Stringer Calibration


When my machine was new, Gamma ELS 7900 - and actually it is still new IMO, I used to recalibrate it about every month. It has been two years now, and I strung a little over 250 times on it (low volume), and I am finding that I have to recalibrate earlier and earlier. First, I dropped it to stringing every three weeks (no major change in volume), and now two weeks.

What prompts me to recalibrate is that I am off by one point or pound in the final SBS and RT readings, and when I play with the racquet I can certain feel the softness of the string bed.

Is this normal, and is there anything I can do from a maintenance wise or other steps to improve the situation?

Thank you in advance.
How long do you wait before clamping? I wait until the tensioner stops making noise which is between 15-20 seconds. If you clamp too quickly, you might as well get a LO. How often do you clean your clamps? Could be some slippage due to buildup or lack of adjustment.

If you find that your calibration is off that frequently, it means that your electrical components are failing, ie the load cell components. I am assuming that your calibration tool is itself accurate. Being paranoid, I actually calibrated my luggage scale before using it to check my pulled tensions.
To add to what esgee48 wrote (he made a very good point about cleaning the clamps which could slip when they get dirty), you need to determine if this is an issue with the machine or with the calibrator.

If you are off one point in DT readings, it could mean that either your previous calibration was off, or that your machine was calibrated correctly but drifted since last calibration.

Lets start with an easier assumption, that it is the calibrator. I want to share a personal observation here. I use ********** 2090 electronic calibrator. I found that the type of string which you use to attach it to a clamp on one side and tensioner on the other plays a significant role. I had troubles with tennis strings snapping where they were attached to wire clips on both sides of the calibrator, and regular thin rope was giving me drifting and irreproducible readings. I ended up with getting good results with kevlar string, with diameter similar to that of a tennis string. I would imagine it could also start measuring incorrectly if its battery were getting low. I do not know what what the reason of issues with regular rope - either the knots were sliding, or rope was extending under load, but the fact is that until I found a better rope - kevlar string (not a tennis string, a household kevlar string), I had issues with repeatability of calibration.

If you use a spring calibrator, it is conceivable that its spring may get weaker over time and use and it may require calibration of his own. If the string gets weaker, if will show you a higher tension than it really is.

The last possibility, when you mention a drop in RT, do you measure it right after stringing? RT can drop by 1 unit after 2 days of storage or less, depending on the string. Also, RT depends on both string, racquet, and string gauge. It is not just tension. Comparison makes sense only for the same frame, same string, same tension.

ADDED LATER: This stupid forum censors even brand names which Tennis Warehouse does not carry. Why wouldn't it also censor "Wimbledon" and "US Open"?


I appreciate your responses @esgee48 and @AndI thank you.

Both of you make good points certainly. I use a spring-based calibrator with double kevlar strings. The clamps, base of the clamps and everything else are clean and in good condition, there is no slippage. Also, I take measurements of SBS and RT right after stringing for all racquets to stay consistent.

Based on your experiences, there is the possibility that the calibrator's spring maybe getting to be loose, it reads generally 2-4# higher than the tensioner on the ELS. I will call @Gamma Tech and/or possibly get a new calibrator to test it out. I find that Gamma is a high quality equipment, I don't expect the calibration to drift 2-4# every other week based on the low volume I have; however, it is best to ask them I guess.

Thank you very much.
I am sure that Gamma stands behind its calibrators, if this is what you have. If it is something different, you might consider that "2090 precision electronic calibrator" because it is not that much more expensive (only $65), but its digital scale is much easier to read. It comes from the factory calibrated, and, according to the manual, it can be sent to the manufacturer for recalibration if/as needed. It is made by the same company which makes Wise tensioner, so the quality can be trusted.

I tried to share an image of the calibrator from Tennis Warehouse Australia, but apparently this is also censored here. Great.

One afterthought: how long has it been since you replaced the string attached to the calibrator? It could wear out over the time, meaning, a worn string could start slipping more, all other conditions being equal. A good test could be to clamp and tension the calibrator 10 times in a row to check how repeatable the readings are. If it varies from measurement to measurement, I would start with the string.

What I found works well for me is kevlar string used e.g. for flying kites, the one I use is sold on Amazon as "400lb 100% Dupont Kevlar Braided Line, 1.7 mm Dia, cut and abrasion resistant, low stretch" by Spearit for $13.99 for 50 ft. This diameter is greater than that of a tennis string, but this string/line is soft and clamps just right. My first attempt with a thinner kevlar braided line, either 1.3 or 1.4 mm, did not work well (slippage). With this kevlar string (sorry, line, to emphasize it is not a tennis string), I am getting the most stable calibration ever, actually, better than with tennis strings. Never thought that such small thing would make such a big difference - when I tried a nylon braided rope, it was a disaster, you calibrate, then check, and you are still several pounds off, you measure twice and get different results. With tennis strings it was better, but not perfect. With this line, calibration does not shift at all over time, it is super stable. You wrote you use Kevlar, I just wanted to be more specific. (I did not try Kevlar tennis string since I did not have it at hand).

It would be great if you let us know what your final finding will be.
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