Doug and Brian,
        I might have some useful input on yur prolem.  I am a process
engineer using a belt driven (furnace/oven).  Attached to the furnace is
an oxygen analyzer capable of measuing Parts per million.

        Recently I set up a SPC rouine for the oxygen content in the
furnace.  First I would like to say that consistancy may be just as
important as quantity of oxygen.  Our furnace is opperating on average at
6.4 ppm with a standard deviation of 2.5 ppm. an operator records the data
in a chart once an hour and each time anything is placed into the furnace
the content is recordered.

        If I were you I would first calculate what the residual
concentration should be.  Assuming that the furnace is turned on every
morning.  Then the concentration should follow the following equation.

        Cf = Ci/(2^T/t)
Cf is the final concentration (ie at time T). Ci is the initial
concentration (ie concentration of O2 in the atmosphere)  T is time. And t
is the time constant to complete one volume exchange.  To calculate t you
need to know your flow rate and cavity volume.  This analysis assumes that
you have pure N2 with no O2.

The equation is quite simple after one volume exchange T=t and the
concentration is one half of what is was origionially.  Two time constants
gives you one fourth.  This will give you the precentage.

So!!!!!  Calculate your desired Cf and see how many time constants away it
is.  If everything is right you will probably be between .5 and three
hours.

The equation doesn't calculate any oxygen leaking into the system.
However to a first aproximation you can just add the back ground to the
calculated concentration.

In this particular case the gas is at 3 pppm and the furnace is at 9 ppm
this is equivlent to a 6 ppm leak.  So for the furnace with 20 ppm gas I
would expect a volume of 26 ppm.

So why the equation if its so easy?  Because gas costs money (although
nitrogen is cheap compared to say CF4).  So find out from the supplier
what the standard deviation on the gas is and use a flow rate that will
get you to the average plus one standard deviation before you run your
first piece through the furnace.

Also if I were you I would run some test at home before sending it abroad.
Try 60 ppm or maybe 120 ppm.

I hope this does more good than harm.

Paul

BTW we don't use CF4 for solder reflow that was a comment from my provious
life (I mean job)

> ------------------------------
>
> Date:    Wed, 29 Apr 1998 15:39:37 -0700
> From:    ETS <[log in to unmask]>
> Subject: Re: ASSEMBLY: Nitrogen Reflow
>
> Doug,
>
> I appreciate your input but it does not really answer my question. I am
> aware that by putting high amounts of N2 in the machine the customer can
> reach the same PPM level as the source Nitrogen but no customer in their
> right mind is going to want to pay for that. You may ask your customers to
> do that but we look for affordable N2 processing methods for our customers.
> Sorry, had to put at least one plug for my company just to return the favor!
> ;-)

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