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Hello, a common test used for determining the cleanliness of aluminium melt is the denstity index unit where you cool two molten Al samples, one under air and one under vacumn under a pressure of 80 mBar. Can someone explain why does the vacumn level have to be 80 mBar?

Kruttika Kher

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Kruttika Apshankar-Kher
Director
Energy Systech
Pune
08436241631
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The pressure of the vacuum will affect the result: your measurement is effectively the void volume at 80 mbar versus the void volume at ambient pressure.  If you halve your vacuum pressure to 40 mbar without accounting for it, you'll approximately double your inferred gas content.

80 mbar is a low enough pressure for the density index test to easily discriminate gassy versus clean melts, but a high enough pressure that you can easily achieve it with a diaphragm pump and measure it with a Bourdon gauge.  You need a standard, and it's a sensible value to pick.

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Nicholas Taylor
Research Associate
University of Cambridge
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Thank you Mr. Tayor for your explanation. I would also like to know whether there is any international historic agreement in which 80 mBar was agreed upon.

Warm regards,

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Kruttika Apshankar-Kher
Director
Energy Systech
Pune
08436241631
------------------------------

Original Message:
Sent: 09-25-2023 09:36
From: Nicholas Taylor
Subject: Density Index of aluminium samples for hydrogen determination

The pressure of the vacuum will affect the result: your measurement is effectively the void volume at 80 mbar versus the void volume at ambient pressure.  If you halve your vacuum pressure to 40 mbar without accounting for it, you'll approximately double your inferred gas content.

80 mbar is a low enough pressure for the density index test to easily discriminate gassy versus clean melts, but a high enough pressure that you can easily achieve it with a diaphragm pump and measure it with a Bourdon gauge.  You need a standard, and it's a sensible value to pick.

------------------------------
Nicholas Taylor
Research Associate
University of Cambridge
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