A CLOSER LOOKTools
Check nutrient strength. Keep the scale straight.
HM Digital COM-100
Waterproof EC, TDS and temperature meter
Buyer feedback ↓The COM-100 measures conductivity, estimated dissolved solids and temperature in water. Its selectable scales are useful when nutrient instructions specify a particular EC or TDS convention.
- EC / TDSWater measurementsTemperature also displayed
- 185 mmHeightMeter body≈ 2.2 bank-card lengths
- 34 mmDiameterMeter body≈ 1.6 nickel diameters
Amazon US · Waterproof EC, TDS and temperature meter · Retailer link
THE QUICK TAKE
Is it right for you?
Buy it if you want EC and selectable TDS scales in one pen. Match the displayed unit and scale to the nutrient instructions, then compare readings taken the same way. The replaceable sensor is useful for a meter you expect to keep.
Choose something else if you need pH or a breakdown of individual nutrients. Conductivity cannot identify which salts are present. Cleaning and calibration still matter, and switching TDS scales can change the number without changing the water.
01 / BUYING NOTES
Before you buy.
Read the unit before the number.
Use the same EC unit or TDS scale as your nutrient instructions. The MODE control changes how the result is displayed; a different number is not automatically a change in nutrient strength.
Keep the sensor clean and sealed.
Rinse the probe after use and check calibration with an appropriate standard solution. The manual also requires the blue sensor collar and battery cap to be tightened for the waterproof seal.
COM-100 manual and scale guide ↗ · Manufacturer specifications ↗ · Exact US listing ↗ · Conductivity alternative ↗
02 / AMAZON REVIEWS
The buyers’ view.
Buyers describe straightforward calibration, although one owner found readings slow when moving between liquids at different temperatures. Reliability experiences are mixed: one owner reports years of service with routine cleaning and battery changes, while another repeatedly found dead batteries after switching the meter off for storage. A separate owner reported that it worked well and held calibration after replacing faulty supplied batteries.
What buyers like
- accuracy42
- quality36
- calibration7
The small numbers are review mentions. Topics and sentiment are from Amazon’s customer-review overview.
Amazon US · Checked . Ratings may include other colors and sizes.
View source ↗03 / REVIEWS & VIDEO
See it in use.
See how it works, then read the findings behind the buying advice.
HM Digital Com 100
Pacific Northwest Garden Supply shows the COM-100’s controls and explains EC versus the available TDS scales. This is a product demonstration, not an independent accuracy test.
Watch on YouTube ↗Video and written sources are labeled by creator and tested scope. Manufacturer demonstrations explain use; they are not independent tests. No in-house hands-on testing is claimed.
Read source summaries Product scope is labeled · key findings in bold
The manual tells growers to match the nutrient supplier’s requested scale. It also distinguishes measured conductivity from the calculated TDS reading.
The demonstration shows the meter’s physical controls and explains why different TDS conventions can produce different numbers. It is useful for understanding the display, without establishing long-term accuracy.
04 / COMPARE
Different priorities.
Choose the tradeoff that fits how you will use it.
Swipe to see every alternative →
THIS GUIDEHM Digital COM-100COM-100 water tester |  ALTERNATIVEApera EC20Conductivity tester kit | |
|---|---|---|
| Amazon price | $46.00 | $57.90 |
| Best for | EC plus selectable TDS scales | EC readings with a supplied calibration kit |
| Design | Replaceable conductivity sensor | Conductivity and temperature |
| Use | Match the unit and calibration standard | Automatic calibration; rinse the probe |
| Consider | No pH or individual nutrient identification | No pH measurement |
| Explore | Read buying guide →Amazon source ↗ | View on Amazon ↗ |
About these everyday comparisons
Approximate comparisons to familiar objects: a bank card’s 3.37-inch long edge, a Letter sheet’s 11-inch long edge, or a US nickel’s 21.21 mm diameter / 5 g weight. Water comparisons exclude the container’s weight. Capacity comparisons describe volume, not exterior size or how food fits. Reference pictures identify the object; use the labeled ratio for the comparison.
05 / PRICE HISTORY
The price over time.
HM Digital COM-100 · Waterproof EC, TDS and temperature meter · Amazon US
The chart legend identifies Amazon and new-offer prices when available. New offers can include marketplace sellers. Coupons, shipping and size/color selections can change the final price. Blank periods are missing observations, not a zero price.
06 / RELATED FINDS
A few useful extras.
For the way you plan to use it.
Measure pH separately
Apera PH20 pH Tester Kit
The PH20 answers the acidity question that an EC/TDS pen cannot.
Rinse between samples
BEADNOVA Squeeze Watering Bottles
A squeeze wash bottle can hold clean rinse water for the probe.