Ultraviolet Transmittance (UVT): The Critical Water Quality Metric You're Probably Not Measuring (But Should Be)

Clear-Flow Team
Sep 24
5 min read

UV Transmittance – or UVT – might be one the most important water quality metrics you've never heard of. While most commercial pool operators focus obsessively on chlorine residuals, pH balance, and alkalinity, they're missing a measurement that directly impacts the effectiveness of now standard UV disinfection systems.

If your aquatic facility uses UV for secondary disinfection (and increasingly, facilities that don't are the exception rather than the rule), UVT determines whether your UV system is actually doing what you think it's doing. Poor UVT can render even the most expensive and even oversized UV systems virtually useless, while optimal UVT makes every aspect of your disinfection investment work more effectively.

What Exactly is Ultraviolet Transmittance?

Ultraviolet Transmittance (UVT) measures how much ultraviolet light passes through your pool water without being absorbed or scattered by dissolved substances and suspended particles. More technically speaking, UVT is a measurement of the amount of ultraviolet light at 254 nanometers (nm) that is able to pass through 10 mm of water. It's expressed as a percentage – 100% UVT is seen when pure water allows all UV light to pass through, while 0% UVT would mean water so cloudy or contaminated that no UV light penetrates.

For commercial pool applications, UV manufacturers test and certify their equipment using water with a minimum of 94% UVT. Translation? In order to achieve the disinfection rates that were marketed to you as an operator, you must maintain a UVT of 94% or higher. For every single digit percentage drop in UVT, an exponential drop in UV efficiency will follow.

Here's what makes UVT different from other water quality measurements: it's not just measuring what's in your water – it's measuring how effectively your UV disinfection system can actually work with the water you have.

Why UVT Matters More Than You Think

Most pool operators understand that UV systems kill pathogens, but fewer understand that UV effectiveness drops dramatically as UVT decreases. Manufacturer validation curves demonstrate that a UV system designed to achieve 99.9% pathogen reduction at 94% UVT may deliver closer to 90% at 85% UVT. This is not a system failure, but rather a function of physics: as UVT decreases, light penetration is reduced and less dose reaches the target organisms.

This is where things get expensive fast. When UV systems can't perform as designed, operators typically compensate by increasing chemical sanitization. More chlorine, more frequent chemical additions, more complex water chemistry management – all to compensate for UV systems that aren't working at capacity. To make matters worse, the significant energy consumption required of your expensive equipment not functioning as it promised to, creates additional financial headaches for your organization.

But here's the real problem: most facilities aren't measuring UVT, so they don't realize their UV systems are underperforming until water quality issues force expensive emergency interventions and upgrades.

The Hidden UVT Killers

It's not just cloudy or turbid water that impacts transmittance. Several common commercial pool contaminants dramatically reduce UVT, often without obvious visual indicators. Here are just a few:

Dissolved Organic Compounds: Body oils, sunscreen, cosmetics, and other organic materials absorb UV light even when present in concentrations that don't affect visual clarity. These compounds build up over time and are often resistant to traditional chlorine oxidation.

Fine Suspended Particles: Particles smaller than 10 microns might not create visible cloudiness but scatter UV light significantly. Traditional sand filtration misses most of these particles, allowing them to accumulate and steadily degrade UVT.

Combined Chlorine: While combined chlorine levels indicate sanitation issues, they also absorb UV light. High combined chlorine levels can reduce UVT even in visually clear water.

Metal Ions: Iron, copper, and other dissolved metals from equipment corrosion or source water contamination absorb UV light and can accumulate to levels that significantly impact UVT.

"To the naked eye the pool water was clear, but to the UV units it wasn't clear enough... We were at a loss on how to fix our UVT levels." – Luke Diede, Aquatics Recreation Supervisor at Berthoud Recreation Center

How to Test UVT: The Right Tools and Techniques

UVT testing requires specialized equipment – specifically, a UV spectrophotometer calibrated for 254 nanometer wavelength measurement. This isn't equipment most facilities have on hand, but it's becoming increasingly available through pool service companies and water quality laboratories.

Professional Testing: Some commercial pool service companies can provide basic testing as part of comprehensive water quality analysis. Expect to pay a small fee per test, but consider this essential baseline data for any aquatic facility with UV disinfection.

On-Site Monitoring: Advanced commercial facilities are making the investment in their own UVT meter to provide their team with on-demand readings. These systems typically cost $2,000-5,000, but provide invaluable operational data.

Free Testing: Operators who are interested in gaining a baseline understanding of where their UVT currently sits are welcome to send samples into our team at Clear-Flow for free testing. Please get in touch with us for instructions on where to send your water sample.

Testing Frequency: For most commercial facilities, monthly UVT testing is sufficient during normal operations, with increased frequency during high-use periods or when water quality issues arise.

Improving UVT: Strategies That Actually Work

The most effective approach to UVT improvement is enhanced filtration. Since total dissolved solids (TDS) and finer suspended particles are often primary limiters of UVT, improving your filtration system's ability to capture more particles typically provides the most dramatic UVT improvements.

Electronic flocculation technology creates larger particle clusters from fine suspended matter, allowing existing sand and even regenerative media filtration systems to capture particles they would normally miss. Facilities implementing electronic flocculation often see UVT improvements of 5-15% within the first few weeks of operation.

The Economics of UVT Optimization

Poor UVT forces expensive compensatory measures: increased chlorine usage, more frequent backwashing or perlite media changes, and higher energy costs from UV systems that have to work harder to achieve designed performance levels.

Facilities that optimize their UVT systems typically see:

Reduction in chlorine usage
Improvement in UV system efficiency
Reduced backwash frequency and media changes due to better filtration performance
More stable water chemistry requiring fewer adjustments

The payback period for UVT optimization is typically 8-15 months, with ongoing operational savings continuing indefinitely.

"We were seeing the UV working properly again, TDS levels had dropped significantly, and our chemical usage was down. Installing [Clear-Flow] is hands down the best thing we could have done for the quality of our water." – Luke Diede, Aquatics Recreation Supervisor at Berthoud Recreation Center

The Future of UV Transmittance

As UV disinfection has become standard for commercial pools, UVT testing is transitioning from an unspoken advanced practice to operational necessity. Facilities that aren't measuring UVT are operating blind with regard to their UV system effectiveness.

The most forward-thinking commercial pool operations are implementing UVT testing as part of more comprehensive water quality management. This provides real-time feedback on UV effectiveness, filtration performance, chemical effectiveness, and overall system operation.

Want to understand how UVT measurement can improve your UV system performance and reduce operational costs?

Our technical team provides complimentary UVT testing, system performance analysis, and improvement recommendations.

Clear-Flow's electronic flocculation technology improves UV transmittance by enhancing particle capture in existing filtration systems, resulting in more effective UV disinfection for commercial pool operations.