HM Digital EC/TDS HydroTester Model COM-80

The HM Digital COM-80 is a water-resistant EC/TDS HydroTester that measures electrical conductivity and total dissolved solids across four scales. It ships factory-calibrated to 1413 µS, includes automatic temperature compensation, and is suited for hydroponic reservoirs, pools, aquariums, and drinking-water testing. A large LCD displays readings and temperature simultaneously, and push-button digital calibration lets you recalibrate to any point within the meter’s range without tools.

HM Digital COM-80 Specifications

All specifications are sourced from the manufacturer’s product description and labeled attributes for the COM-80 HydroTester (SKU HMDCOM80).

Specification	Value
Brand	HM Digital
Model	COM-80
Measurement Parameters	EC (electrical conductivity), TDS (total dissolved solids), Temperature
Measurement Scales	Four scales: EC µS, EC mS, TDS ppm, TDS ppt
EC Range (µS)	0–9990 µS
EC Range (mS)	0–9.9 mS
TDS Range (ppm)	0–5000 ppm
TDS Range (ppt)	0–5.0 ppt
Temperature Compensation	Automatic (ATC)
Calibration	Factory calibrated to 1413 µS; field-recalibration via push-button digital calibration
Water Resistance	Water-resistant
Display	Large LCD; simultaneous EC/TDS and temperature readout
Features	Auto-off, data-hold, low battery indicator
In the Box	COM-80 meter, protective cap, batteries
Weight	0.15 lb (0.07 kg)
SKU	HMDCOM80
GTIN-12 (UPC)	891144000229

Why Choose the HM Digital COM-80

The COM-80 is designed for growers and water-quality testers who want accurate EC and TDS readings from a compact, straightforward instrument. Here is what sets it apart from basic single-scale meters:

• Four measurement scales in one meter. The COM-80 reads EC in both µS and mS, and TDS in both ppm and ppt. Switching between scales means you can match the units your nutrient charts or water-quality standards call for without needing a separate conversion.
• Automatic temperature compensation (ATC). EC and TDS readings shift with temperature. The COM-80 compensates automatically so you get a corrected value rather than a raw reading that drifts as reservoir or tap temperature changes.
• Factory-calibrated and field-recalibratable. The meter leaves the factory calibrated to a 1413 µS standard solution. If drift occurs over time, push-button digital calibration lets you recalibrate to any point within the measurement range — no screwdriver or trim-pot required.
• Simultaneous temperature display. The large LCD shows EC or TDS and the current temperature at the same time, giving you both values in a single glance instead of toggling between modes.
• Practical daily-use features. Auto-off conserves battery life between measurements, data-hold freezes the reading on the screen so you can record it after withdrawing the probe, and a low-battery indicator tells you when power is running low before accuracy is affected.
• Water-resistant construction. The housing resists splashing and brief contact with water, which matters when working around open reservoirs, sump tanks, or irrigation lines.

Best Uses for the HM Digital COM-80

The COM-80 covers a wide range of dissolved-solids and conductivity testing needs. The following are the most common applications:

• Hydroponic nutrient monitoring. Check reservoir EC or TDS before and after adding nutrients to confirm concentration is within the target range for your growth stage. The ppm scale is the format most nutrient feeding charts use, while the mS scale aligns with EC-based schedules.
• Reverse osmosis and source water verification. Measure the TDS of your source water and your RO-filtered output to confirm your filtration system is working as expected. If you use a compact reverse osmosis system to prepare your feed water, a baseline TDS reading from the COM-80 tells you how much buffering capacity you have before adding nutrients.
• Pool, spa, and aquarium maintenance. The COM-80’s measurement range covers typical salinity and dissolved-solids levels found in pools, spas, and reef tanks, making it a single tool that works across both garden and home water applications.
• Drinking water quality checks. Verify that tap or filtered drinking water falls within acceptable TDS limits. The ppm scale is the standard unit used in most drinking-water guidelines.
• Water ionizer output testing. Confirm the output of water ionizers and remineralization filters is delivering the expected mineral load before use in plant feeds or consumption.

HM Digital COM-80 vs Similar Products

The COM-80 occupies the entry-level segment of HM Digital’s meter lineup. Competing pen-style EC/TDS meters in the same price range typically offer one or two measurement scales and manual screwdriver calibration. The COM-80 differentiates by covering all four scales (µS, mS, ppm, ppt) and using push-button digital calibration rather than a hidden trim-pot, which reduces calibration error caused by hand-tremor adjustments.

