74
Advanced
™
Osmometer Model 3250 User’s Guide
The following procedure is recommended for testing and/or replacing
the sample probe:
1. Place an empty sample tube in the freezing chamber
(Figure 3, Item 8) to catch any extraneous material that might fall
in.
2. Unplug the instrument; it is not sufficient to just turn it off.
3. Remove the head cover by removing the two top screws and pulling
the cover straight up.
4. Unplug the sample probe connector (Figure 7, Item 10). The probe
leads are electrically accessible through the side slots in the
connector. Use care not to touch or bend the connector pins. Soil
or salt from your fingers can increase the electrical resistance of the
connection.
5. With the probe at room temperature (20°C - 25°C), the resistance
between the probe leads should be in a range of 2000 - 2800
ohms. Immerse the tip of the probe into a container of ice and
water (~0°C). The resistance should rapidly increase to a range of
5500 - 6100 ohms.
With the tip of the probe immersed in a concentrated NaCl solution
(do not wet the probe leads or connector with the salt water), the
resistance between either probe lead and the solution should be
greater than 20 megohms. Carefully rinse the probe clean following
this test.
Probe damage is indicated by any deviation from the resistance
parameters given above.
If sample probe replacement is indicated:
6. Loosen the two setscrews (Figure 7, Item 9) holding the mandrel in
the head chassis. Lower the probe and mandrel down over the stir
wire.
7. Fit a new sample probe by pushing the probe connector and wires
up through the hole in the head chassis before seating the mandrel.
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