The ERT 300 is an iconic little machine to measure the dynamic string tension of your racquet. This is used by stringers worldwide.

The ERT 300 is back

I have used my second-hand ERT 300 ever since I started stringing. It allows me to be exact in my stringing over and over again and check it’s time to restring my racquet. The ERT measures DT, which means dynamic tension or stringbed stiffness.

You can buy the ERT 300 from the official website and if you use the code tennisnerd, you get 10% off.

What is DT?

The DT value is physically defined as the ball force in kp (kilopond) that is required to press the string bed by 1 cm for a short time (ball strike).

The DT value defines the playing characteristics of power and control.  A low DT value generates more power through ball acceleration. A higher DT value provides more control but also a firmer feel overall.

The following factors influence the DT value: stringing weight (reference tension set on the machine), accuracy and condition of the stringing machine, technique of the stringer, material and diameter of the string, head size, and stringing pattern of the racquet.

It will tell you when to restring your racquet

Over the years of testing hundreds of racquet and string combinations, I have developed a good feel for when it’s time to restring. But the DT can be a great assistance. According to the ERT 300 official website, restringing is necessary after a DT loss from 10% to a maximum of 20% of the initial value. Let’s say you start with a DT of 35. If it drops to 29, it might be time for a restring. If the value is lower than 26 the machine will tell you that it’s time to restring, no matter what, but some players play with spaghetti-like tensions (read about Adrian Mannarino).

I like a DT value around 35-36, according to the ERT guideline, which makes me a dynamic and offensive player, which I can relate to overall.

If you go lower than that, you are most likely a more defensive player, and if you go higher, you can play at very high swing speeds and really attack the ball. Overall, you know I recommend to consider a lower tension for your game since high tensions and stiff racquets can lead to arm issues. Playing with a dead polyester string is never a good idea. No control and no comfort.

In short, I am a big fan of the ERT 300. It retails at the same price as most new racquets these days (not cheap), but it’s extremely useful if you take your tennis seriously.