EC vs PPM: What Your Nutrient Meter Is Actually Reading, and Why 1.5 EC Can Be 750 or 1,050 ppm
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EC vs PPM: What Your Nutrient Meter Is Actually Reading, and Why 1.5 EC Can Be 750 or 1,050 ppm
You read a nutrient chart that says 1,000 ppm. Your meter says 1.4. Both can be right, and the only thing between them is a conversion factor that most growers never see. Mix up the factor and you can feed a room 40 percent more or less than you meant to.
ModernFarms sells Bluelab's pens and Truncheons, TrolMaster's reservoir EC sensors, and the reference solution that keeps them honest. This guide explains what EC and ppm really are, how to convert between them, and which of our meters show what. Where a listing doesn't say something, we won't guess.
The 30-Second Answer
EC is the measurement. Ppm is a calculation made from EC. The conversion: multiply EC by 500 for the 500 scale or by 700 for the 700 scale. CF is EC times 10. The trap: 1.5 EC is 750 ppm on one scale and 1,050 ppm on the other, a 40 percent gap from the same water. The fix: record EC, and write down which ppm scale a chart or meter uses before you trust a ppm number. The gear: the Conductivity Pen is $99.00, the Truncheon V2 is $117.00, and the OnePen is $187.64.
What the Meter Actually Measures
A conductivity meter measures how easily electricity passes through your nutrient solution. Dissolved salts carry current, so more dissolved nutrient means higher conductivity. That number is EC, usually shown in milliSiemens per centimeter or just as a plain decimal like 1.8.
Ppm, parts per million, claims to say how much dissolved solid is in the water. A meter can't count those particles directly, so it takes the EC and multiplies it by a factor. Different manufacturers picked different factors, which is how one reservoir ends up with two honest ppm numbers.
The Conversion Table
| EC | CF | ppm (500 scale) | ppm (700 scale) |
|---|---|---|---|
| 1.0 | 10 | 500 | 700 |
| 1.5 | 15 | 750 | 1,050 |
| 2.0 | 20 | 1,000 | 1,400 |
| 2.5 | 25 | 1,250 | 1,750 |
The gap is always 40 percent, because 700 divided by 500 is 1.4. If a feed chart written for the 700 scale says 1,050 ppm and you hold a 500-scale meter, you will see 750 and think the room is underfed. If you then add nutrient until it reads 1,050, you have fed 2.1 EC, not 1.5. That mistake is common, and it costs nutrients, leaf tips, and sometimes a crop. Some meters use other factors, so treat the two scales above as the common ones, not the only ones.
Which of Our Meters Show What
| Meter | What the listing says | Price |
|---|---|---|
| Bluelab Conductivity Pen | Measures conductivity and temperature. Selectable units, backlit LCD, hold function, automatic temperature compensation. Calibration optional. | $99.00 |
| Bluelab Truncheon Nutrient Meter V2 | Shows EC, CF, ppm 500, and ppm 700 on one line. Range up to 3.6 EC. Factory calibrated. | $117.00 |
| Bluelab Commercial Truncheon | Built for crops with higher-than-normal EC. Covers EC, CF, ppm 500, and ppm 700. No calibration required. | $117.00 |
| Bluelab OnePen | Replaceable probe measuring pH, conductivity, and temperature. | $187.64 |
| TrolMaster Aqua-X Reservoir EC/Temp Sensor PCT-1 | Reads EC and temperature in the reservoir for the Aqua-X system. | $67.99 |
The Truncheon is the one that removes the ppm argument. It puts all four scales on one display, so you can read a chart in whatever units it uses. The V2 listing gives its top of range as 3.6 EC, which is 1,800 ppm on the 500 scale and 2,520 on the 700 scale, matching the math above. The Commercial Truncheon is listed for higher EC crops. Both list at $117.00, so ask us which suits your feed range before you buy. The Commercial Truncheon is down to a couple of units at the moment.