Within the HM Digital catalog, the COM-80 sits below the COM-100, which adds a wider measurement range, and above single-parameter TDS pens. If your work is primarily nutrient solution testing and you do not need pH measurement, the COM-80 covers the EC/TDS side without paying for pH circuitry you will not use. Growers who also need pH measurement should consider pairing the COM-80 with a dedicated pH pen or a combo unit.

For growers who want a calibration reference for their EC instruments, the Apera GroStar EC calibration kit provides a 2.77 mS/cm standard solution that is compatible with the COM-80’s digital calibration function.

Compatibility & Operating Range

The COM-80 is a general-purpose water-quality pen and does not require a specific power source or controller integration. It operates on the batteries included in the box. The water-resistant housing is suited to working environments where the meter is exposed to splashing but is not designed for full submersion.

The measurement range of 0–9990 µS / 0–9.9 mS covers typical hydroponic nutrient solution concentrations from seedling through late flower, as well as drinking water, pool, and aquarium levels. Readings above 9990 µS (9.9 mS) — such as undiluted concentrate or very high-salinity water — fall outside the instrument’s rated range.

The automatic temperature compensation adjusts readings to a standard reference temperature, which means the meter functions accurately across the temperature variations common in indoor grow rooms, greenhouses, and outdoor use without requiring manual temperature correction from the user.

For growers running recirculating deep water culture or NFT systems, the COM-80 can be used alongside aeration equipment. If you are using an Active Aqua air pump to oxygenate your reservoir, note that EC readings are best taken when the pump is briefly paused, as active aeration can introduce micro-bubbles that affect the conductivity reading at the probe tip.

Usage Guidelines

The following steps describe standard operating procedure for the COM-80 in a hydroponic or water-testing context. All steps are derived from the features described in the manufacturer’s product documentation.

1. Prepare the sample. For hydroponic use, take the sample from a well-mixed reservoir at a stable temperature. Stir or aerate briefly, then let the water settle for 30–60 seconds before dipping the probe. For tap or filtered water, let the water run for a few seconds to clear standing water from the line before sampling.
2. Power on and select scale. Press the power button to turn on the COM-80. The display will show the last active scale. Cycle through the four available scales (EC µS, EC mS, TDS ppm, TDS ppt) to select the unit that matches your reference charts.
3. Insert the probe. Submerge the probe tip into the water sample to at least the minimum immersion depth marked on the housing. The probe tip should be fully wetted. Avoid touching the probe to the container walls or floor.
4. Read the stabilized value. Wait for the reading on the LCD to stabilize — usually within a few seconds. The display simultaneously shows the EC or TDS value and the current water temperature. Use the data-hold button to freeze the reading if you need to withdraw the meter before recording.
5. Record and compare. Compare the reading to your target range for the application. For hydroponic nutrient solutions, cross-reference against the feeding chart for your specific nutrient line and growth stage. Adjust nutrient concentration or dilute with RO or low-TDS source water as needed.
6. Rinse after use. Rinse the probe with clean water after each measurement to prevent carryover contamination between samples. Pat dry or shake off excess water. Fit the protective cap before storing.
7. Auto-off. The COM-80 will shut off automatically after a period of inactivity to conserve battery life. Replace batteries when the low-battery indicator appears on the display.

Calibrating the HM Digital COM-80

The COM-80 ships factory-calibrated to a 1413 µS standard calibration solution. Under normal use the factory calibration remains valid for extended periods, but periodic recalibration is recommended if readings appear to drift or after extended storage. The following procedure applies the push-button digital calibration built into the meter.

1. Obtain a calibration standard. Use a known-concentration EC calibration solution. The factory standard is 1413 µS; you may calibrate to any other certified solution within the meter’s range. The Apera GroStar EC calibration kit (2.77 mS/cm) is one compatible option.
2. Rinse and dry the probe. Rinse the probe with deionized or distilled water and pat dry with a lint-free cloth. Residual sample water from a previous measurement will dilute the calibration solution and introduce error.
3. Immerse the probe in the calibration solution. Place the probe tip fully into the calibration solution. Allow the reading to stabilize — typically 30–60 seconds at room temperature.
4. Initiate calibration. With the stable reading displayed, press and hold the calibration button as described in the manual included with the meter. The display will confirm calibration has been applied. The meter stores the new calibration point digitally — no screwdriver or trim-pot adjustment is needed.
5. Verify the calibration. After calibrating, confirm the displayed reading matches the labeled value of your calibration solution within the meter’s stated accuracy. If it does not, repeat the process with fresh calibration solution. Contaminated or expired solution is a common cause of calibration failure.
6. Rinse again. Rinse the probe with clean water before returning to sample measurements.

Sources & Further Reading