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APPENDIX C
– USING THE SINGLE EVENT CAPTURE MODE
This is to how to calculate a scale factor and to show sources of measurement error.
In this example, the distance between sensors is 1 inch and we want the readings displayed in Miles Per Hour (MPH). The fastest
measurement we intend to make is 130 MPH.
First calculate the scale factor. With a scale of one, the tachometer will display readings in pulses per second.
The scale factor can be calculated as:
1 Pulse
1 Inch
3600 Seconds
1 Foot
1 Mile
0.05681818 Miles
——— x ——— x —————— x ———— x ———— = ————————
Second
Pulse
Hour
12 Inches
5280 Feet
Hour
There will be an error because scale factors can only be five digits. Therefore, the scale factor is rounded to 0.0568.
You will also have an error in the placement of the sensors. The tape edges won’t be exactly 1 inch apart. Assuming the edges
were really 1.01 inches apart there would be 1% error. So at 130 MPH your reading would be 128.7 MPH.
The internal clock inside the tachometer runs at 2 MHz. All measurements are synchronized to this internal clock giving a ±0.5
microsecond uncertainty. As RPM, MPH, etc increase, the measurement time decreases. As the measurement time decreases, the
small measurement uncertainty becomes a larger percentage of the measurement.
At 130 MPH there would be 0.00043706293706 seconds between pulses. (Seconds = scale factor / 130 MPH.) This is equal to
about 874 clock cycles for a 2 MHz internal reference clock.
873 clock cycles = 130.16766 MPH
875 clock cycles = 129.87013 MPH
Therefore, we have an error of ±0.148765 MPH from the clock resolution at 130 MPH. The sensors must be placed further apart
for better resolution.
