SUPERPAVE Digest 304
Topics covered in this issue include:
1) Calibration of Vacuum Degassing Oven
by "Mathisen, Kim (DOT/SOIL)" <soil001@gov.nb.ca>
2) Re: Calibration of Vacuum Degassing Oven
by khobson@fd9ns01.okladot.state.ok.us
3) Re: Calibration of Vacuum Degassing Oven
by Tom Reese <prentex@iamerica.net>
4) Re: Calibration of Vacuum Degassing Oven
by khobson@fd9ns01.okladot.state.ok.us
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To: "'Superpave Newsgroup'" <superpave@lists.cc.utexas.edu>
Subject: Calibration of Vacuum Degassing Oven
From: "Mathisen, Kim (DOT/SOIL)" <soil001@gov.nb.ca>
Date: Fri, 14 Jan 2000 09:16:08 -0400
We have purchased a vacuum degassing oven and would like to calibrate the
vacuum gauge on the oven. Can you calibrate with a residual pressure
manometer and a barometer?
1.
Our vacuum gauge read 25" Hg.
2.
The manometer read 7.9" Hg.
3.
The barometer read 29.3" Hg.
Should the gauge read 21.4" Hg ?(barometer reading minus the manometer
reading)
Kim Mathisen, P.Eng.
NBDOT Central Lab
Soils and Minerals Building
975 College Hill Road
Fredericton, N.B. E3B 4J7
(506)453-8967 phone
(506)457-4831 fax
kim.mathisen@gov.nb.ca
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To: superpave@lists.cc.utexas.edu
Subject: Re: Calibration of Vacuum Degassing Oven
From: khobson@fd9ns01.okladot.state.ok.us
Date: Fri, 14 Jan 2000 10:18:35 -0600
This brings up an interesting topic Kim. AASHTO PP1 can be misread. It is the
ole absolute and gauge pressure issue. Note 12 of PP1 states that the standard
gauge pressure should be 25 to 26.5 inHg. This is about 85 to 90 kPa for those
reading the gauge pressue in kPa. I assume this means standard when atmospehic
pressure is 1 atmosphere. I like to think in atmospheres. If we keep this up
we'll have a Mars crash.
Some computations:
12.5 kPa - 1 atm = -0.88 atm = -26.2 inHg which is say 26 inHg gauge vacuum.
This was rounded to 26.5 inHg gauge which is near 85 kPa gauge.
17.5 kPa - 1 atm = -0.82 atm = -24.8 inHg which is say 25 inHg gauge vacuum.
The 25 inHg gauge is near 90 kPa gauge.
As for your manometer, It sounds right. Apply Bernoulli's equation to compute
it. I'll have to dust off my hydraulic and PE books to confirm it. Consult the
manufacturer. By doing it that way, your gauge reading on the vacuum oven
appears to be way off. Did you calibrate the barometer? Check for bubbles in
the mercury. Check connections. Check the local weather station's barometer for
confirmation. Could be that some frictional losses are not accounted for.
Our in-house vacuum system is not sufficient to obtain the required vacuum. The
automatic vacuum oven from Gilson with self-contained vacuum pump looks good.
Anytime we can automate things is generally " a good thing".
Kenneth Hobson
khobson@odot.org
Bituminous Branch OKDOT
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To: superpave@lists.cc.utexas.edu
Subject: Re: Calibration of Vacuum Degassing Oven
From: Tom Reese <prentex@iamerica.net>
Date: Fri, 14 Jan 2000 15:25:42 -0600
Since we manufacture the only vacuum oven designed specifically for the PP-1
degassing requirement, this question came to us last fall for one of our customers.
We did extensive reading and a bunch of calculating and produced a table that we
feel may be an adequate guide for PP-1 users. Our customers vary in the level of
certification; some are required to have a barometer on site; others must rely on
the weather bureau.
One caveat with weather bureau reporting -- make sure they don't give you the
altitude-corrected barometric reading used by pilots to set their altimeters ! ! We
had a customer call us with a reading up in the mountains that would have been a
very high-pressure reading at sea-level . . . it was the pilot's version of
barometric pressure.
The concept of absolute pressure is that you are measuring the amount of positve
pressure above that of a perfect vacuum, rather than the pressure below the ambient
pressure that a local barometer would indicate.
The following is a discussion not of calibration, but of compensation for ambient
barometric pressure, which needs to be taken into account according to AMRL. The
problem arises from the PP-1 requiring measurement in "absolute" pressure while
using what most vacuum oven suppliers provide, a gauge indicating relative
pressure. Thus, your altitude and current bariometric conditions affect the
relative pressure gauge's indication and require calculation as to that local
pressure is equivalent to 15 kPa absolute.
What we provide for our vacuum degassing oven customers is a chart with the data
listed below. It is provided with the assumption that the customer is using a
relative pressure gauge indicating in/Hg below actual atmospheric. We do not claim
that the chart is totally correct; the readings shown for standard barometric
pressure at given altitudes have not been corrected for the difference between the
"standard" temperature for that altitude [generally well below 15 C] and the ambient
temperature at the point of measurement, more typically approximately15 C.
