# Dissolved Oxygen Meters: How to Choose One for Field Water Monitoring in Canada

> How to choose a dissolved oxygen meter for field water monitoring: optical, galvanic and polarographic probes, range, compensation and calibration, with the instruments ERE carries compared.

- **URL:** https://www.ereinc.com/blogs/field-notes/dissolved-oxygen-meter-buyers-guide-for-canada

A dissolved oxygen (DO) meter measures how much oxygen is dissolved in water, reported as a concentration in mg/L or as percent saturation. For field work the buying decision comes down to four things: probe technology (optical, galvanic or polarographic), measuring range, compensation for temperature, salinity and pressure, and how much calibration and maintenance you are willing to carry. This guide compares the DO instruments ERE carries from Hanna Instruments, Ohaus and LaMotte.

**Key takeaways**
- Probe technology sets your maintenance. Galvanic and polarographic probes read through a membrane and electrolyte, consume a little oxygen and need water moving past the membrane. The optical probe on the [Ohaus Starter 400D](/products/starter%E2%84%A2-400d-dissolved-oxygen-portable) measures by luminescence, and Ohaus states it consumes no oxygen, so the sample does not need stirring.
- Match the range to the water. Freshwater at 5 °C holds about 12.77 mg/L at saturation (CCME), so a 0 to 20 mg/L instrument covers natural waters with headroom. Wider ranges suit water oxygenated beyond air saturation.
- Know your limit. CCME's lowest acceptable freshwater DO runs from 5.5 to 9.5 mg/L depending on water type and life stage, and EPA treats levels below 5 mg/L as stressful for fish.
- Set pressure or altitude and salinity correctly before you calibrate. The reading depends on both.

**In this guide**
- [What a dissolved oxygen meter measures](#what-it-measures)
- [Canadian guideline levels](#guideline-levels)
- [Optical, galvanic or polarographic probes](#probe-technology)
- [The DO meters ERE carries, compared](#compare)
- [Choosing the measuring range](#range)
- [Temperature, salinity and pressure compensation](#compensation)
- [Calibrating a DO meter](#calibration)
- [Taking a reliable field reading](#field-reading)
- [Probe consumables](#consumables)
- [Where DO fits in site and wastewater work](#site-work)

## What Does a Dissolved Oxygen Meter Measure?

A DO meter measures the oxygen dissolved in water, in milligrams per litre (mg/L, the same as ppm) or as a percentage of the amount the water could hold at that temperature and pressure (percent saturation). The U.S. EPA's [dissolved oxygen indicator page](https://www.epa.gov/national-aquatic-resource-surveys/indicators-dissolved-oxygen) describes DO as a direct indicator of an aquatic resource's ability to support aquatic life, and says its surveys measure it with a calibrated probe meter, usually together with temperature and pH.

The number is not a fixed property of the water. The Canadian Council of Ministers of the Environment (CCME) explains in its [freshwater dissolved oxygen guideline](https://ccme.ca/en/res/dissolved-oxygen-freshwater-en-canadian-water-quality-guidelines-for-the-protection-of-aquatic-life.pdf) that oxygen solubility is governed by pressure, temperature and salinity, among other factors, and gives 12.77 mg/L as the solubility in freshwater at 5 °C and standard atmospheric pressure (101.3 kPa). That is why every meter below has a temperature sensor and a way to correct for salinity and for pressure or altitude.

## What Dissolved Oxygen Level Do Canadian Guidelines Call For?

The CCME freshwater guideline for the protection of aquatic life sets the lowest acceptable DO concentration at 5.5 to 9.5 mg/L, depending on whether the water is warm or cold and on the life stage of the organisms. Its Table 1 gives:

 | Water type | Early life stages | Other life stages

 | Warm water | 6 mg/L | 5.5 mg/L

 | Cold water | 9.5 mg/L | 6.5 mg/L

As a rough field benchmark, EPA notes that DO below 5 mg/L is generally stressful for fish, below 3 mg/L is too low to support fish, and below 1 mg/L is hypoxic and usually devoid of life. Permit conditions or provincial criteria for your site may differ, so confirm the limit that applies before you settle on a range or a data-quality target.

## Optical, Galvanic or Polarographic: Which DO Probe Technology Should You Choose?

Choose an optical probe when you want the least routine maintenance and no flow requirement. Choose a membrane probe, galvanic or polarographic, when your workflow already supports membrane and electrolyte care and you can keep the sample moving past the sensor. ERE carries all three.

