Why Doesn't my Gauge Work?

What your Beverage Content Gauge is Actually Telling You.

Author: Dan Broaddus

I get a lot of confused calls from my customers, and from end users, asking me what the reading on the Tank Pressure Gauge is actually telling them. Well, hopefully it's telling them a truth. Unfortunately, like a lot of truths, it may not be the truth that they are hoping to hear.

What your gauge is telling you depends on the type of gauge, and what it is reporting on. The first question is: what sort of gas is in the tank? The second question is: which gauge are you looking at? 3rd: what does the gauge measure, and in which units?

The gases we most commonly deal with, for beverage dispense, are carbon dioxide (CO2) and nitrogen (N2). Sometimes, we might be dealing with beer gas, AKA Aligal, AKA G-gas. These are simply Trade and Trademark names for specific Blends of CO2 and N2, which may or may not be the best choice for your beer system, but are often a "better" choice than pure CO2... or the ultimate sin, compressed air. I will tackle that one in a different blog, after some additional legal and scientific study.

Critical Temperature & Critical Pressure

Let's keep to the basics, for now. CO2 and N2 behave very differently. All gases interact with our environment differently, because they have very different molecular structures, and very different masses. One of the ways we determine how these gases interact with our world is by figuring out what their critical temperatures and critical pressures are. Bear with me through this next little bit. It's quite important.

Critical temperature is that point, above which a gas cannot exist in liquid form. Critical pressure is that point, below which a gas may not exist in liquid form, when at critical temperature. Critical pressure will lower as temperature drops below its own critical point. If critical temperature is exceeded, but pressure is maintained, many inert gases will act as super fluids, existing in a state that is not quite gas or liquid.

CO2, compared to other gases, has a fairly low critical pressure, and a fairly high critical temperature. At the temperatures in which we interact with it, it will store as liquid in your cylinder, or bulk tank.

Nitrogen however, has a very low critical temperature and a very high critical pressure. So when we are interacting with nitrogen, it is stored as a high-pressure gas. When you are dealing with a beer gas, these facts do not change. The CO2 will be in liquid form, underneath a vapor space of high pressure N2. How exactly they "mix "is a subject for that aforementioned article-to-come.

Pressure Gauge Locations

Equipped with our rudimentary facts about these gases, I feel we should first tackle bulk CO2. These are those big, stainless steel tanks, permanently installed at a restaurant or concession. They usually hold between 200 and 300 lb of liquid CO2.

On these tanks you will find two gauges. One will be a pressure gauge that usually covers a range from 10 psi, to 160 PSI. It will usually be set between 90 and 105 PSI. This gauge is measuring the tank’s output pressure.

You will also find a gauge reminiscent of a gas gauge in an old car. It is usually rectangular, though sometimes circular. It will usually be at the center of the top of the tank. And it will have a needle that denotes such things as 1/4, 1/2, 3/4, and F, for full. This gauge is measuring the approximate liquid level of the remaining CO2. The accuracy is not often superb, nonetheless, it will usually give you the information you require.

Sometimes this regulator is attached directly to the valve on the tank. Sometimes it is mounted on a wall, or a bag in box rack, and connected by a high pressure hose. These regulators usually have a tank gauge, and one or more output gauges. Sometimes, they only have an output gauge. On rare occasions, they will have no gauges at all, but these are irrelevant to our conversation.

Pure C02

If you are using Pure CO2, unfortunately, your tank gauge is all but useless. Since your CO2 stores as liquid, if even the tiniest amount of liquid remains in the tank, the needle on your tank gauge will remain at whatever the critical pressure is for the temperature of the tank at that moment. Usually, this is between 800 and 950 PSI.

If the gauge is dropping below 700 PSI, be prepared to change to another tank within the hour. Pressure this low means that you have evaporated all the liquid CO2, unless the tank is very cold. Depending on the size of the cylinder, and the volume of usage, this could last only a few minutes, or might last several hours.

The only way to tell how much CO2 is left in a small cylinder is to weigh it. In North America, CO2 cylinders are usually classified by Content Net Weight. Most cylinders will have a stamp informing you of the tare weight. This is the weight of your cylinder when empty.

However, if dealing with a cylinder above 20 lb, the risk of moving it usually outweighs the potential reward of weighing it. A ruptured gas cylinder will take off like a rocket or torpedo, will easily pass through a cinder block wall, and still be carrying enough force to pop your head like a grape! So unless you are Wile E. Coyote, or have experience moving cylinders safely, don't do it. Of all the Awards you may aspire to, I hope that a Darwin Award is not amongst them .

Nitrogen

If you have nitrogen on site to feed your gas blender, the gauge is an accurate indicator of the gas remaining. If your cylinder says it has 1,000 lb, which is just shy of 70 bar, then the gas remaining is equal to 70 times the volume of your cylinder.

An example would be to take a q sized cylinder, which stands about two and a half feet tall, and holds (very) roughly 5 gallons/19L. Chances are your gas blender puts 40% nitrogen into the blend, which is dispensed at 24 psi, or 1.655 bar. 40% of this would be .662 bar. Using our example of 1,000 lb/69 bar, your 5 gallon vessel will be able to displace an additional 345 gallons of product.

Mixed Gas

If you have a mixed gas cylinder, your gauge is sending you mixed signals...literally. As the nitrogen pressure depletes, the gauge will continue to drop, until such time as you reach the critical pressure of the CO2. At this time, you will find yourself in the same place as you do with pure CO2.

Choosing a Gas System

So if you are someone who enjoys recreational mathematics and wants to dig in to such things as the expansion rate of carbon dioxide, and/or have the necessary skills to move and weigh compressed gas cylinders... go nuts! If not, you might consider choosing a gas system that is more user-friendly, and will reduce worry. One might argue, that as a restaurateur, you have enough on your plate. (Go ahead and moan. It only makes me stronger.) But seriously, there is enough keeping you awake at night.

When acting as a consultant, or simply selling a beverage system, sometimes I have no say in the gas system. Oftentimes, large chains have standing agreements with beverage gas providers, and the gas system is predetermined. As a consultant, when I am engaged in the conversation early enough and asked my opinion, I will recommend mini bulk CO2, and pneumatic beverage pumps.

The wonderful thing about pneumatic beverage pumps is that the gas used to drive the pump never touches the product. This means that you can use clean, dry compressed air from an oilless compressor. This will reduce your CO2 consumption, as well as reduce the dangers of a CO2 leak, which can be fatal, and more so than simple asphyxiation, or even Carbon-Monoxide poisoning.

However, things break down, or the power might go out. If you take the extra few minutes during installation to vent the exhaust of all the pumps to a safe, well ventilated space, you can also install a changeover valve that can switch you from Air to CO2 as the source for powering your beverage pumps. This way, in the event of an air compressor failure, you can still serve beer.

If you have nitrogenous ales or coffees on offering, there are nitrogen generators available, many of which also provide air to power pumps. Or, you can simply keep a cylinder of nitrogen and a single mix Blender. And if you want to know why I prefer pumps... that blog is likely posted by now.

To sum it up, what your Tank Gauges tell you, most of all, is this: Your beverage gas system requires planning, knowledge, and expertise, as well as a bit of foresight. And my last bit of advice is this: Beer Distributor Reps are interested in selling you beer, Gas providers are interested in selling you gas, and Beverage Equipment technicians want to install beverage equipment. Get advice from all three, but weigh the advice against their interests. Many are honest and give good measure. Those who are not will quickly fail the sniff test when measured by this method. I hope I have helped.

Cheers,

Dan Broaddus
Technical Advisor
Canadian Beverage Supply & Chill-Max Beverage Solutions