How do I trap large amounts of water before it gets to my rough pump?

[MsMombo]

I'm trying to freeze dry samples in 2 liters of water, using a Welch 1397 roughing pump. Most of the water stays in the pump oil, which I have to change after each run. I've heard there are condensers that can trap the water before it gets to the pump. Is this the way to go? If so, where can I get one? I need something relatively cheap -- it doesn't need to be fancy.

Thanks,
Catherine

 

[butelja]

You need a cold trap in the line going to the pump. I assume this is being done on a lab scale of production. What we commonly use is a flask submerged in a bath of dry ice chips and kerosene. The flow is routed through this flask on the way to the vacuum pump.