Cable Interference & Drift Test
These two tests – Cable Interference and Drift – are intended for pin meters. Cabled interference comes into play when a meter has a cabled attachment for a hammer probe.
Handling the cabled attachment can affect the reading’s accuracy. Drift also must be considered when a meter’s pins are driven into the wood and within two or three seconds the meter readings start to move or drift.
If a meter is susceptible to cabled interference or drift, the accuracy of the meter readings becomes questionable.
Pin meters that come equipped with a cabled attachment may have their readings affected by interference or handling of the cable. In the real world, people who use pin meters may not be aware that there’s an electromagnetic field surrounding the cable.
Simply holding or moving the hammer probe attachment itself can alter the field and, therefore, alter the readings. Coiling or uncoiling the cable, or even holding one’s hand over the meter, can also interfere with the electromagnetic field. Readings often can change as much as two percent just based on how one holds the hammer probe or positions the cable.
Meter readings may start to drift lower after the pins are inserted into wood with high moisture content. Meter drift is less of a problem at lower moisture content levels. Some manufacturers recommend taking readings within the first 2 to 3 seconds of driving the pins into the wood to avoid drift.
But this can be problematic when the meter’s display has no backlight, or when lighting is poor, or when the display panel is upside down after pin insertion. If the reading is not observed in time, one may inadvertently obtain a reading that is different than the true value.
Steps 2-5 must be completed as quickly as possible to minimize the possibility of drift.
- Select three pieces of hardwood: one piece at low MC (about 7-8%), one at medium MC (about 10-12%), and one at high MC (15% and above).
- After driving the pins into the piece, make sure the cable is as straight as possible and take a reading with the meter.
- Place your hand over the cable, and take a reading.
- Coil the cable and take a reading.
- While the cable is still coiled, place your hand over the coil and record the reading.
- Drive the pins in again, and with the cable straight, record the initial reading.
- After 3 seconds, record another reading.
- Repeat steps 2-7 with the second piece of wood.
- Repeat steps 2-8 with each pin meter.
Cord Variance Testing