The 1X(times) frequency is the best indicator for overall machine health than anything else on the market. Everything else is just noise.
The VTU1X-102 has the capability to monitor two separate machines. Usually used to monitor and engine and compressor. It take in to signals from external accelerometers and outputs two 4-20mA signals that represent the amount of movement or looseness in the monitored equipment.
No protection system is foolproof, they are either, insensitive, hard to implement, hard to adjust, or cause nuisance trips.
The term overall RMS is misleading, in that this technology usually only measures a limited range of frequencies. But, while limited, that limited band of frequencies may include 10 to over 100 harmonic frequency bands which may or may not include the most important 1X. Many times this technology is implemented with filters that remove the 1X frequency, which even if included, would only contribute 1/10 to 1/100 of the RMS total, which makes up the indicated output signal. As explained in this paper, the variance in all these other harmonics, as speed changes, can lead to nuisance trips while hiding real changes in machine health
Knock sensors are a sophisticated technology that monitors for impacting caused by reciprocating parts that are broken or so loose that they are impacting. This technology claims to be able to shut down within a small number of impact events, thereby limiting damage. Because of the many different parts of a reciprocating machine, it is hard to adjust for the different signatures caused by parts that might be damaged to the point of knocking. 1X technology not only will catch knocking but has a better chance of alarming on looseness before it gets to the point of knocking.
One and the oldest and cheapest technologies available have been shown to be either too sensitive or non-protective. These devices give no indications that something is starting to go bad, only that it already has. While maintenance personnel may like the convenience of adjusting protection levels with a screwdriver and a hammer, validating a switch with a hammer only represents that the machine will stop after a catastrophic failure has occurred. Plus, because they are measuring overall vibration, which can cause false trips when adjusted sensitive enough to protect, they are usually “de-tuned” to the point that they are ineffectual.