Confused About Valve Compression Numbers?

“Compression” is what horn technicians and players decided to call a valve or tube’s ability to accept the air pressure coming from your lungs without leaking. Since the actual air pressure inside the horn is low, the term compression is not entirely accurate but has become a standard term when talking about leaks. This leaves many people confused when they see used horns listed alongside “valve compression numbers.” For example, I found three advertisements from the same site which say:  

“The compression reading is a 1.0 which is fabulous.”

“This one has an excellent valve compression of 1.0.”

“The valves have been rebuilt, which is why the horn has amazing compression (0.6 out of 8).”

Is a 1.0, amazing, excellent, or fabulous? Without additional context, these numbers are essentially meaningless. Here is a layman’s explanation of these numbers and what they do or do not tell you.

Enter the Magnehelic...

These numbers come from a device called a magnehelic gauge. Originally developed to measure low pressure HVAC systems, a magnehelic measures the differential pressure between two closed systems. So, you input a pressure of a known amount on one side of the machine, and measure the pressure on the other side. The difference in pressure between the two sides is measured in “inches of water.” This weird unit comes from another kind of differential pressure device called a manometer. You can look that up yourself if you’re really that interested. It’s a very cool device. These kinds of devices are calibrated to atmospheric pressure.

The numbers go from 0 to whatever the gauge is meant to read (in inches of water.) The most popular gauge currently being used goes from 0-10, but is commonly set up in our industry to measure from 0-8. Woodwind technicians also use a machine like this to test the pads on various instrument. On this gauge, when the testing side of the hose is perfectly sealed, the gauge will read 0 (the difference in pressure between the two sides being none.)

When it is open completely, it will read 8. The result from the machine is highly dependent on the setup and calibration of the gauge; mag readings from two different shops may or may not be exactly the same.

When the dial reads 0, there are no holes and the joint is fully sealed.

When the dial reads 0, there are no holes and the joint is fully sealed.

When the dial reads 8 or greater, the joint is fully open and unsealed.

When the dial reads 8 or greater, the joint is fully open and unsealed.

Why do we even care?

It’s a feisty debate among players and technicians as to what range of “compression score” is noticeable in the playability of a horn. A related debate is, at exactly what score are the valves ready to be rebuilt?

My personal feeling is that a 1 or below is standard for brand new horns and that something over 4 should be rebuilt. On my custom horns, I shoot for a reading that is 0.2-0.6. Every maker has to balance compression with valve speed. A lower score valve has a tighter tolerance which means that a lighter valve oil must be used. These evaporate much more quickly and can give the impression that the valves are slower than a 20 year old horn where the valves have been worn down and are reading a higher number. If you are used to blown out valves with enormous tolerances, a tight valve will feel “slower”. It would be up to you to decide if all the benefits of a tighter valve can overweight that speed that you are used to.

Tight valves have less leakage.

Tight valves have less leakage.

As valves age, and wear increases, air leakage worsens.

As valves age, and wear increases, air leakage worsens.

If you have any JoyKeys on your horn, you should not concern yourself with valve compression. Even if your valves have a perfect score of 0.2, your horn overall will be more like a 2.5 or 3 because of the JoyKey. In that case, you’re better off ignoring the compression, and focusing on the technical aspects (noise level, speed, etc.) of the valves. Many folks would argue that 3.0 is still in the range where it may not be noticeable in the playing.

A scale of Magnehelic readings. The green is where a new horn should be. As wear increases, the ranges will be around the yellow area. Horns generally should be rebuilt in the red.

A scale of Magnehelic readings. The green is where a new horn should be. As wear increases, the ranges will be around the yellow area. Horns generally should be rebuilt in the red.

The bottom line is, don’t get too hung up on valve compression scores, but if you want nice tight valves with a long life ahead of them, stick to something under a 1. Just be sure that whoever is measuring them knows what they are doing. While it’s just one number, it’s nice to be able to have an objective way to measure one aspect of a horn’s quality.

-Jacob

Released in December 2017 Newsletter

Jacob Medlin