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LitreMeter on finding the right Nitrogen flow meter

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19 March 2021

When considering a Nitrogen flow meter, all the application requirements, together with the applicable safety factors need to be considered. LitreMeter look at a unique enquiry that led us to work with partners Sierra Instruments, on developing the right nitrogen flow meter for a laboratory.

Nitrogen flow meter requirement
The requirement was for a laboratory, which had six glove boxes; each fed from a ring main. The usage varied greatly from a small "puff” of Nitrogen every time someone puts their hands into the gloves, to a high flow re-pressurisation of the "ante-chamber” when a sample enters or leaves the main chamber.

The solution
The proposed solution was to utilise the Sierra Instruments 100 Series mass flow controller (MFC), fitted with a Compod. This meter was able to measure the Nitrogen usage to an acceptable ±1.0% based on an agreed flow rate of 200 SLPM. This flow rate would be sufficient to re-pressurise the anti-chamber in an acceptable time, plus providing some spare capacity.

Installing the nitrogen flow meter
The MFC would be fitted in the laboratory supply line, upstream of the ring main feeding the glove boxes. A reservoir fitted downstream of the MFC adds volume to the pipework, plus the addition of a pressure transmitter are the keys to the systems operation.

The Compod fitted to the MFC, adds the required functionality for the application, allowing it to use a 4-20mA signal from a pressure transmitter.  The general arrangement for a single glove box is shown below, but will equally apply if there are six or more glove boxes.

System operation
The customer’s pipework including the reservoir should be considered as a contained vessel.

From the nitrogen source, which is regulated to 9.7 bar g. the Compod fitted to the MFC is programmed with two downstream pressure "set points” of 8.5 bar g and 6.5 bar g.

On start up, the MFC pressurises the system at 200 SLPM. When the system pressure reaches 8.5 bar g the MFC valve closes.

The pressure contained within the system including the reservoir, supplies the glove boxes with the Nitrogen contained within the system. When the pressures drops to 6.5 bar g. the Compod opens the MFC valve and the "system” is re-charged at the same constant flow rate until the system pressure again reaches 8.5 bar g. when the Compod closes the MFC valve once more. Obviously, this cycle then repeats, thereafter.

Accuracy
For this application the delivery flow rate is always 200 SLPM. The MFC is calibrated specifically for this flow rate, so the accuracy is ±1.0% of the indicated flow rate. Therefore, the totalised "standard litres” supplied, are also to the same accuracy.

Commissioning
With the associated freely available software for the Compod loaded onto a laptop, the system can be pressurised and the pressure transmitter monitored for leaks over time.

Remote Totalisation
The system can also provide a pulsed signal with respect to the "standardised volume” used for remote data logging.

Summary
The system described was to monitor the gas usage into laboratory, but could easily be adapted for other applications.