Impact of Magnet Size on Frequency Response

For route-based measurements, CTC always recommends analysts have a variety of magnets in their toolbox. The reason for this is magnet choice can have a large impact on the high-end frequency response of your sensor. The magnetic mounting base choice depends on several factors, including:

The flatness of the machine's surface
The sensor mounting base diameter
The pull strength of the magnetic base

The size and case style of your sensor will have a large impact on the right magnet choice, as the outside diameter of the magnet must be greater than the mounting base of the sensor.

Side exit sensors typically have larger mounting base diameters than their top exit equivalents. As a result, side exit sensors will require a larger magnet than the top exit version of the same size class.

A stainless steel, top exit miniature sized AC140-1D Condition Monitoring Sensors with CE and UKCA certification engravings
AC140
Miniature Industrial Top Exit Accelerometer, 100 mV/g

A stainless steel, side exit, miniature sized AC140-1D condition monitoring sensor with CE and UKCA certification engravings
AC144
Miniature Industrial Side Exit Accelerometer, 100 mV/g

AC140 has a mounting base diameter of 0.50 in. (12.7 mm) and can be used with 0.75 in. OD magnets such as MH136-1A:
Line Drawing showing the Dimensions of the cylindrical MH136-1A flat surface mounting magnet with integral stud coming out the top

Due to the side exit profile, AC144 has a mounting base diameter of 0.85 in. (22 mm) and requires 1 in. OD magnets such as MH122-1A:

drawing of a magnet in a stainless steel circular case, with a tapped hole in the center of the top, showing the height dimension of 0.68 inch and the diameter dimension of 1.00 inch

An analyst is certainly able to use an MH122-1A magnetic mounting base with 1 in. OD for both sensors, however, the MH122-1A will lower the high-end frequency response that an AC140 is able to achieve when paired with a smaller magnet. This is because when the accelerometer is subject to high-frequency vibrations, the system resonant frequency of the accelerometer/magnetic base is lowered by the size and mass of a larger magnetic base, causing the accelerometer to have a reduced ability to accurately measure high-frequency signals. The increased mass can result in a lower resonant frequency, limiting the upper-frequency range of the accelerometer. As a result, while any magnet will limit the high-end frequency response of the accelerometer compared to stud mounting it, opting for a smaller magnet with a lower mass wherever possible will allow the analyst to maximize the high-frequency response of the accelerometer.

a mini size CTC AC140 top exit condition monitoring sensor on a CTC MH136-1A magnetic mounting disc

CTC's AC140 Condition Monitoring Sensor mounted on MH136-1A Magnet Mount Base

a graph showing the frequency response working principle of accelerometer sensor AC140 on an MH136-1A magnet mount base

a miniature size, stainless steel, top exit AC140-1D condition monitoring sensor mounted on a circular MH103-1B mounting magnet

CTC's AC140-1D Condition Monitoring Sensor mounted on MH122-1A Magnet Mount Base

a graph showing the frequency response working principle of accelerometer sensor AC140

This test shows the high-end frequency response of an AC140-1D was about 4 to 5 kHz less when paired with the MH103-1B (1 in.) OD magnet compared to the MH136-1A (0.75 in.) OD magnet. As a result, it is recommended that analysts have several magnet styles on hand, in order to meet the needs of each application and sensor in use.

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