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Respiratory Research in the Department of Medicine, Alfred Hospital

Mefar Dosimeter Pressure Check.

The Mefar Dosimeter

>Figure 1: The Mefar Dosimeter.

Introduction

The driving pressure developed by the Mefar dosimeter (fig 1) directly influences aerosol output from nebulisers (the greater the pressure the higher the output; (1). It is therefore essential that driving pressure is checked on regular basis to ensure that it remains constant and consistent between centres.

The recommended driving pressure for the dosimeter is 180 KPa. Dosimeters working at this pressure produce satisfactory aerosol output (1,2). In practise the dosimeter should be checked weekly to ensure the driving pressure is within the range (1)

Procedure for Checking the Driving Pressure.

  1. Driving pressure can be checked by attaching a pressure gauge to the output hose of the dosimeter (in place of the nebuliser). (The pressure gauge should have an accuracy of +/- 2%, and we recommend a dial diameter size of at least 100mm.)
  2. The programmed settings on the dosimeter should be set as follows:
    3. Number of activations=07. Pause time=6.0 seconds. Activation time=1.0 second.
  3. Press the "RESET" button on the dosimeter to prepare the dosimeter for activation. Press the "EMERGENCY MANUAL THERMISTOR" button to activate the dosimeter.
  4. The dosimeter should be activated twice before any readings are made.
  5. The dosimeter should then be activated 5 times. The peak pressure developed during each activation should be recorded. The mean driving pressure should be recorded and retained for future reference.

Adjustment of Driving Pressure

If the dosimeter is working outside the range, 176-184 KPa, it needs to be adjusted to operate at the correct pressure. In order to avoid electrical hazard task is best performed by an electronics technician as the cover of the dosimeter must be removed

The Driving Pressure is Adjusted as follows:

  1. Disconnect the dosimeter from mains electricity supply.
  2. Remove the outside cover. Access is gained by the removal of Allen bolts.
  3. Locate the pressure control (see figure 2 and 3).

    4. Figure 2: Inside the Dosimeter.
    Pressure Control being adjusted with a screwdriver.

  4. With a screwdriver turn the screw on the pressure control half a turn clockwise to increase pressure or half a turn anticlockwise to decrease pressure.
  5. Replace the outside cover.
  6. Re-check pressure.
  7. Repeat steps 1-6 until correct operating pressure (180 KPa) is obtained.
  8. Replace outside cover.
  9. Dosimeter is now ready for use.
Units

1 KPa =1 Kg cm2

References

  1. Ward RJ, Liakokos P, Reid DW, Leonard RF, Johns DP, Walters EH A Critical Evaluation of the Mefar Dosimeter . Accepted by Eur Respir J 1998

  2. Ward RJ, Reid DW, Leonard RF, Johns DP, Walters EH. Nebulizer calibration using lithium chloride: an accurate, reproducible and user-friendly method. Eur Respir J 1998 Apr;11(4):937-41
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