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....ισως επηρεαζουνε και αλλα πραγματα τη μετρηση....

σπυρο,το σκεπτικο ητανε αυτο:

http://www.tdsmeter.com/faqs/#different-readings

Why do I experience different readings in the same water with the same meter?

Reasons for varied readings include:
Ions: The nature of charged positive ions (which is what the TDS meters are measuring) is that they are always moving. Therefore, there may always be variances in the conductivity, and thus a different reading.
Temperature: Even with ATC, temperature changes by a tenth of a degree may increase or decrease the conductivity. Additionally, the temperature coefficient (what the reading is multiplied by to adjust for temperature differences) changes slightly depending upon the range of ppm. Our meters and virtually every meter under $500 has a single temperature coefficient, regardless of the range. (The new COM-100 offers three temperature coefficient options, but each is linear once selected.)
Air bubbles: Even a tiny air bubble that has adhered to one of the probes could potentially affect the conductivity, and thus the reading.
Lingering electrical charges: Electrical charges off fingers, static eletricity off clothes, etc. on the meter and lingering electrical charges in the water will affect the conductivity of the water.
Beaker/cup material: Plastic cups retain lingering electrical charges more than glass. If the meter touches the side of the glass or plastic, it could pick up a slight charge. If the plastic is retaining a charge, it could also affect the water.
Volume changes: The amount of water in the sample may affect the conductivity. Different volumes of the same water may have different levels of conductivity. Displacement may affect the conductivity as well.
Probe positioning: The depth and position of the probe in the water sample may also affect the conductivity. For example, if a meter is dipped into the water, removed and then dipped into the water again, but in a different spot, the reading may change.