I might be wrong since I don't know this piece of equipment but I think ROL might be a better measure. For 63/37 it used to be 20 ppm ROL (residual oxygen level) was about what you needed for a reflow oven. This is to take full advantage of all that nitrogen had to offer. So you would measure the oxygen ppm, in your application, with an oxygen meter.
Sounds like you are operating off a tank or more correctly a dewar. A dewar is a tank with an evaporator inside. The nitrogen is liquid and changes to a gas as it passes through the evaporator. The evaporator converts the liquid nitrogen to gas. I assume you know why the tank (dewar) goes empty even if no nitrogen is used. If not, I will explain. As the liquid nitrogen changes to as gas, this happens as the nitrogen warms, pressure builds up and the dewar must vent the excess pressure. This venting consumes liquid nitrogen.
I will bet you are paying a lot more for delivery of nitrogen dewars than you would if you had a bulk storage tank. You probably could reduce nitrogen cost if you went bulk. However, your bulk tank will vent just like the dewar will. This is why it is important to size your bulk tank to your consumption of nitrogen. If you don't use the nitrogen, the tank will vent the nitrogen.
Thanks,
Chris
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