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Standard Operating Procedures

Liquid Nitrogen

Introduction

This method describes everything related to liquid nitrogen. How to obtain it, how to safely transfer from vessel to vessel, and what to do with empty large dewars.

Safety

Liquid nitrogen is -196 °C. It evaporates very quickly and can remove oxygen from the local area. The major safety concerns, then, are frost bite (cryogenic burns) and asphyxiation. Wear protective clothing (close-toed shoes, pants, gloves, and safety glasses) when transferring liquid nitrogen from vessel to vessel. You are expected to have read and understood the SDS for liquid nitrogen. EH&S has an online course called Liquid Nitrogen Safety that you should take.

Large 230 L liquid nitrogen dewar
Large, 230 L, liquid nitrogen dewar

Large dewars

The large, 230 L, dewars are delivered by Praxair to the hallway outside rooms 302 / 303 on Tuesday mornings. If you have ordered a second dewar after Tuesday morning, it will most likely arrive Thursday morning.

The large, 230 L, dewars have three valves on top. One for using liquid nitrogen, one for using nitrogen gas, and a third one that allows users to pressurize the headspace in the event it doesn't have head pressure. We need a head pressure for the liquid nitrogen to come out. It is usually about 20 psi.

hand-held liquid nitrogen dewars
Hand-held liquid nitrogen dewars

Hand-held dewars

The hand-held dewars are used to transfer liquid nitrogen from the large, floor-rolling, dewars to your place of work. The larger of the hand-held dewars can be used as your daily allotment and the smaller hand-held dewars can be used to fill the open-top dewars. You can use a funnel to transfer between hand-held dewars.

open-top liquid nitrogen dewars
Open-top liquid nitrogen dewars

Open-top dewars

Open-top dewars come in a wide range of sizes. They are made of stainless steel or glass. Be careful with the glass varieties. Since all dewars have an evacuated space between the outside and inside walls, the glass dewars will make a loud pop if dropped and glass will travel far. When you are finished with the liquid nitrogen in an open-top dewar, use a funnel to dump the remaining liquid back into a hand-held dewar.

Filling from large dewar

liquid nitrogen station
Liquid nitrogen station
filling a hand-held dewar
Filling a hand-held dewar from the large, 230 L, dewar

Use the liquid nitrogen station area to fill hand-held dewars from the large, floor-rolling, dewar. You will find safety goggles, gloves, cryo-hose, and a wrench here. Attach the hose to the liquid port if it is not already. Put the other end of the hose into the hand-held dewar you wish to fill. Don goggles and gloves. Hold the hose with one hand to keep it steady and then open the valve with your other hand. The liquid nitrogen will boil immediately upon hitting the warm hose and it will sputter and puff and whistle. This will calm down after the hose cools. Continue filling until full or until the desired amount is dispensed. If the large dewar begins to whistle and does not stop whistling, the large dewar is empty. Turn off the valve, remove the hose, tear off the bottom part of the tag, and role the dewar into the hallway.

Filling among hand-held dewars

To fill among the various hand-held dewars, use one of the funnels. Place the funnel in the dewar you wish to fill. Lift the dewar you are filling from and pour until the desired amount has been transferred. If the dewar you are filling is warm, it will take some time to first cool it down. You will know its warm because the liquid nitrogen immediately boils when entering the dewar. You will observe lots of splashing and it will seem difficult to make any progress at all. The best way to cool a warm hand-held dewar is to use the large floor-rolling dewar and hose. Once the hand-held is cool, pouring in as originally intended will be much easier.