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When encountering DO measurement problems …

Check the meter

This can be checked with a pH simulator or using a pH electrode and buffers to see whether the input functions correctly. If required, check the temperature input with a Pt1000 sensor. If not available, use an electronic resistor with a value between 1000 Ohm (=0°C) and 1300 Ohm (almost 78°C).

Check the electrode

1. The condition of the electrode's parts:

  • The correct electrolyte solution should be used! Use only the solution that is supplied with the electrode or the membrane kit (article code SZ02K).
  • The membrane (Teflon) should be intact, use a magnifying glass to check for visual damage, replace when required. If not sure, try with a new membrane.
  • The plastic body and the screw threads should be clean and intact.
  • The O-ring should be present, intact and correctly placed.
  • When the electrode membrane cap is removed, check the presence and the condition of the 2 metals (see picture).

  • At the tip of the inner electrode stem, the Platinum metal plate (diameter around 5 mm) should be clean and properly attached to it. Be careful when touching it as the attachment is very fragile.
  • On the inner electrode stem, the Lead anode can be found. When completely covered with a white precipitate, it can be removed with a soft grinding paper. It also possible to clean it chemically by placing it in 0.1N HCl for 10 to 15 minutes (see Maintenance).

2. Some tests of the electrode with an Ohm meter:

  • Remove the membrane cap and rinse the electrode tip. Gently dry it by dipping a soft tissue.
  • When measuring the Ohmic resistance between the 2 contacts of the BNC plug, you should measure a NTC resistance value that varies with the temperature (decreasing when heating up). Normally is it a 10K resistance (10000 Ohm @25°C) but might be a 5K resistance (5000 Ohm @25°C) also for older models. Value? ……. Ohm
  • This same NTC value should be measured between the two electrode metals Lead and Platinum at the electrode type. Value? ……. Ohm
  • Another measurement is between the BNC plug contacts and the metals (Lead and Platinum) at the electrode tip. Be careful when touching the Platinum metal cap at the tip as it easily detaches from the electrode which would destroy it permanently. Each metal is connected to one of the BNC contacts and should measure around zero Ohm.
  • Value between the BNC outer conductor and Lead metal? ……. Ohm (should be zero)
  • Value between the BNC central pin and Platinum tip? ……. Ohm (should be zero)
  • Value between the BNC outer conductor and Platinum tip? ……. Ohm (should be NTC value)
  • Value between the BNC central pin and Lead metal? ……. Ohm (should be NTC value)
  • A resistance measurement between the temperature banana plugs should give a resistance value around 1100 Ohm @ 25°C. This resistance should increase when becoming warmer. Value? ……. Ohm

3. When 1. and 2. are found to be OK. The electrode should be reassembled. Watch very carefully for the correct placement of membrane, O-ring and membrane cap. After reassembling, filled with the electrolyte, a Voltage should measured at the BNC contacts. While polarising, this Voltage will be decreasing and will end with a value between 30 and 90 milliVolt at room temperature. Value? ……. mV

do_troubleshooting.txt · Last modified: 13/06/2018 08:19 by pieter