This article will introduce you to the interfacial tension test commonly used on transformer and insulating fluids to establish their condition.
The world of transformer maintenance might seem complex, but understanding the science behind it can go a long way. One crucial concept is interfacial tension, a critical parameter in electrical oil analysis that plays a significant role in ensuring the longevity and performance of transformers and their fluid. Let’s decode this concept and explore its relevance to transformer oils.
What is interfacial tension (IFT)?
Interfacial tension, in layman’s terms, is the force that maintains the boundary between two immiscible or partially miscible liquids, such as oil and water. This force helps maintain the distinct identity of the two liquids, preventing them from mixing haphazardly.
In the world of transformer maintenance, interfacial tension is a reliable indicator of oil health. When a transformer oil sample is contaminated, especially with water, particulates or thermal degradation products, the interfacial tension between the oil and water changes. A decrease in interfacial tension can signal an increase in contamination. This early warning allows your laboratory to help the electrical maintenance professionals to take timely action, preventing potential damage to the transformer.
Interfacial tension might seem like a high-level scientific concept, but its practical implications in transformer maintenance are undeniable. Keeping a close eye on this parameter allows you to safeguard the longevity and efficiency of your transformers, potentially saving substantial costs over time. It’s a powerful reminder that every drop of transformer oil carries essential information that can contribute to a well-functioning electrical grid. The lab can carries the key to help you unlock this information.
So how does the lab check my oils interfacial tension?
Understanding and monitoring interfacial tension offers a glimpse into the health of your transformer oil and, by extension, the transformer itself. So, how do we measure interfacial tension? Various methods exist, with the most commonly used in oil analysis being the Du Noüy Ring method. This technique involves immersing a platinum ring into the oil sample sitting on top of pure water and measuring the force required to lift it out again.
The most common methods are ASTM D971 and as part of the the IEC60422 tests. Here’s how it works: The platinum ring is dipped into the oil and then slowly lifted up. The test measures how strongly the oil clings to the ring, a bit like trying to lift a wet ring out of a cup of honey versus a cup of water. This ‘clinginess’ is what scientists call interfacial tension (IFT). If you look in the animation below on a sample of oil you can see the clinginess between the bottom water layer up to the bottom white line and the oil on top of it (also colourless).
A high IFT—think of it as a strong ‘hug’ from the oil to the ring—is a good sign. It’s like the oil showing off its strength, saying “I’m healthy and free from nasty contaminants!” New transformer oils, at their best, give a hug equivalent to 40-50 dynes/cm.
But if the IFT is low, below 20 dynes/cm, it’s a warning sign. The oil isn’t clinging to the ring as strongly, which suggests it’s not feeling so well. Maybe it’s old, or perhaps it’s contaminated with something. It’s like when your grip is weaker when you’re not feeling your best.
In such a case, the oil might need some TLC—maybe a treatment or even a replacement. But remember, this test is just one part of the oil’s health check-up. It should be paired with other tests to get a full picture of how the oil is doing.
Clean lab technique is essential to a good result
As a little side story the test is so sensitive to contaminants that a single ungloved finger can cause a drop in interfacial tension. Hence although it looks quite simple you need a trained lab professional to ensure you get an accurate answer. See box below.
On our first interfacial tension tester I purchased for the lab the platinum ring for the tester came in a special moulded holder to keep the ring clean and dust free as well as protect it as it as the slightest knock can cause it to bend. The case for the ring looked like something you might find at NASA with various seals etc to keep the ring clean and airtight when it in use and at the time I recall thinking this seemed overkill. I found to open the case required a push with a fingernail to release the clip, which meant taking my gloves off to get it out. Once undone I used tweezers to hang the ring. The test with deionised water which should be 70 came out slightly lower than expected and I couldn’t work out why. I cleaned everything and it came out perfectly, but because I was curious why it came out low the first time I repeated everything I did from the start and it came out low again. Clearly something was contaminating the process, but what? We worked out that the tweezers I had been holding ungloved in my hand whilst unscrewing the bottle had slight residues of oils (as in skin oils not lube oils) from my hand and when these tweezers were touching the platinum ring they were transferring this across. Clearly the level of sensitivity of the test to contamination was phenomenal. We adjusted the process to involve washing of the tweezers as well as the ring and cup before every sample and the water standard comes out perfect every time now.An example of how sensitive interfacial tension is to contamination.
Remember, the science behind our everyday operations isn’t just for scientists—it’s for everyone involved in these processes, including you. By understanding these principles, you can make informed decisions and optimise your maintenance practices. Here’s to the power of knowledge in ensuring transformer health! If you want to find out more about how to monitor your transformer oil then get in touch using the contact us button below.