For a full comparison of the pH side, see our guide to OnePen vs Combo Meter vs Combo Meter Plus, and the complete Bluelab buyer's guide.
Calibration and Checking
Bluelab's 2.77 EC solution listing says most of its EC and ppm equipment is factory calibrated, except the EC and ppm Pens. The solution works as a reference: dip the probe, and if it reads near 2.77 EC the probe is clean and honest. If it doesn't, clean the probe first. The 250 ml bottle is $51.00 and the 500 ml is $84.24. The listing says to pour a small amount into a cup, discard it after use, and keep the lid tight, so the bottle doesn't get contaminated.
Buying a reference solution is a choice, not a requirement. If you have one meter and feed the same recipe every week, a reading that suddenly looks odd is the moment to check it. If you run several rooms, a bottle that every meter gets checked against is cheap insurance. Our probe care guide covers the pH side of the same habit.
Temperature Quietly Changes the Number
Conductivity rises as water warms, which is why the Bluelab listings mention automatic temperature compensation. The Conductivity Pen listing also notes that the nutrient solution temperature matters for roots and recommends feeding at 65 to 72 degrees F. A reservoir that drifts warm can make a meter and a controller disagree, so note the temperature next to the EC. Our grow room cooling guide covers the air side.
How to Choose
If you want one meter that reads every chart you will ever meet, the Truncheon V2 at $117.00 shows all four scales. If you want a pen for spot checks that also reads temperature, the Conductivity Pen at $99.00 does that. If you also want pH from the same tool, the OnePen at $187.64 covers pH, EC, and temperature. If the reservoir needs continuous logging, the PCT-1 sensor at $67.99 works with the TrolMaster Aqua-X system, and we'd want your setup before recommending anything else. For a larger facility, ModernFarms finances projects over $25,000 for licensed operators; our financing guide explains what we can and can't tell you about terms.
Start the conversation
Tell us what you grow, which nutrient line you feed, and what meter you have now. Email info@modernfarms.store or call 818-504-8165, and we'll tell you whether you need a new meter or just a conversion.
Frequently Asked Questions
What is the difference between EC and ppm?
EC (electrical conductivity) is what the meter actually measures. Ppm is a number calculated from EC using a conversion factor. Two meters can read the same water and show different ppm because they use different factors, while their EC readings match.
How do I convert EC to ppm?
Multiply EC by 500 for the 500 scale or by 700 for the 700 scale. An EC of 1.5 is 750 ppm on the 500 scale and 1,050 ppm on the 700 scale. CF (conductivity factor) is EC multiplied by 10, so 1.5 EC is 15 CF.
Which ppm scale does my meter use?
Check the manual or the screen. Bluelab's Truncheon V2 listing shows EC, CF, ppm 500, and ppm 700 on one line, and the Commercial Truncheon lists the same scales. The Conductivity Pen is listed with selectable units. If your meter doesn't say, use EC and avoid the problem.
Should I use EC or ppm?
Use EC if you can. It is the measured value, and it doesn't change between meters. If your nutrient chart is written in ppm, find out which scale the chart assumes before you copy a number.
Does temperature change an EC reading?
Yes. Conductivity rises with temperature, which is why the Bluelab listings mention automatic temperature compensation. Test at a consistent temperature when you can.
Do I need to calibrate an EC meter?
It depends on the meter. Bluelab's 2.77 EC solution listing says most of its EC and ppm equipment is factory calibrated, except the EC and ppm Pens, and that the solution is mainly a reference to check the probe is clean.
Can I finance equipment for a larger facility?
Financing is available for projects over $25,000 for licensed facilities. A meter won't reach that, but it can be part of a larger order. Rates and terms are not published, so ask in writing.
This article is general information, not technical, financial, or legal advice. Conversion factors are standard multipliers; check your meter's manual for the scale it uses and your nutrient maker's chart for feed targets. Prices and stock reflect ModernFarms' live catalog as of October 8, 2026. We don't upsell, and if your current meter already shows EC, you may not need a new one.