercury = 15lbs/sq" = 760
Torr = 32ft of water = 1 bar = 1000mbar = 1013 hectopascals. (all approx.)

Assuming that the question relates to vacuum chucks.

A metric/imperial conversion shows that for every 67mbar below 1000mbar you
will have 1lb/sq" holding the bowl in place.

If you get down to 67mbar (1000-14x67mbar) you will have 14lb/sq" holding
the bowl in place. Reducing the pressure below 67mbar is an attempt to gain
the extra 1lb/sq" to hold the bowl in place. I would suggest that if the
anyone struggling for the last 1lb/sq" is living life on the edge.

I'm sure Bill Noble's advice would be to make sure the pump has the
throughput to the maintain the target vacuum whilst handling leaks of the
seals and porosity of the wood
and release of water from the wood at reduced pressure.

I hope this helps.

BillR


"Peter Wells" wrote in message
...
On Wed, 20 Dec 2006 01:00:17 -0500, Bill in Detroit
wrote:

How can I correlate mbars, as in :

Ultimate vacuum mbar 1.10-4

with inches of mercury so as to make a comparison between various makes
of vacuum pumps. I'm not in the market today but I'm reasonably
confident that I eventually will be ... so I'm trying to understand the
lingo.

Just asking wikipedia about millibars (mbars) brought me to a nice chart
comparing them to hectoPascals. On this side of the pond, that's still
apples & oranges.

Not sure I understand you: a hectopascal IS a millibar. Shipping
forecasts and reports here (in France) now use the hectopascal as
standard for all atmospheric pressures.
For medical purposes (the only area where it is still used in Europe,
apparently), 1 mm mercury = 133.322 pascal, or 1.33322 hectopascal.

As 1"=25.4 mm, my calculation makes that 1" mercury = 33.864
hectopascal.

Unless I've screwed up somewhere...


only one p in my real address / un seul p dans ma véritable adresse

"Peter Wells" wrote in message
...
On Wed, 20 Dec 2006 01:00:17 -0500, Bill in Detroit
wrote:

How can I correlate mbars, as in :

Ultimate vacuum mbar 1.10-4

with inches of mercury so as to make a comparison between various makes
of vacuum pumps. I'm not in the market today but I'm reasonably
confident that I eventually will be ... so I'm trying to understand the
lingo.

Just asking wikipedia about millibars (mbars) brought me to a nice chart
comparing them to hectoPascals. On this side of the pond, that's still
apples & oranges.

Not sure I understand you: a hectopascal IS a millibar. Shipping
forecasts and reports here (in France) now use the hectopascal as
standard for all atmospheric pressures.
For medical purposes (the only area where it is still used in Europe,
apparently), 1 mm mercury = 133.322 pascal, or 1.33322 hectopascal.

As 1"=25.4 mm, my calculation makes that 1" mercury = 33.864
hectopascal.

Unless I've screwed up somewhere...


Millibar. One thousandth, right? Nope. BTW, 29.92" or 760 mm is assumed
standard sea level pressure in Hg . Mystery is why there are 1014 one
thousandths of a bar in a standard atmosphere.

How can I correlate mbars, as in :

Ultimate vacuum mbar 1.10-4

1 mBar = 0.03 inches mercury

D

with inches of mercury so as to make a comparison between various makes
of vacuum pumps. I'm not in the market today but I'm reasonably
confident that I eventually will be ... so I'm trying to understand the
lingo.

Just asking wikipedia about millibars (mbars) brought me to a nice chart
comparing them to hectoPascals. On this side of the pond, that's still
apples & oranges.

Bill


--
When the rich wage war it's the poor who die.
Jean-Paul Sartre, The Devil and the Good Lord (1951) act 1


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On Wed, 20 Dec 2006 01:00:17 -0500, Bill in Detroit wrote:

How can I correlate mbars, as in :

Ultimate vacuum mbar 1.10-4

with inches of mercury so as to make a comparison between various makes
of vacuum pumps. I'm not in the market today but I'm reasonably
confident that I eventually will be ... so I'm trying to understand the
lingo.

Just asking wikipedia about millibars (mbars) brought me to a nice chart
comparing them to hectoPascals. On this side of the pond, that's still
apples & oranges.