Column A: Altitude in feet
Column B: Standard Atmospheric Pressure, In/Hg [relative pressure]
Column C: Standard Atmospheric Pressure, mm/Hg [relative pressure]
Column D: Standard Atmospheric Pressure, kPa [relative pressure]
Column E: 15kPa absolute pressure expressed as in/Hg corrected relative pressure
A
Alt/Ft
0
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
4,500
5,000
5,500
6,000
6,500
7,000
B
in/Hg [rel]
29.92
29.39
28.86
28.34
27.82
27.32
26.82
26.33
25.84
25.36
24.89
24.43
23.98
23.53
23.09
C
D
mm/Hg [rel] kPa [rel]
760.00
101.3
746.51
99.5
733.04
97.7
719.84
96.0
706.63
94.2
693.93
92.5
681.23
90.8
668.78
89.1
656.34
87.5
644.14
85.9
632.21
84.3
620.52
82.7
609.09
81.2
597.66
79.7
586.49
78.2
E
in/Hg @ 15 kPa [abs]
-25.49 ± 0.74
-24.96 ± 0.74
-24.43 ± 0.74
-23.91 ± 0.74
-23.39 ± 0.74
-22.89 ± 0.74
-22.39 ± 0.74
-21.90 ± 0.74
-21.41 ± 0.74
-20.93 ± 0.74
-20.46 ± 0.74
-20.00 ± 0.74
-19.55 ± 0.74
-19.10 ± 0.74
-18.66 ± 0.74
Using the Altitude / barometric pressure conversion chart. Use the most accurate
available method to determine you ambient barometric pressure. Altitue is the least
accurate. Weather bureau readings in in/Hg, mm/HG or kPa relative are preferable.
Whichever method is used, locate the appropriate colum and then the number closest
to your own. Then reference the right-hand column [column E] to see what relative
vacuum pressure in inches mercury will provide 15kPa absolute pressure. Record or
mark that value as the 15kPa absolute pressure.
Example: If your local barometric pressure is 26.33 in/Hg, the 15kPa equivalent
reading [in/Hg] is -21.90 ± 0.74 in/Hg.
I hope this helps.
One small commercial plug -- our oven has the kPa absolute scale printed around the
gauge indicating in in Hg relative. The gauge with its in/Hg scale can be rotated
to align the correct in/Hg value opposite 15kPa absolute for operator convenience
[and less chance for error].
Tom Reese
Prentex Alloy Fabricators, Inc.
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To: superpave@lists.cc.utexas.edu
Subject: Re: Calibration of Vacuum Degassing Oven
From: khobson@fd9ns01.okladot.state.ok.us
Date: Fri, 14 Jan 2000 16:08:56 -0600
Thanks for the contribution Tom.
Kim, the other thing to consider in your case is temperature. The oven
temperature and pressure, location of the gauge on the oven, height of the
manometer, and atmospheric pressure and temperature, the ideal gas law PV=nRT.
The flow of course is air and mercury. So, density of air and mercury may need
to be considered. Hmm, now we are getting complicated...
As for advertisements, feel free to correct me on equivalent or alternative
products and procedures. I allow and welcome advertisements on my web forum on
asphalt at:
http://www.telepath.com/khobson/odot/ubb/ubbcgi/Ultimate.cgi?action=intro
Kenneth Hobson
khobson@odot.org
Bituminous Branch OKDOT
SUPERPAVE Digest 305
Topics covered in this issue include:
1) Re: Calibration of Vacuum Degassing Oven
by khobson@fd9ns01.okladot.state.ok.us
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To: superpave@lists.cc.utexas.edu
Subject: Re: Calibration of Vacuum Degassing Oven
From: khobson@fd9ns01.okladot.state.ok.us
Date: Wed, 19 Jan 2000 16:38:00 -0600
Kim, a general procdure for working manometer problems:
1. Start at one end (or any meniscus if the circuit is continuous) and write the
pressure there in an appropriate unit (say pascals) or in an appropriate symbol
if it is known.
2. Add to this the change in pressure, in the same unit, from one meniscus to
the next (plus if the next meniscus is lower, minus if highter). (For pascals
this is the product of the difference in elevation in meters and the specfic
weight of the fluid in newtons per cubic meter.)
3. Continue until the other end of the gage (or the starting meniscus) is
reached and equate the expression to the pressure at that point, known or
unknow..
Below are some crude text examples (best I can do in text email):
Manometer Case 1:
|------- |
|
|
|
|2
|1
|
|
|
A
|
B3
Manometer Case 2:
A---------|
|
|
|------------B 3
|
|
|2
|1
|
|------|
Where;
A to 1=h1,
1 to 2=h2,
2 to 3=h3,
At A, S1 or Gamma 1,
1 to 2 is the mercury with S2 or Gamma 2,
At B, S3 or Gamma 3.
Gamma=Rho*Gravity or Density*Gravity.
So, for case 1:
pA-h1*gamma1-h2*gamma2+h3gamma3=pB or
pA-pB=h1*gamma1+h2*gamma2-h3*gamma3 or
hA-hB=h1*S1+h2*S2-h3*S3 units of length H2O
So, for case 2:
pA+h1gamma1-h2*gamma2-h3*gamma3=pB or
pZ-pB=-h1*gamma1+h2*gamma2+h3*gamma3 or
hA-hB=-h1*S1+h2*S2+h3*S3 units of length H2O
Don't forget to convert the gage pressure to absolute so that you compare apples
to apples.
Rho=density, so RhoAir is the density of air. Here temperature is important.
RhoAir=p/(R*T). Where R is the universal gas constant and T is absolute
temperature.
Hope this doesn't "muddy" it up for you. Let us know how it works out for you.
Kenneth Hobson
khobson@odot.org
Bituminous Branch OKDOT