the mk1 has a wheel, this should be numbered, although the numbers do wear off with age, you can get new wheel stickers from deben, click it on an wind it all the way around, thats your 15ft setting, as you walk around waving it, you should get the knocking noise as your pointing in the right direction, by passing the locator over the area that the ferret is in/under you can pinpoint where the spot it, then you start turning the wheel back, as the signal (knocking) gets to a point of dissapearing, you then read off the number,printed on the wheel, which gives you a depth in feet, as to how far