The Odd Struckness Meter
The Odd Struckness Meter (OSM) is a small hand-held box which is a new tool for the bell-hanger and anyone interested in bell maintenance.
It allows accurate measurements to be made of the swing times of a bell and its clapper. This data can then be used to help minimise oddstruckness.
The OSM shows which bells in a ring need adjusting, and by how much, and takes all the guesswork out of this process.
It uses an optical sensor (photohead) to detect the bottom dead centre of the swing of the bell, and an internal microphone to detect the moment at which the clapper strikes.
The OSM has very few controls. One button selects the operating mode (swing time and odd-struckness measuring), and a knob sets the sensitivity of the microphone. A LCD screen is used to display the measurements and two LED indicators are provided to show when the microphone and photohead are active.
There is room for two 9V batteries (one in use, one spare) in the battery compartment, and a message is displayed on the LCD screen when the OSM is switched on if the battery needs replacing.
Measuring the swing time
The OSM can be used to take accurate measurements of the period of swing of a bell. These measurements are taken with the bell down and swinging just a little.
The time taken for the bell to swing one way (t1) and the other (t2) are added up and displayed. This is the period of swing of the bell.
When swing period measurements are plotted for every bell in the ring, they should follow a slight curve from treble to tenor, roughly in the same way as the bell weights increase from treble to tenor.
The following graph is produced from measurements taken at Ashchurch, Glos.
This shows that the 6th swings slightly too quick, and the 3rd slightly too slow with the other bells with smaller deviations.
The OSM also measures odd-struckness and whether a bell strikes early or late with respect to the other bells in the ring. These measurements are taken when the bell is up and rung open. The OSM measures the time between bottom dead centre (bdc) of the swing and the moment when the clapper hits the bell
The t1 time is the bdc to handstroke clapper strike time, and the t2 time is the backstroke time. Then t1-t2 is the amount of odd-struckness, and t1+t2 gives an indication of how early or late the clapper hits.
The following graphs show the measurements made at Ashchurch.
This shows that the treble 4th and 5th are not oddstruck, but the 4th strikes too early. The backstrokes are generally slightly slow, but the tenor is very slow at handstroke.
(Positive numbers indicate the bell is slow at hand/quick at back)
Note that the tenor is some 70ms slow at hand. After the clapper had been corrected, the OSM was used to show that this had been reduced to 19ms.
The amount of oddstruckness noticeable by the human ear is surprisingly small. An error of just 20ms can be detected, and anything greater than 40ms can be very obvious.
The use of tapered washers can help reduce the oddstruckness on many bells. With headstocks fitted with "twiddle" pins, it is possible to reduce the amount of oddstruckness to better than 5ms.
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