Many people have been unable to calibrate the DTP41/T because the sliding block on the bottom of the instrument was not in the proper position. To be successfully calibrated the block must be positioned so that the transmission lamp is located below the instrument optics.

When looking at the bottom of the instrument you will see two slots which allow access to this block. The slot on the left is identified with the letters BL and an arrow pointing to the right, the slot on the right is identified with a T/W and an arrow pointing to he left. The block must be positioned so that the slot on the right is open, you should be able to clearly see the block in the left hand slot.

The block should be in this position for calibration no matter whether you are going to use the instrument for reflection readings or for transmission readings.

One further note: this was not a requirement with early firmware releases of this instrument.