Dissolved Oxygen

In Aquaponics, Dissolved Oxygen (DO) is extremely important. We all know that fish require dissolved oxygen in order to breathe. But plant roots require adequate dissolved oxygen to survive as well. Even the beneficial bacteria in our filters need oxygen. Having a way of testing for dissolved oxygen lets you make sure you have enough aeration for your fish and plants to thrive. It will also let you answer questions like “How much oxygen do airstones put in my system?” or “Does my system need more aeration?”

Overview of Dissolved Oxygen Testing Options

Dissolved Oxygen testing can be done with a chemical test, electrochemical sensor, or through an optical based sensor.

Chemical testing methods are accurate, and don’t need calibration. The up-front cost can be low, but at $0.51 – $1.20 per test the cost will  add up. The main disadvantage of the chemical methods is you only learn the DO for where you sampled. Comparing DO levels in a tank, sump, and raft bed would need at least 3 separate tests.

With a sensor based meter, you can simple move the DO probe from area to area and wait for the readings to stabilize.

Electrochemical sensors come in two varieties: Galvanic and Polarographic. Polarographic sensors last longer, but require a 5-15 minute warmup. Galvanic sensors work immediately, but won’t last as long as a Polarographic model, as they age even when the meter is  turned off. Luckily both sensor types can be maintained. In continuous use, probe membranes last 2-8 weeks. Plan on spending up to $100 a year in maintenance costs.

Optical Dissolved Oxygen, or Luminescent Dissolved Oxygen probes have the advantage of requiring very little maintenance. The sensor caps should be replaced every year (approx $100), but there are no membranes, special fluids, calibrations, or storage constraints to worry about.

My Opinion

I use a Hach HQ30d meter with Rugged LDO probe. If you are careful, you can buy a similar setup on eBay for around $200. Assume that the sensor cap will be past the calibration expiration date. A new sensor cap is $108, but the sensor will continue to work with an expired cap and you can check the accuracy by performing a calibration. Make sure the eBay vendor states that the probe connects to the meter, and displays a reading.  (PS: Need help figuring out if an eBay auction for a Dissolved Oxygen meter is any good? Ask in the comments. I’ll try to reply individually by email, and won’t post the comment until the auction ends, so nobody snipes it from you.)

If you’re not comfortable with used equipment and are on a budget, the Milwaukee MW600 is a great deal (149@Amazon).

The only real duds here are the chemical tests.

Chemical test / Winkler Titration

  1. Hanna HI3810 (0.0-10.0; Resolution 0.1)
    $56 / 110 = $0.51ea
    HI3810_400
  2. LaMotte 5860-01 (0.0 – 10.0; Resolution 0.2)
    $52 / 50 = $1.04 ea
    Lamotte_5860-01
  3. Hach Digital Titrator +8332 (1 – 10 mg/l)
    $159@Hach + $84@Hach
    Hach Digital TItrator + Hach8215

Chemical Test / Photometric

  1. LaMotte Smart3 + 3688-SC (0.0-10.0; MDL 0.6)
    $40 / 100 = $0.40 ea
  2. Hach DR900 + 8166 (0.3 – 15.0)
    $30 / 25 = $1.20 ea
    This is the easiest chemical dissolved oxygen testing method. Individual vacuum filled ampules have the reagent in them – just snap the neck under-water to fill the ampule, and put it into the meter to read.
    HachDR900 + Hach8166

Electrochemical

  1. OxyGuard Handy Polaris Galvanic $934
    The original handheld, waterproof Dissolved Oxygen Meter. The OxyGuard meters are reported to need less maintenance than other electrochemical probes.
    handypolaris
  2. YSI Pro20 Polarographic or Galvanic $947@Amazon
    MO-YSI-Pro20-DO
  3. YSI 550A Polarographic $759@Amazon
    YSI_550a
  4. YSI EcoSense 200A Polarographic $495@Amazon
    ysi_do200a_cable
  5. American Marine Pinpoint II Polarographic $229@DrFostersSmith
    The Pinpoint II and Milwaukee MW600 are roughly equivalent devices. They both require a two-point calibration (more expensive units don’t need zero calibration) and while the probes are waterproof, the meters are not. If you’re the type to knock something into the tank… neither of these units will survive that.
    Don’t forget to purchase zero-calibration solution as neither meter comes with it.
    pinpointii_Oxygen-Monitor
  6. Milwaukee MW600 Polarographic $149@Amazon
    Update: Milwaukee now ships these units with calibration instructions that don’t require zero-calibration solution! I’d recommend this unit over the Pinpoint II because of that. There is a great video showing how to calibrate and install membranes on this unit here.
    Milwaukee_Dissolved_Oxygen_Meter_MW-600-500x500

Optical

  1. Hach HQ30d + Rugged LDO Sensor
    Meter: $750@Hach +
    Probe: $750@Hach +
    Sensor Cap: $108/yr@Hach
    Hach_HQ30d + Hach_LDO + Hach_LDO_Cap
  2. YSI ProODO
    Meter: $604@Fondriest +
    Probe: $556@Fondriest +
    Sensor Cap: $87/yr@Fondriest
    ysi_proodo + YSI-ProODO-Cable + YSI-ODO-Membrane-Kit

Know of, or use another method of testing for dissolved oxygen in Aquaponics? Let me know in the comments and I’ll update this post.

 

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