gaseous nitrogen flow

I guess you really want to transmit, not just measure. If measuring were all, you would fit a ball in taper tube gage and that would be that.

Brian

For the help of the OP and other this is commonly referred to as a rotameter, no?

Yep: though balls can be found in low flow rate rotameters, the beefier ones use a cone or cylindrical pellet with a skirt of some kind - and to stabilize this float, a helical groove is sometimes cut, so as to spin the float - giving reason for the 'rotameter' tag.

Brian Whatcott Altus OK

Dear John Miller:

Grainger, Dwyer and others have a nice thermal dispersion probes that you could insert in the gas stream (in a known geometry), and totalize the analog value that results. Their output is sfpm. A few hundred dollars, a DC power supply, and a computer.

David A. Smith

Dear John Miller:

Can you rig your own, or do you need suggestions? You could capture instantaneous flow into a PC every second or so through a DataAq card. And assume a straight line between the points to integrate... result, a totaled flow. sfpm * area (in feet of the body the probe is inserted in) * 1 second intervals * 60 intervals = scf

David A. Smith

If that's the case I don't understand your 10-15% accuracy requirement stated in your original post.

Dear John Miller:

The method I described will not work at all with mixed phase nitrogen, or even straight liquid nitrogen. It will only work with the gaseous nitrogen, "regardless" of temperature. I just want to be sure you understand that.

David A. Smith

Could you describe your storage method and distribution methods. That might help in trying coming up with suitable methods of determining a suitable method of measuring what you discharge. For LN2 and even gases distributed by cylinder weight and pressure/temp measurements of a given dear/cylinder seem to be the best methods off hand.

At the NASA research center where I work N2 cylinders from stock are a standard charge., whether you return it partially full or empty. As such you have a hard time getting a k-bottle at full pressure unless you specifically ask for you or order special deliveries yourself. The stock charge though is so cheap in comparison, one really needs to justify the full pressure requirement. LN2 is delivered by truck to specific sites, but is costed in an overhead charge, so I can't really comment on it's delivery cost.

Only one sensor has been proposed, feeding all users in any combination. A totalizer on it could give no more information than the bill of lading for the delivery truck does.

If the users are squabbling about how the bill for the building is apportioned, then you need to measure not only how much gas is flowing at any given time, but also where it is going.

It might be sufficient to install pressure switches or position switches at strategic locations, and just count purge cycles for each user, factored by an assumed representative gas usage per cycle per machine. That assumes a relatively trustworthy user community, which is not always achievable.

-Mike-

Liquid nitrogen is dispensed into known volume dewars sized from 2 - 500L. There is a built in transfer loss charge. The gas is used only by the chemistry dept with that dept either picking up the tab or making some kind of percentage distribution to users. The existing non fuctional meter is about 60 ft down stream from the storage tank. There is a foating ball type flow meter at the same location, it's of minimal use.

Someone else asked about the line pressure. The only gauge is on the storage tank and is set at 50 psi. Thiss provides the force for both liquid and gas delivery. If therre is no gas or liquid the tank has it's own venting system.

Dear John Miller:

Follow up: A thermal dispersion probe (and body) will need to be in each delivery line to which you deliver gas (that you care about monitoring). I have used these probes in both air and oxygen streams, in case you ever need this. They are *calibrated* in air, unless you request otherwise. They have the same reading, regardless of which direction down the duct the gas is flowing, as long as the velocity is the same. So if you have flow reversals, you'll need to watch for zero crossing.

David A. Smith

talk to your local gas company and see if they'll sell you a meter....

You may need minor modifications and/or recalibration to deal with the fact that your gas is at higher pressure than what's normally used for natural gas