## Polarographic probes: Hanna HI9146, Hanna edge and LaMotte DO Tracer

A polarographic probe applies a small fixed voltage across the sensor, and oxygen that diffuses through a membrane reacts and produces a current proportional to the reading. The manual for the [Hanna HI9146](/products/hanna-ha-9146-04-ha-9146-10) specifies a polarization voltage of about 800 mV and warns that a non-polarized probe reads incorrectly, because the electrolyte still holds oxygen. The [Hanna edge](/products/edge%C2%AE) DO probe is described in ERE's product record as a Clark-type digital polarographic probe with an easy-to-replace membrane cap. The [LaMotte DO Tracer](/products/dissolved-oxygen-tracer) also uses a polarographic sensor with a bonded membrane cap: it needs about three minutes of polarization on first power-up, keeps the electrode biased for seven days after the meter has been on, and LaMotte's short-form instructions read fill before first use, polarize weekly, calibrate daily.

## Galvanic probes: Ohaus Starter 300D

A galvanic probe generates its own signal: oxygen diffuses through the membrane and is consumed at the cathode, producing a current. The manual for the [Ohaus Starter 300D](/products/starter%E2%84%A2-300d-dissolved-oxygen-portable) describes this principle and notes that, although little oxygen is consumed, the sample must still flow past the membrane at a minimum of 5 cm/s to prevent false readings.

## Optical probes: Ohaus Starter 400D

The Starter 400D's STDO21 probe emits blue light that makes a sensing element luminesce, and oxygen in the sample changes the luminescence lifetime, which the probe measures. According to the Starter 400D manual, the probe does not consume oxygen, so the sample does not need stirring, and there is no membrane, anode or cathode to maintain. Only the sensor cap is replaced, every 12 to 18 months, after which you enter the new cap's parameter and recalibrate. Ohaus states that optical probes drift little and can hold a calibration for many months.

The trade-off is range: the 400D reads 0.00 to 20.00 mg/L, while the HI9146, the edge and the 300D extend to 45 mg/L.

## How Do the Dissolved Oxygen Meters ERE Carries Compare?

The table sets the five field instruments side by side. Every value comes from the manufacturer's manual or ERE's product record.

 | Instrument | Probe technology | DO range | Compensation | Also worth knowing

 | Hanna HI9146 | Polarographic, PTFE membrane | 0.00 to 45.00 mg/L; 0.0 to 300.0 % | Automatic temperature; salinity 0 to 80 g/L; altitude 0 to 4,000 m | 4 m or 10 m cable kits include 2 spare membranes, 30 mL electrolyte and a hard case; 300 g

 | Hanna edge (DO) | Clark-type polarographic, digital probe | 0.00 to 45.00 mg/L; 0.0 to 300 % | Automatic temperature 0 to 50 °C; salinity 0 to 40 g/L; altitude −500 to 4,000 m | 12 mm slim meter with data logging; also reads pH and conductivity with their own electrodes

 | Ohaus Starter 300D | Galvanic (STDO11) | 0.00 to 45.0 mg/L; 0 to 400 % | Automatic or manual temperature; salinity 0 to 50 ppt; barometric pressure entered by hand | 30-result memory; IP54; ST300D-B is the meter only

 | Ohaus Starter 400D | Optical (STDO21) | 0.00 to 20.00 mg/L; 0.0 to 200.0 % | Temperature sensor in the probe; salinity set in ppt; barometric pressure measured automatically | 99-result memory; IP54; no stirring needed; ST400D-B is the meter only

 | LaMotte DO Tracer | Polarographic, bonded membrane cap | 0 to 20.00 mg/L; 0 to 200.0 % | Automatic temperature; salinity 0 to 50 ppt; altitude 0 to 6,000 m | 110 g pocket meter, IP67; 25-result memory; optional 1 m and 5 m extension cables

If you want DO alongside other parameters in one instrument, the [Hanna HI9829 multiparameter meter](/products/9829-multiparameter-water-quality-meter) lists DO (to 500 % saturation) with pH, EC and turbidity. Our [water quality meter guide](/blogs/field-notes/water-quality-meters-for-environmental-work-a-buyers-guide-for-canada) covers that class.

## What Measuring Range Do You Need?

A 0 to 20 mg/L range covers most natural waters. Cold freshwater at saturation holds about 12.77 mg/L (CCME), so a 20 mg/L instrument leaves headroom. The wider ranges of the HI9146, the edge and the Starter 300D, up to 45 mg/L and 300 to 400 % saturation, suit water oxygenated beyond air saturation, for example by pure-oxygen injection.

Compare accuracy on the same basis. The HI9146 lists ±1.5 % of full scale, the DO Tracer ±2 % of full scale, and the Starter 400D ±0.2 mg/L below 8 mg/L and ±0.3 mg/L from 8 to 20 mg/L. A percentage of a 45 mg/L full scale is a larger absolute error than the same percentage of 20 mg/L: ±1.5 % of 45 mg/L is about ±0.7 mg/L, while ±2 % of 20 mg/L is ±0.4 mg/L.

