How often should I clean my filter sand?

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I have a deviation between the manual pH measurement (measuring tape, colorimetric measurement or Photolyser) and the display in the measuring and control device.

• Error in colorimetric measurement with phenol red indicator (deviation up to 0.4 pH) due to salt
  

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The pH value of the device differs from the manual measurement

• Incorrectly performed manual measurement
• Probe dirty or aged
  

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My AquaCat does not climb walls

• Filter bag full
  
• pH value is too high
  
• Biofilm on walls
  
• Device too heavy (AquaCat 3000 and Viking TURBO)
  
• Drive belts loose
  

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An "Alarm" LED is flashing on my measuring and control device, and/or the display "Sample water alarm" or "Sample water error", the dosing is interrupted.

• The sample water monitoring in the flow valve has responded.
  

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