Float tanks tend to use a lot of parts borrowed from the pool and spa industry and, whenever that happens, it’s always important to question whether that pool part works accurately with our salt water solution. We’ve recently gained some insight into that question on one piece of equipment in particular: the flow meter.

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A company called H2Flow, maker of the FlowVis (a flow meter that is widely used in the float industry), recently tested their device with float water. They found that the density of the solution did, in fact, make a difference, and that their standard pool flow meter was quite a bit off from the real flow rate.

As we will explore below, this can have some ramifications for float tank sanitation, and may make you want to change how long you run your pump between clients.

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One of the foundational realities of float tank sanitation is cleaning the solution between each float. Because the solution isn’t drained after each float, a diligent approach to ensure the highest level of water sanitation is essential. Whatever combination of sanitation methods are used to achieve this (UV, H2O2, ozone, bromine, etc.), each tank must also be filtered between floats.

Float tanks accomplish this in one of two ways. Some float tanks pump the entire volume of their solution into an external holding tank after each float, passing it through the filter and the other sanitation equipment along the way. Once 100% of the solution has emptied into the holding tank, it can be refilled into the float tank; this is typically known as the “reservoir” method, and lets you confidently know that all the water has gone through the filter in a single pass.

Other float tanks use the “recirculation” method, where there is no holding tank. In this method, the solution goes right back into the float tank as it’s being pumped through the filtration system. This means that the filtered salt solution mixes back in with the solution that is still waiting to be filtered, making it hard to know when all of it has actually gone through the system. The answer, therefore, is to filter for a longer period of time, giving the all of the water a better chance of getting through the system at least once.

For the recirculation method to be fully effective, we need to pump and filter the volume of the tank a number of times. Each cycle is called one “turnover”. The standard is generally 3 to 5 turnovers per cleaning cycle, and this number is often dictated by local health guidelines.

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Now, consider this scenario:

Your local health department has mandated that you have a minimum of 4 turnovers between each float. The capacity for your system to do this will depend on the volume of liquid in your tank, the power of your pump, and systemic head loss (resistance). In order to make sure that 4 turnovers are actually occurring, you attach a flow meter, which monitors the speed at which the liquid is flowing through your filtration system.

By taking the volume of your tank and dividing by the gallons per minute (GPM) shown on the flow meter, you are able to calculate how long you would need to run your system to filter the volume of each tank. Multiply that by 4, and you’ll have the amount of time necessary to do 4 turnovers.

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” W-BAP!!   MATH!! “

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Thanks to testing by our friends over at H2Flow, we now know that their regular flow meters generally read 5-10 GPM higher than the actual rate with the saturated salt water. To say this another way, they overmeasure the amount of liquid flowing through a system. While it would be convenient to just transpose the perceived flow rate to the actual rate, the relationship between the flow meter reading and the actual rate is nonlinear. A 45 GPM reading on the flow meter correlates to about 40 GPM, whereas a 80 GPM perceived rate equates to about 70 GPM in reality.

As you may have deduced by now, an inaccurate flow meter reading will throw off the calculation of how long you need to run the pump in order to meet the target number of turnovers.

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Consider this example:

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A 200-gallon tank needs to complete 4 turnovers between each float. That’s 800 gallons of salt solution.

The perceived flow rate on the standard, pool industry flow meter reads 45 GPM.

This makes the expected turnover time 4 minutes and 26 seconds for 200 gallons and, therefore, 17 minutes and 44 seconds for all 4 turnovers.

However, we now know that 45 GPM is actually 40 GPM.

This means it actually takes 5 minutes to do a single tank turnover.

Those 4 turnovers actually take a full 20 minutes.

That’s over 2 extra minutes that the pumps should be running between each floater.

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While H2Flow is the only company we know of that has actually tested their flow meters for accuracy in float water, it’s not hard to imagine that other similarly designed flow meters would be inaccurate as well.

Luckily, H2Flow didn’t stop when they discovered that regular flow meters were inaccurate: they developed a float industry specific flow meter, the FlowVis FV-C Saline, which is specifically designed for the salt solution in our float tanks. It’s Level-1 NSF Certified, meaning that it has over 98% accuracy when it comes to measuring the flow rate of salt water. It should be noted that it is calibrated for tanks with a specific gravity of 1.25. At a higher specific gravity, the actual flow will be slightly less than displayed.

At Float On, in the face of this knowledge, we began retrofitting all of our systems with the FlowVis FV-C Saline flow meter. With our now-accurate readings, we extended our filtration time by several minutes in our smaller tanks and by up to 5 minutes in our larger rooms to be sure that we are getting our intended turnover target. We urge all other centers who are currently using the older H2Flow meters to do the same.

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If you have an existing FlowVis, H2Flow has made upgrading it easy. Here are the instructions they asked us to pass along:

For those users who have already installed a FlowVis but who have the standard scale shown on the left of the above image, (FV-C-S), please contact H2flow at: carol.mckenzie@h2flow.net or 419-841-7774, to receive a free updated scale along with installation instructions. If, on the other hand, you have installed a FlowVis that has a check valve feature included (denoted by the head of the scale saying “Pr”), you will need to order a replacement flapper, indicator arm, and scale from H2flow. Please use the above email address and Carol will help you order the required kit at a cost of $36. Please note that, after you complete the upgrade, you will no longer have a check valve feature.

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While the fact that our flow rates have been wrong up until now will be disconcerting to a number of people, the good news is that this generally reflects a technical evolution in our industry. Floating is getting big enough that we’re getting custom equipment made for our tanks. Hopefully H2Flow’s meter is just the first of much more equipment that will be made specifically with the unique needs of float tanks in mind.

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Happy flowting!