You might find
http://www.srh.noaa.gov/elp/wxcalc/formulas/pressureConversion.html
helpful.

Bill



--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

Dan Bollinger wrote:
How can I correlate mbars, as in :

Ultimate vacuum mbar 1.10-4

1 mBar = 0.03 inches mercury
D

Thanks, Dan. I couldn't find the math to make the conversion and without
some simple 'rule of thumb' to use in comparisons, I couldn't tell if
what I was looking at was in any respect appropriate - or not.

I've been to Bill Nobles site a number of times, but he uses in/mm and
most of the other sites a casual Googling turned up used other units and
no one was providing any sort of a Rosetta stone.

Bill


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William Noble wrote:
if you recall that one atmosphere is 14.7 PSI, this chart will help:
http://www.britishmetrics.com/html/pis-bar.htm
and if you further reacall that one atmosphere is 30 inches of mercury
(aprox), then it should all be obvious. You don't need a vacuum less than
.07 bar or so, that's 70 milibar, or 7X10E5 greater than the value you have
below. You REALLY REALLY REALLY do NOT NOT NOT want a high vaccuum pump.

you may find my article on vacuum pumps and chucks helpful - you can
download it from my web page (www.wbnoble.com, click on vacuum pumps) or you
can download it from the tips section of www.woodturners.org - it's the same
article both places.

bill (not in Detroit)


Thanks, Bill. I don't think I've needed to know either of those numbers
more than once or twice since graduating from High school in 1970. And
possibly stretching all the way back to Jr. High.

I see various units offered in the catalogs, your offerings, and the
other offerings on the internet and I was trying to puzzle out just what
sort of unit would have more than enough suction without sucking all the
money out of my pocket or requiring an underground bunker for noise
abatement.

Out of curiosity, how far away from what I need would a good shop vac
be? I mostly want to finish turn the bottoms of bowls in the 6-10" range
with flat edges that have already been sealed.

Hmmm ... maybe I could put a barometer in a wooden box with a plexi face
and test for myself. That should be good enough to give me a ballpark
feel for what's going on.

Bill
--
I not only use all the brains that I have, but all that I can borrow.
Woodrow Wilson (1856 - 1924)


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On Wed, 20 Dec 2006 01:00:17 -0500, Bill in Detroit
wrote:

How can I correlate mbars, as in :

Ultimate vacuum mbar 1.10-4

with inches of mercury so as to make a comparison between various makes
of vacuum pumps. I'm not in the market today but I'm reasonably
confident that I eventually will be ... so I'm trying to understand the
lingo.

Just asking wikipedia about millibars (mbars) brought me to a nice chart
comparing them to hectoPascals. On this side of the pond, that's still
apples & oranges.

Bill

I just tried google with the following search parameters..

convert millibars to hg

The second URL in the results was

http://www.csgnetwork.com/pressinmbcvt.html

amd it had the following notes

To convert inches of mercury to millibars, multiply the inches value
by 33.8637526
To convert millibars to inches of mercury, multiply the millibar value
by 0.0295301.

Hi Bill

Bill I'll try to make things a little simpler, that works just fine for
everyday approximation, like a circle is about 3/4 the area of a
square, some will cringe but yes thats good enough for what we need in
most cases.

Also with vacuum, take all air away and you would have 15 # pressure
per inch square

The 15 # is the same as 30" mercury, or 75 cm mercury and that is also
1 bar.

To get 1# of pressure per square inch you'd need 2" mercury or 5 cm of
mercury.

So now you have a let's say 10" Diameter bowl, that would be 10x10=100,
and we take 3/4 of that for the area of a circle and we have 75 square
inches.

Now if you had a vacuum of just 4" or 10 cm of mercury and no leakage
you'd have 150 pounds holding your bowl down, that should be lots,
but.............

Next you have a 6"D bowl, that would be 6x6=36, and 3/4 of that is 27"
square with no leakage and 4" or 10 cm of mercury vacuum and 54#
holding it down should also be enough to clean up the bottom

Now we take a 3"D bowl and 3x3=9, 3/4 of that gives us 6.75" square,
no leakage, and 4" mercury vacuum 10 cm or 100 mm same as before, and
now we'd have only 13# holding it down, I would suggest that's not
enough.

