Transparency Meter Calibration

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					          Transparency Meter – Standard Procedure Calibration


Wear protective clothing and eye protection

Electrical hazards

A risk assessment must be carried out before starting this work

Calibration Procedure

Making and calibrating the instrument

1. Set up the electrical circuit as follows: -

The first decision to take is the value of the fixed resistor. You want the
voltage range across the fixed resistor to be as wide as possible. Set the
resistance substitution to 1000. Place the LDR at the end of the tube,
and put a 36W lamp at the other end. Record the voltage readings in this
light with the LDR uncovered.

2. Place 20 microscope slides between the lamp and the LDR, and take
   the voltage reading again. These two readings will show your range of
   output voltages

3. Change the value of the resistance substitution box, and repeat the
   process (repeat this for a range of resistance values – this will give
   you information about the best value fixed resistor to choose)
4. Calibrate your light meter by taking the reading with no microscope
   slides as the brightest level (this calibration must be taken within the
   tube, as it needs to take place in the dark) – this is your relative
   intensity 1 reading

5. Measure the distance between the lamp and LDR – this is distance X

6. Keep the LDR fixed in position, but move the lamp so the distance
   separating it and the LDR is 1.4 X (the relative intensity at this point
   is 0.5). Record the output voltage reading

7. Now move the lamp twice the distance so it is 2X (the relative
   intensity at this point is 0.25). Record the output voltage reading, and
   repeat for 3X; 4X and 5X – fill all your results in the calibration
   results table similar to the one below (do this in excel as you will need
   to plot your results): -

     Distance (cm)           Relative intensity            Voltage (V)
           X                          1
         1.4X                        0.5
          2X                        0.25
          3X                        0.11
          4X                       0.063
          5X                        0.04

8. Plot a graph of relative intensity (x-axis) against the voltage across
   the fixed resistor (V) (y-axis). This is your calibration graph

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