Real World Hydroponics ORP Testing Applied

For our ORP meter review, we put Hanna Instruments HI98120 handheld ORP & temperature tester through some real world hydroponics applications for over 6 months. The benefits of managing ORP in hydroponics and soilless applications are becoming increasingly better understood. ORP (oxidation reduction potential) in these terms is a good measure of your water or nutrient solutions ability to oxidize unwanted spores or microbes.  

Above: Healthy roots resulting from ORP management in hydroponics reservoirs and source water supply

ORP is essentially a measure of your hydroponic nutrient solution’s “health”. Water that can lead to root infections will typically provide a low ORP reading, while water that is less likely to contribute to root issues will have a higher reading. In practical terms, maintaining an ORP range from 300-400mV (milliVolts) is typically the target.

ORP values are typically increased in hydroponic reservoirs or fertigation systems with hypochlorous acid (HOCl-, hydrogen peroxide (H2O2), peroxyacetic acid  or ozone (O3). Hypochlorous acid is considered more favorable to work with and poses less risk of over application or harm to people, animals or the environment.

Not All ORP Meters are Created Equal

ORP is kind of an odd duck of a measurement to work with. Most growers with experience in managing ORP use it as a guideline, rather than an absolute value to be achieved. Chasing a predetermined ORP value when stronger forces like microbial infections are present can create too great a load of chloride or ozone for the roots, causing damage, as an example.

TIP: If you see sudden changes or drops in ORP values it is likely time to look more closely at what’s causing it.

There are a very wide range of ORP Meters and testers available. As far as reliability goes, we often get what we pay for.  However, very accurate ORP meters can get very expensive and require frequent calibration to ensure reliability.

Hanna’s handheld ORP meter and temperature tester doesn’t break the banks at around $200–but the price tag is high enough to want to know if the ORP tester performs as claimed before buying; hence our real world ORP meter review.

Hanna Instrument HI98120 is a hand held tester, not a continuous monitor.  Additionally, it is not user calibratable. However, it comes dialed in from the factory while being supplied with a reference solution so you can verify the level of accuracy with readings. The ORP meter probe is replaceable, so there is no need to replace the entire tester eventually (accuracy may drift with time).

ORP Meter Tested & Applied

The meter was used to check source water and nutrient solutions used in a variety of hydroponics applications including Aeroponics, DWC (deep water culture, NFT (nutrient film technique) & RDWC (recirculating deep water culture).

Hypochlorous acid (Clear Line) was applied with each reservoir change-out or occasionally in between reservoir changes (to raise ORP levels). It was also applied to filtered source water storage.

With the ORP meter we were able to note changes in the source water supply as well as in our water cisterns (holding tanks). Sometimes this can be a good indicator of when to consider changing water filter cartridges beyond as regularly scheduled.

We were also able to note the presence of unwanted microbial loads, after learning the natural rhythm of the ORP meter values from the working life in a hydroponic reservoir following a change out or partial change out.  The same can hold true for source water storage. If the ORP value begins to decline suddenly, when it typically would not be, it’s a good indicator that there is an unwanted microbial load.

The ORP meter remained accurate enough for our purposes, as we occasionally gauged the accuracy with the ORP reference solution supplied with the tester. Note that we were vigilant about ensuring the probe tip remained in the electrode storage solution, i.e. not letting it dry out in storage which could decrease accuracy and working life.

Using the ORP meter can also tell you if microbial applications intended for reservoirs (i.e. beneficial microbes) were viable; we could see a slight drop in ORP values during such applications.

We find the Hanna Instruments HI98120 ORP and Temperature tester provides confident readings for budget minded hydroponic ORP management purposes and recommend it for such applications. For the money, it provides a competent measure of crop protection in nutrient solution applications and source water management. ORP testing also helps make better use of hydroponic additives like hypochlorous acid and beneficial microbes

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