## Why Do Temperature, Salinity and Pressure or Altitude Matter?

The amount of oxygen water can hold changes with temperature, salinity and pressure, so a DO meter needs all three to turn the sensor signal into a correct reading, especially in percent saturation. The instruments differ in how they handle the pressure term. The Starter 400D measures barometric pressure automatically (50 to 115 kPa). The Starter 300D relies on you to enter the correct barometric pressure before calibration and measurement, and its manual warns that a wrong value affects the calibration and the accuracy of the reading. The Hanna and LaMotte instruments use an altitude setting instead: 0 to 4,000 m on the HI9146 and 0 to 6,000 m on the DO Tracer.

For brackish or saline water, set the salinity value. The Starter manuals note that seawater is about 35 ppt, and every instrument in the table accepts a salinity setting.

## How Do You Calibrate a Dissolved Oxygen Meter?

Calibrate at 100 % saturation, in air or air-saturated water, on the schedule the manufacturer specifies, and add a zero-oxygen point only when you will measure very low DO. Intervals differ by instrument: LaMotte calls for calibration daily, and Hanna recommends a weekly slope calibration for the HI9146.
- **Hanna HI9146:** one or two points, 0 % in HI7040 zero-oxygen solution and 100 % in air. Hanna says the zero calibration is very stable and is needed only when the probe or membrane is replaced, while the slope calibration is more critical and is recommended weekly. Allow at least 15 minutes of probe conditioning first.
- **Ohaus Starter 300D:** a 1-point calibration is normally enough. The 2-point calibration is rarely performed and uses a zero-oxygen solution prepared from the DO Zero Oxygen Chemical.
- **Ohaus Starter 400D:** 1-point or 2-point. Ohaus suggests 1-point for samples at or above 2 mg/L and 2-point below 2 mg/L. Recalibrate after replacing the sensor cap.
- **LaMotte DO Tracer:** calibrate daily and polarize weekly, per the manufacturer's short-form instructions.

The USGS [National Field Manual chapter on dissolved oxygen](https://pubs.usgs.gov/publication/tm9A6.2) provides guidance on the scientific basis of the measurement, equipment selection and maintenance, calibration, troubleshooting and measurement and reporting procedures.

## How Do You Take a Reliable Field DO Reading?

Prepare and condition the probe, calibrate, set salinity and pressure or altitude, immerse it with water moving past a membrane probe, wait for a stable reading and record DO with temperature. A practical sequence:
- **Prepare the probe.** Fill the membrane cap with electrolyte on membrane probes, or inspect the sensor cap on an optical probe, following the manual.
- **Condition the probe.** Let a polarographic probe polarize, and let the probe reach thermal equilibrium with the sample, which can take several minutes.
- **Calibrate.** Calibrate at 100 percent saturation in air or air-saturated water, add a zero-oxygen point only for very low dissolved oxygen, and record the altitude or barometric pressure and salinity settings.
- **Immerse with flow.** Cover the sensor completely and keep the sample moving past the membrane on membrane probes. An optical probe does not need flow.
- **Wait and record.** Let the reading stabilize, then record dissolved oxygen in mg/L and percent saturation together with the sample temperature.
- **Rinse and store.** Rinse the probe with clean water and store it with its protective or wetting cap, as the manual directs.

Flow requirements are manufacturer figures and they differ: Hanna specifies 0.3 m/s for the HI9146 and Ohaus a minimum of 5 cm/s for the Starter 300D, so follow the manual for your probe.

## What Consumables Does a DO Probe Need?

Membrane probes need electrolyte and replacement membranes, and an optical probe needs a new sensor cap every 12 to 18 months, according to Ohaus. ERE carries [30 mL electrolyte solution for galvanic DO probes](/products/dissolved-oxygen-electrolyte-solution-galvanic-do-probes) and [replacement DO membranes for galvanic DO probes](/products/replacement-do-membranes-galvanic-do-probes) (5-pack); confirm compatibility with your probe model when you request a quote. The HI9146 kits include 2 spare membranes and 30 mL of electrolyte, and the LaMotte DO Tracer kit includes a spare membrane cap and electrolyte.

Ohaus probes are also sold separately. The [Ohaus Starter electrodes](/products/ohaus-starter-electrodes) include the STDO11 galvanic DO electrode (1 m cable) and the STDO21 optical DO probe (1 m or 5 m cable), so a probe can be replaced, or a longer cable chosen, without buying a new meter. The ST300D-B and ST400D-B are meters only, so ask which configuration a quote includes.