If you now increase the vacuum to let's say 20" of mercury, you'd have
10# of pressure per square inch, and on that 3"D bowl the hold down
would be 67.5# and that would be lots IMO.

So do you need all that high vacuum ??, I would say no, but you do need
enough volume though, as there is always leakage and if you don't have
the volume you'd never get any vacuum.

A good shopvac will give you enough vacuum for a larger bowl, however
you better keep in mind that the vacuum motor needs air to flow by to
cool it, and if you have very little air going past, like with using it
as a vacuum holding pump your shopvac's motor will burn out, period.

Hope this helps you and anybody else that has some questions about all
of this.

Have fun and take care
Leo Van Der Loo



Bill in Detroit wrote:
How can I correlate mbars, as in :

Ultimate vacuum mbar 1.10-4

with inches of mercury so as to make a comparison between various makes
of vacuum pumps. I'm not in the market today but I'm reasonably
confident that I eventually will be ... so I'm trying to understand the
lingo.

Just asking wikipedia about millibars (mbars) brought me to a nice chart
comparing them to hectoPascals. On this side of the pond, that's still
apples & oranges.

Bill


--
When the rich wage war it's the poor who die.
Jean-Paul Sartre, The Devil and the Good Lord (1951) act 1


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a really good shop vac might draw 5 inches of mercury. The table in my
article will show you the holding force, but it's basically 1/6 atmosphere,
right? or about 2.5 PSI of force - if you figure you need 100 pounds of
force to hold your object (that's just a number pulled out of nowhere, but
it is probably plausible), then your shop vac would be OK with anything
whose diameter (remember area = pi times radius squared) is greater than 7
inches or so

try this table (view it in a fixed pitch font) - hopefully I made no errors
in the spreadsheet

Force for indicated diameter in inches
in mercury psi 1 2 4 6 8 10
1 0.49 0.3848451 1.53938 6.157522 13.85442 24.63009 38.48451
2 0.98 0.7696902 3.078761 12.31504 27.70885 49.26017 76.96902
3 1.47 1.1545353 4.618141 18.47256 41.56327 73.89026 115.4535
4 1.96 1.5393804 6.157522 24.63009 55.41769 98.52035 153.938
5 2.45 1.9242255 7.696902 30.78761 69.27212 123.1504 192.4226
6 2.94 2.3090706 9.236282 36.94513 83.12654 147.7805 230.9071
7 3.43 2.6939157 10.77566 43.10265 96.98097 172.4106 269.3916
8 3.92 3.078760801 12.31504 49.26017 110.8354 197.0407 307.8761
9 4.41 3.463605901 13.85442 55.41769 124.6898 221.6708 346.3606
10 4.9 3.848451001 15.3938 61.57522 138.5442 246.3009 384.8451
11 5.39 4.233296101 16.93318 67.73274 152.3987 270.931 423.3296
12 5.88 4.618141201 18.47256 73.89026 166.2531 295.561 461.8141
13 6.37 5.002986301 20.01195 80.04778 180.1075 320.1911 500.2986
14 6.86 5.387831401 21.55133 86.2053 193.9619 344.8212 538.7831
15 7.35 5.772676501 23.09071 92.36282 207.8164 369.4513 577.2677
16 7.84 6.157521601 24.63009 98.52035 221.6708 394.0814 615.7522
17 8.33 6.542366701 26.16947 104.6779 235.5252 418.7115 654.2367
18 8.82 6.927211801 27.70885 110.8354 249.3796 443.3416 692.7212
19 9.31 7.312056901 29.24823 116.9929 263.234 467.9716 731.2057
20 9.8 7.696902001 30.78761 123.1504 277.0885 492.6017 769.6902
21 10.29 8.081747101 32.32699 129.308 290.9429 517.2318 808.1747
22 10.78 8.466592201 33.86637 135.4655 304.7973 541.8619 846.6592
23 11.27 8.851437301 35.40575 141.623 318.6517 566.492 885.1437
24 11.76 9.236282402 36.94513 147.7805 332.5062 591.1221 923.6282
25 12.25 9.621127502 38.48451 153.938 346.3606 615.7522 962.1128
26 12.74 10.0059726 40.02389 160.0956 360.215 640.3822 1000.597
27 13.23 10.3908177 41.56327 166.2531 374.0694 665.0123 1039.082
28 13.72 10.7756628 43.10265 172.4106 387.9239 689.6424 1077.566
29 14.21 11.1605079 44.64203 178.5681 401.7783 714.2725 1116.051
30 14.7 11.545353 46.18141 184.7256 415.6327 738.9026 1154.535


