The accuracy of measurements made with your Plan Wheel depends on the inherent accuracy of the device and on your technique. These are described in this application note.
The accuracy of any measurement is related to the actual length rolled on the print. The longer the distance rolled, the more accurate the measurement. This is shown in the chart below:
Actual Distance Rolled
1" (2.5 cm)
2" (5 cm)
5" (12.5 cm)
10" (25 cm)
20" (50 cm)
Example: You roll a dimension and get a reading of 40 feet. The actual distance you rolled on the print is 10 inches (1/4" = 1' scale). The reading is accurate to within ±1.05% times 40 feet or ±0.42 feet.
Note that the percentage accuracy is not affected by the scale factor used. Also, the accuracy shown in the chart is the maximum possible error for a single measurement. The actual error may be any number up to this maximum.
When you roll more than one dimension and accumulate the total, the maximum possible error will be a kind of average of the errors for each individual measurement. The errors will tend to cancel out; that is, the more separate dimensions you roll, the more likely you will be to have a smaller error than the maximum possible.
As a practical matter, if you do a takeoff with multiple segments, some of them longer and some of them shorter, you can feel safe that your result will be within a few percent of the correct answer.
With good technique, you should get the accuracy described in the chart. First, make sure you start and stop as accurately as possible. To do this, look down the flat part of the scaler tip as shown below:
It does not matter if the scaler is tilted as you roll it. But try to start and stop with the same tilt. If you are looking down the tip as shown above, this will help.
Also, for best accuracy, set the scaler's SENSE mode to plus/minus (+-). In this mode, if you "shake" a little when you put the wheel down at the start point or when you stop at the end point, it will not add distance like it will in plus (+) mode.
The wheel on the scaler is adjusted at the factory for a light friction. This stops the wheel from rolling if you pick it up at the end of the measurement. Yet you only need to apply slight pressure during the measurement to be sure the wheel rolls without slipping. If you are rolling on a slick surface, such as glass, be sure to apply enough pressure to avoid slipping.
There is no way to calibrate the Plan Wheel, the calibration is fixed. To test the Plan Wheel for accuracy, roll it along a scale or ruler. Remember that a drawing may be slightly reduced or enlarged when it is copied.
The mathematical formula for overall accuracy is:
Overall Accuracy = 0.25% + (0.08 / d), where d is the actual distance rolled, in inches, or
Overall Accuracy = 0.25% + (0.2 / d), where d is the actual distance rolled, in centimeters.
The 0.25% term is the accuracy of the wheel diameter. The second term reflects the resolution of the display. The display is updated every 0.04" (1.0 mm) of actual distance rolled. You may be off by this amount at the start point and the end point of the measurement, so the measurement may be off by ±0.08" (±2.0 mm