How to Calibrate an ORP Meter


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Water is often overlooked when poured from the sink or sprayed from the hose. Visibly, water can look extremely clear and still have many unknown chemicals floating around. To measure water quality, many are quick to jump to pH levels, dissolved oxygen, so on and so forth. 

All measurements are great when identifying the overall quality of the water, but one measurement often overlooked is the oxidation-reduction potential (ORP).

What is oxidation-reduction potential (ORP)?

ORP is a measurement of electron activity in a substance (in this case water). Oxidation is the process of losing electrons and reduction is the process of gaining electrons. Therefore, the overall strength of this electron exchange amongst the substances in water will contribute to the ORP level.

Similar to pH, which measures the hydrogen ion concentration, ORP measures the overall strength of electron activity. In the case of water, the higher dissolved oxygen concentration will correlate to a higher ORP level since oxygen inherently contributes to the loss and gain of electrons among other substances.

This general electron exchange is commonly known as a redox reaction and can become confusing when identifying oxidizing and reducing agents since it is backwards of what actually happens (i.e. oxygen itself gets reduced and gains electrons, but it causes oxidation in another substance so it is an oxidizing agent). 

Either way, an ORP meter will do all this for you and produce an insightful value in millivolts (mV), so no need to worry about the flip-flop naming convention. Just think, the higher the dissolved oxygen the higher the ORP reading in normal water.

Why does oxidation-reduction potential (ORP) matter in water?

ORP is affected by any substance that exchanges electrons. In water, this action is mostly performed by dissolved oxygen, as dissolved oxygen is a very important characteristic of healthy water sources be it natural, hydroponic tanks, wastewater effluents, etc.

Organisms that break down contaminants and other unwanted substances floating in water rely on dissolved oxygen to do so. Therefore, the lower the ORP levels the lower the oxygen, or similar oxidizing substances, and this equals a more contaminated water source. 

Conversely, a higher ORP reading will equate to a more “sanitized” water. Drinking water will be adequately sanitized at around 650 mV on the ORP scale, whereas swimming pools will have a higher ORP level (due to the extra chlorine acting as an oxidizing agent) around 700 mV to 750 mV.

In this way, it is extremely important to have a basic understanding of ORP measurements in water whether it be tap water, swimming pool water, or a hydroponic setup. Make it easy on yourself and use an ORP probe.

How does an ORP meter work?

As discussed above, an ORP meter works similarly to a pH probe but instead of measuring hydrogen ions, it measures the relative amount of electron activity. The graphic below provides an example of the industrial ORP sensor that can attach directly into piping for continuous monitoring. 

An ORP meter operates using a reference solution inside the probe (usually potassium chloride, KCl) and compares that to the test sample to determine an ORP level in mV. These probes measure how strongly electrons are transferring in the solution and equate that to an overall voltage—positive voltage being oxidizing, and negative voltage being reducing. 

The reference solution and test solution are separated by a platinum probe tip and silver wire. Since the platinum is unreactive it allows the silver wire to read the electron activity in the test solution, without reacting itself, and compare it to the reference solution. 

Now that you understand the basic principle of an ORP meter, let’s discuss how to calibrate and use the probe for whatever application you need.

How to calibrate an ORP meter for accurate measurements

To begin, without calibration your ORP meter will not provide reliable readings of ORP in the solution. Thankfully, calibration is fairly straightforward with the use of ORP calibration solution. Atlas Scientific provides 225 mV calibration solution, as it provides a common oxidative calibration since most water samples will be in the positive voltage range. 

Other calibration solutions and powders can be created and/or purchased, just be sure to note the exact mV and accuracy of the calibration solution so that it aligns with the measurement accuracy you require.

Once the calibration solution is obtained, gently rinse the ORP sensor with distilled water and then submerge it in the calibration solution. Lightly stir the probe for about 30 seconds as readings stabilize. Once the readings have stabilized, run the calibration command and make sure the probe reads 225 mV or the correct mV reading for your calibration solution.

Now you are ready to measure ORP! 

When do you re-calibrate an ORP meter?

A common question revolves around recalibration of ORP probes, ranging from the industrial to the consumer-grade ORP meter. Since most samples will be widely different it is recommended to recalibrate and clean the probe between different test samples. Also, since the probe measures electrons, it is more susceptible to contaminant build-up on the probe tip. To clean, gently scrub the probe tip—with materials no more abrasive than a toothbrush—alongside some diluted bleach and your probe will be in top shape. 

However, if continuous monitoring and submersion are occurring, like in a hydroponic tank, recalibration only needs to occur once a year for the first two years, then every six months after year two.

Additionally, ORP probes cannot dry out due to their fluid solution operating principle. This is not a worry for indefinite submersion and measurement setups. In the case of intermittent testing, be sure to keep the probe tip wet with ORP storage solution. The industrial probe from Atlas Scientific comes equipped with a probe-tip cap to easily store this solution and screw onto the ORP probe.


Oxidation-reduction potential (ORP) is a great measurement to determine the overall water quality from a natural source, sink tap, hydroponic tank, or whatever the water application. It provides general insight into the overall dissolved oxygen level and level of sanitation (positive ORP readings in the 600-800 mV range are ideal for sanitation). ORP meters come in different grades from consumer to lab to industrial depending on the level of accuracy and intensity of the measurement environment. Additionally, if you’d like to understand more about your water quality check out Atlas Scientific’s industrial pH/ORP/temperature probe to conduct three measurements with one probe. 

If you are unsure exactly which ORP device will best suit your needs, or you would like to learn more about other water measurements like pH levels and electrical conductivity, do not hesitate to reach out to the world-class team at Atlas Scientific.

ORP Probes & Calibration Solutions

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