"Bill in Detroit" wrote in message
...
William Noble wrote:
if you recall that one atmosphere is 14.7 PSI, this chart will help:
http://www.britishmetrics.com/html/pis-bar.htm
and if you further reacall that one atmosphere is 30 inches of mercury
(aprox), then it should all be obvious. You don't need a vacuum less
than .07 bar or so, that's 70 milibar, or 7X10E5 greater than the value
you have below. You REALLY REALLY REALLY do NOT NOT NOT want a high
vaccuum pump.

you may find my article on vacuum pumps and chucks helpful - you can
download it from my web page (www.wbnoble.com, click on vacuum pumps) or
you can download it from the tips section of www.woodturners.org - it's
the same article both places.

bill (not in Detroit)


Thanks, Bill. I don't think I've needed to know either of those numbers
more than once or twice since graduating from High school in 1970. And
possibly stretching all the way back to Jr. High.

I see various units offered in the catalogs, your offerings, and the other
offerings on the internet and I was trying to puzzle out just what sort of
unit would have more than enough suction without sucking all the money out
of my pocket or requiring an underground bunker for noise abatement.

Out of curiosity, how far away from what I need would a good shop vac be?
I mostly want to finish turn the bottoms of bowls in the 6-10" range with
flat edges that have already been sealed.

Hmmm ... maybe I could put a barometer in a wooden box with a plexi face
and test for myself. That should be good enough to give me a ballpark feel
for what's going on.

Bill
--
I not only use all the brains that I have, but all that I can borrow.
Woodrow Wilson (1856 - 1924)


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William Noble wrote:
a really good shop vac might draw 5 inches of mercury. The table in my
article will show you the holding force, but it's basically 1/6 atmosphere,
right? or about 2.5 PSI of force - if you figure you need 100 pounds of
force to hold your object (that's just a number pulled out of nowhere, but
it is probably plausible), then your shop vac would be OK with anything
whose diameter (remember area = pi times radius squared) is greater than 7
inches or so

try this table (view it in a fixed pitch font) - hopefully I made no errors
in the spreadsheet

Bill, not my day.

Are the numbers 1-30 along the left hand side just row numbers? If so, I
think I could reconstruct the spreadsheet. If not, could you supply the
missing labels? Read R - L as I have the least clutter in the far right
column.

TIA

Bill


--
Make yourself an honest man, and then you may be sure that there is one
rascal less in the world.
Thomas Carlyle (1795 - 1881)


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well, I didn't save the spread sheet - but here's what I did

left most column - inches of mercury
next colum, PSI (due to air pressure pushing on the item) - = inches of
mercury * 30/14.7
next columns, force due to pressure - numbers across the top are diameter,
calculated by =(diameter/2)^2*psi

I believe I put a similar table near the front of my treatise on this
subject (see www.wbnoble.com or www.woodturners.org)





"Bill in Detroit" wrote in message
...
William Noble wrote:
a really good shop vac might draw 5 inches of mercury. The table in my
article will show you the holding force, but it's basically 1/6
atmosphere, right? or about 2.5 PSI of force - if you figure you need
100 pounds of force to hold your object (that's just a number pulled out
of nowhere, but it is probably plausible), then your shop vac would be OK
with anything whose diameter (remember area = pi times radius squared)
is greater than 7 inches or so

try this table (view it in a fixed pitch font) - hopefully I made no
errors in the spreadsheet

Bill, not my day.

Are the numbers 1-30 along the left hand side just row numbers? If so, I
think I could reconstruct the spreadsheet. If not, could you supply the
missing labels? Read R - L as I have the least clutter in the far right
column.

TIA

Bill


--
Make yourself an honest man, and then you may be sure that there is one
rascal less in the world.
Thomas Carlyle (1795 - 1881)


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