# There is no option to give Carrier gas interms of lenear velocity in all my Agilent GC instruments.

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Hi ,I am using empower 2 for all my GC(Agileny 6850,7890,6890,)My method of analysis says that my carrier gas is in L.Velocity 30cm/secThere is no provision to enter this linear velocity where as we have provision to give flow and pressure only.Dear experts still i cont be understand waters empower is world leading CDS and is unable to overcome these problems.Not only this There is headspace control from software.Reply on thisPlease do needfully.Thanks in advanceSATYA

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## Comments

Hello,

It is understandable that there is difficulty when the analysis information provided is in cm/sec and the input requires ml/min. When provided certain basic parameters the GC has the capability to calculate linear velocity. We chose this method.

You can do the conversion outside of the software if the SOP doesn't provide it.

The suggested workaround is to download and install a small application that will do the conversion for you. A free GC Calculator App for iPhone is available from Agilent at

http://www.chem.agilent.com/en-US/promotions/Pages/GCapp.aspx

The Headspace Control Option is available and currently supports the HS7694 (G1289B and G1290B) and also the G1888. A newer revision of the Headspace Control Option is being worked on that will support the HS7697 headspace sampler.

I hope this information is helpful.

Dot

Hi dot,

Thanks for giving information.But we are unable to download that calculator.

Please guide me

Regards

SATYA

Hi Satya,

To convert from a linear velocity to a volumetric velocity is not overly challenging but it is a bit involved. Agilent does put out a pc version of the calculator that dot referenced (if the issue was that it was an app). They claim it's not windows 7 compatible but I run it on Win 7 with no issues. Here's the URL:

http://www.chem.agilent.com/en-US/Technical-Support/Instruments-Systems/Gas-Chromatography/utilities/Pages/GCPressureFlowCalculatorPCRev205.aspx

If you want the hand calculation here's how it works.

F = A*Uo

Where:

A is column area

Uo is outlet linear velocity

A = p*d^2 / 4

Where:

d is column diameter in

(not mm)cmNote: Dividing by 4 takes into account that we're using diameter instead of radius

Uo = U / j

U is average linear velocity (30 cm/s for you)

j is a pressure correction factor

j = 1.5*((p

_{i}/ p_{o})^2 - 1) / (p_{i}/ p_{o})^3 - 1)Where:

p

_{i}isinlet pressure (typically we measure this in terms of the gauge, so you'll add atmospheric pressure to the inlet pressure to obtain the absolute pressure)ABSOLUTEp

_{o}is the absolute outlet pressure (just atmospheric unless you're running an outlet into a vacuum)Note: pressures in kPa

Put it all together:

F = p*d^2*U / 4*j

F will have units of mL/s, so multiply by 60 to get mL/min. (or just use 1800 cm/min instead of 30 cm/s)

Finally, to correct for the temperature of your oven.

F*T

_{a}/T_{c}Where:

T

_{a}is ambient temp (298K, typically)T

_{c}is oven temp (also in K)If you wanted to do it all in one go your equation would look like this:

Note this is using U in cm/min, you'll have to multiply by 60 if you use the cm/s value

I put all this in an excel sheet if you want to work out an example.

Hope this helps,

Nate

Hello,

Thank you for providing the link to the PC based calculator. Also, the formula as noted it is complex for those who like to try it.

Best regards

Dot