## Where Does Dissolved Oxygen Fit in Site and Wastewater Work?

DO is one of the routine field measurements. The USGS National Field Manual gives it its own chapter alongside temperature, specific conductance, pH, reduction-oxidation potential, alkalinity and turbidity, and notes that DO is routinely measured when water samples are collected. CCME adds that depletion of DO in receiving waters is often a ready indicator of wastewater treatment requirements.

In remediation, oxygen is something you add. Monitoring wells near an oxygen-releasing compound application or an air sparging system are typically checked for DO to see whether aerobic conditions are being established: see our [bioremediation products guide](/blogs/field-notes/bioremediation-products-for-contaminated-soil-groundwater-canada-buyers-guide) and the [air sparging vs bioventing comparison](/blogs/field-notes/air-sparging-vs-bioventing-systems-in-situ-remediation-equipment-compared). For the rest of a field parameter set, see our guides to [pH meters](/blogs/field-notes/ph-meter-buyers-guide-for-canada), [portable conductivity meters](/blogs/field-notes/portable-conductivity-meter-buyers-guide-for-canada) and [turbidimeters](/blogs/field-notes/turbidimeter-selection-guide-for-environmental-monitoring-in-canada), and to [low-flow groundwater sampling](/blogs/field-notes/low-flow-groundwater-sampling-peristaltic-pump-selection-guide-for-canada).

For our full range of field water-quality meters, probes and consumables, see the [Water Quality Instrumentation collection](/collections/water-quality-instrumentation). Multi-brand supply means one quote can cover the meter, the probe and the consumables.

**Need help choosing a dissolved oxygen meter?**

ERE Inc. has been Canada's environmental equipment specialist for 30+ years. Tell us your water type, range and workflow and we will match the meter, probe and consumables.

 [**→ Request a Quote**](/pages/request-a-quote) | [1-888-287-EREC](tel:+18882873732) | [Browse Water Quality Instrumentation](/collections/water-quality-instrumentation) | [sales@ereinc.com](mailto:sales@ereinc.com)

## Frequently Asked Questions

## What is the difference between a DO meter and a DO sensor?

The meter is the handheld unit with the display and electronics. The sensor, also called the probe or electrode, is the part that goes in the water. Some are sold as kits and some separately: on the Ohaus Starter line, the ST300D-B and ST400D-B are the meter only, while the ST300D and ST400D include a probe.

## Does an optical dissolved oxygen meter need calibration?

Yes. The Ohaus Starter 400D supports 1-point or 2-point calibration, and Ohaus advises a 1-point calibration for samples at or above 2 mg/L and a 2-point calibration below that. Ohaus says optical probes drift little and can hold a calibration for many months, and calibration is repeated after every sensor cap replacement.

## Why does a membrane DO probe need water flowing past it?

Membrane probes consume a little oxygen at the sensor, so the water at the membrane has to be replaced. The Hanna HI9146 manual specifies water movement of 0.3 m/s, from a stream or a magnetic stirrer, and the Ohaus Starter 300D manual a minimum of 5 cm/s across the membrane. An optical probe does not consume oxygen, so Ohaus states the sample does not need stirring.

## What is a low dissolved oxygen reading?

EPA describes dissolved oxygen below 5 mg/L as stressful for fish, below 3 mg/L as too low to support fish and below 1 mg/L as hypoxic. CCME's lowest acceptable freshwater values run from 5.5 to 9.5 mg/L depending on water type and life stage. Check the limit that applies in your permit or provincial criteria.

## Related articles
- [Water Quality Meters for Environmental Work: A Buyer's Guide for Canada](/blogs/field-notes/water-quality-meters-for-environmental-work-a-buyers-guide-for-canada)
- [Portable Conductivity Meters: How to Choose One for Field Water Monitoring in Canada](/blogs/field-notes/portable-conductivity-meter-buyers-guide-for-canada)
- [Turbidimeter Selection Guide for Environmental Monitoring in Canada](/blogs/field-notes/turbidimeter-selection-guide-for-environmental-monitoring-in-canada)
- [Low-Flow Groundwater Sampling: Peristaltic Pump Selection Guide for Canada](/blogs/field-notes/low-flow-groundwater-sampling-peristaltic-pump-selection-guide-for-canada)
- [Bioremediation Products for Contaminated Soil and Groundwater: A Buyer's Guide for Canada](/blogs/field-notes/bioremediation-products-for-contaminated-soil-groundwater-canada-buyers-guide)

*Lire en français : [Appareils de mesure de l'oxygène dissous : comment choisir pour la surveillance de l'eau sur le terrain au Canada](/blogs/perspectives-securite/appareil-de-mesure-de-loxygene-dissous-guide-de-choix-pour-le-canada)*