Manufacturers of electrical power systems love Sulfur Hexafluoride (SF6): it’s a colorless, odorless gas which is chemically non-toxic to humans and animals, and has great usefulness in electrical insulation and arc prevention. Fill an electrical distribution switch box with SF6 and seal it, and you’ve increased the safety and longevity of the equipment.
Environmentally, though, SF6 has a drawback: while it doesn’t deplete ozone, it is the strongest greenhouse gas identified by the Intergovernmental Panel on Climate Change—by mass, it’s more than 22,000 times more potent in contributing to global warming than carbon dioxide! So, SF6 is tightly controlled around the world by environmental protection regulations.
As a power generation/distribution company using SF6, when you decommission a switch box, it’s critical to show regulators that you removed the same amount of SF6 as you put in years ago. That means accurate reclamation measurements—and it’s where totalizers, like the ones offered by Alicat on our meters and flow controllers, come in. A totalizer helps our electrical equipment manufacturing customers verify that they’re not venting this potent greenhouse contributor into the atmosphere… Using the totalizer, they keep the planet safe, and avoid a possible penalty.
How a totalizer works
A flow meter or controller reports a flow rate such as standard liters per minute (SLPM)—that is, the amount of standard volumes of mass passing through per unit of time. Supposing you have a 5 liter switching box, and you fill the box at 5 SLPM for 1 minute. You’ve now got 5 standard liters inside. A totalizer would tally up that flow, providing a running total, not just a reading of the rate. On a meter, you could periodically check the total on the front panel, or poll it electronically. On a mass flow controller, a totalizer becomes a precision dispensing instrument which saves you work and improves precision: you might set your totalizer to dispense 5 liters into the switch box, and start the totalizer. The controller then starts flow, counts down the liters until the 5 liters have been delivered (at your chosen setpoint rate) then it closes the control valve, and you’re ready to move on to the next step in your process. You don’t need to watch the flow rate on the display for spikes, and you don’t need a stopwatch.
Who else uses totalizers?
Gas totalizers are everywhere — the gas meter in your house has a totalizer (although it probably is only a volumetric total, calibrated to line pressure), and so does the gas pump for your car.
In industry, gas totalizers are used in:
- Beer brewing: for aeration of yeast mashes
- Gas blanketing: using nitrogen to fill the volume in tanks to avoid the emission of toxic or combustible fumes
- Pharmaceutical manufacturing: to dispense hydrogen in hydrogenation reactors
- Packaging: to fill each potato chip bag with non-reactive gas, keeping it plump without bursting.
- Custody transfer: whether filling a propane tank for commercial sale, or metering gas flows inter-departmentally at a research lab
- High altitude ballooning: by filling balloons with a precisely measured mass of buoyant gas, you can optimize altitude.
- Bio-reactors: regulating gases going in, or being siphoned off to control reactions.
Things to consider when choosing a totalizer
Does the totalizer allow batch processing?
This would be a programming option allowing a totalizing loop to be triggered from the memory of the device. Alicat totalizers offer batch loops. Another way to handle triggering is through remote software, such as our FlowVision application.
Is the valve tuned for your dispensing rates?
The tricky part about dispensing is getting the valve to shut just when you want to, in the way you want to. You may want a very quick valve snap at the end of the delivery, for the convenience of being able to move onto the next step quickly, but for high flow rates, it may be preferable to taper off flow as the total approaches, and then close at the most precise moment.
Either way, you will need tuning of the PID values to optimize the valve behavior. Alicat works to understand your particular situation so that the instrument is shipped from the factory with the right PID tuning. But we also provide the means for you to customize it in the field. Since our MFC’s all include 100 or more selectable gases, it’s not unusual for our customers to change the gases and processes the device is used with–we want you to be able to optimize your controller in each situation.
Which is better for your application: an integrated totalizer and valve (in an MFC), or a flow meter and remote valve wired together?
With a totalizer inside an MFC, the closed-loop relationship between the meter and the valve offers efficiency and precision. Plus, you don’t have to do the programming yourself! It reduces the number of devices you will need to connect to and maintain.
On the other hand, some customers have an existing valve, and just need to totalize flow, or they need the two elements to be physically separated for other reasons.
Do I need volumetric totals, standard mass totals or true mass dispensing?
Generally, each of these types of measurement would be handled by a different kind of flow measurement device. Our flow meters and controllers report volumetric as well as temperature, absolute pressure, and the standard mass flow. Any Alicat with a totalizer can totalize on volume or standard mass. By knowing the density of the gas flowing, we can program in true mass measurements. True mass is expressed in terms of mass per unit of time (e.g., kg/hr). If you’d like true mass data built into your Alicat meter or MFC, contact our applications engineers.
Whether you’re dispensing and reclaiming greenhouse gases, or if you need to calibrate a monitoring sampler (using a totalizer to provide average flow rate over time), or you need to measure grams per minute of oxygen going into a bioreactor for alternative fuel production, a totalizer from Alicat can enable and optimize your process.
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