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Stringmeter readings

Q: Having read The Physics and Technology of Tennis, I came to appreciate how much the actual tension of a string job can vary from the reference tension. To measure this for myself, I bought a Stringmeter, which I understood from your publications to be a cheap and accurate way of measuring string tension. When I got it, I found that every racquet I tested had string tensions of 40 pounds or lower.

At first, I thought this was an amazing indication of how much tension evaporates over time. But my confidence in the Stringmeter was shattered when I used it to measure string tension before, during, and after stringing my own racquet. I discovered that a string pulled to 60 pounds on my machine measured only 45 pounds on the Stringmeter. To figure out whether the error was in my stringing machine or the Stringmeter, I hung a racquet from a rafter, suspended various weights on a string, then clamped it off and measured tension with the Stringmeter. I assumed that a string with 50 pounds hanging from it would be tensioned to 50 pounds. But the Stringmeter only read 38 pounds. I contacted Stringmeter, and they replied that the Stringmeter only measures relative tension, not actual tension.

I bought the Stringmeter under the assumption that it measured actual tension, based on an article on your website “Measuring String Tension: How to Measure String Tension and What It Means,” by Rod Cross, and in The Physics and Technology of Tennis. Can you help me sort this out?

A: First, there are three separate scales on a Stringmeter. Each is calibrated to a different situation. There are separate pound and kilogram scales for a strung racquet and a “stringers free string scale.” It is the latter that must be used to measure a tensioned string that is not in a stringbed. Each scale is calibrated differently depending on the circumstances of the string. Not using this scale is the most likely culprit. Also, according to Dr. Cross, the Stringmeter is calibrated to be accurate actual tension for nylon string only. (It is good as a relative measurement for other strings.) He also lists other common errors for using the device, including not adjusting the arrow to the correct diameter, reading the wrong units scale, or not rotating the Stringmeter far enough. There could also be something wrong with the technique used in measuring the string with the hanging weight.

In our experience, Stringmeter readings are best used in reference to previous Stringmeter readings. In other words, we recommend measuring tension immediately after a racquet is strung and then re-measuring the tension every few hours of play to see how much tension has been lost from when it was originally strung. In our testing, we find that a racquet strung at 60 pounds will never read 60 pounds on a Stringmeter right after it comes off the machine because a significant amount of tension loss occurs during the stringing process. Then, we find that another 10 percent, on average, is lost within the first 24 hours. Then, more tension is lost gradually over time and with play. This is why the strung racquets you tested showed such “low” readings. Cross strings will measure even lower. When you tension a cross string to 60 pounds, you are not just stretching a string to tension, but also pulling against the friction generated at each weave. Therefore you don’t actually elongate the crosses as much.

Finally, keep in mind when using the Stringmeter that there are many factors that influence the tension of strings in a strung racquet. For an in-depth explanation of these factors, see our Study Guide for Certified Stringer and Master Racquet Technician tests, which can be found on our website. The type of machine used, the stringer’s technique, the string’s construction, and the density of the stringbed are just a few of these factors, as well as the rate of tension loss. So, we don’t recommend using a Stringmeter to check the accuracy of a stringing machine. Rather, we recommend using it to measure tension loss.

One other good use for a Stringmeter is to verify the stringer’s consistency. If you string the same racquet with the same string on the same machine using the same technique twice, the resulting Stringmeter reading should be the same for both string jobs. If you get different readings, then something must have been different between the two string jobs.

From the October 2004 issue
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