What meters do I actually need for growing?

Q: What meters do I actually need for growing?

For a first grow you need three meters: a pH meter calibrated with pH 7 and pH 4 buffers, an EC meter checked against a 1.413 mS/cm standard, and a thermo-hygrometer with min/max memory, salt-tested for accuracy. An IR thermometer is a cheap fourth. A PAR meter is worth it once you grow regularly. A meter you don't calibrate is worse than none.

Short answer

You need three: a pH meter (calibrated with pH 7 and pH 4 buffers), an EC meter (checked against 1.413 mS/cm) and a thermo-hygrometer with min/max memory (salt-tested). An IR thermometer is a cheap fourth; a PAR meter pays off once you grow regularly. Established for calibration practice.

Why

pH controls which nutrients roots can take up. A pH meter is only as good as its last calibration: two fresh buffers that bracket your readings, at least weekly ^[1].

EC tells you how strong the feed and runoff are. EC meters drift less than pH probes, but still need checking against a standard and after drops or battery changes ^[2]. TDS meters show the same reading converted to ppm by a factor that varies.

Temperature and humidity drive VPD and disease risk. Min/max memory catches the lights-off humidity spike you never see. A salt test shows whether the RH reading is honest ^[3].

Light is the biggest yield lever indoors ^[4]. A quantum meter turns guessing into setting.

The numbers

Meter	Must-have?	Check against	How often
pH meter	yes (coco/hydro); strongly advised (soil)	pH 7.00 + 4.01 buffers	weekly, before decisions
EC meter	yes	1.413 mS/cm standard	monthly, after drops
Thermo-hygrometer, min/max	yes	salt test, ~75 % RH	every few months
IR thermometer	useful	emissivity ~0.95	—
Quantum (PAR) meter	later	—	—
Jeweller's loupe, 60×	yes, at harvest	—	—
pH probe life			~1–2 years stored wet

What to do

Buy pH, EC and thermo-hygrometer first; buffers and EC standard with them.
Calibrate pH with pH 7, then pH 4; re-read pH 7 (within ±0.05).
Store the pH probe wet in storage solution, never in RO water.
Salt-test the hygrometer before trusting it.
Log readings in your journal every time.
Add an IR thermometer, then a PAR meter as you scale up.

Common mistakes

Never calibrating the pH pen.
Storing the probe dry.
Reading ppm off a TDS meter without knowing its scale.
One hygrometer on the floor, far from the canopy.

Full article: pH and EC meter calibration
How do I check if my hygrometer is accurate?
How can I measure PPFD without a PAR meter?
Is a TDS meter the same as an EC meter?
Terms: pH meter · EC meter · jeweller's loupe

A pH meter measures solution pH with a glass electrode whose voltage changes with hydrogen ion activity; it needs regular calibration and a moist probe.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.An EC meter measures how well a solution conducts electricity, giving nutrient strength in mS/cm or as ppm via a conversion factor.mS/cm (millisiemens per centimetre) is the standard unit of EC for nutrient solutions; 1 mS/cm equals 1 dS/m and 1,000 µS/cm.An IR (infrared) thermometer reads surface temperature from emitted infrared radiation; growers use it to measure leaf temperature for leaf VPD.Photosynthetically active radiation: light in the 400–700 nm waveband that plants use for photosynthesis, counted in photons rather than brightness.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.TDS (total dissolved solids) is the mass of dissolved material in water; grow meters estimate it from EC with a conversion factor and display it as ppm.Vapour pressure deficit: how much more water vapour the air could hold at its temperature, in kPa; it is the main environmental driver of transpiration.Relative humidity is the water vapour in air as a percentage of what it could hold at that temperature, so the same RH dries plants differently at different temperatures.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.A small handheld magnifier, typically 30-60x for cannabis use, used to inspect trichome heads and spot pests and male flowers.A calibration solution is a standard liquid of known pH or EC used to calibrate meters, such as pH 4.01, 7.00 and 10.01 buffers or 1.413 mS/cm EC standard.RO water is water stripped of most dissolved salts by reverse osmosis; it has near-zero EC and little buffering, so pH moves easily and Ca and Mg must be added.

Follow-up questions

Do I need a pH meter for soil?

For quality living or amended soil, a pH meter is useful but less critical, because soil buffers pH and runs at 6.2–6.8. For coco and hydro, where pH should be 5.5–6.5 and drifts quickly, a calibrated pH meter is essential. Either way, calibrate with fresh pH 7 and pH 4 buffers at least weekly and store the glass probe wet, never dry.

Is a TDS meter good enough instead of an EC meter?

A TDS meter measures the same thing as an EC meter, electrical conductivity, then multiplies it by a conversion factor to show ppm. The factor varies, commonly 500 or 700, so two TDS meters can show different ppm for the same solution. Read or convert to EC in mS/cm, which is unambiguous, and check the meter against a 1.413 mS/cm standard.

How long does a pH probe last?

A glass pH probe typically lasts about one to two years when stored wet in storage solution or pH 4 or 7 buffer. Storing it dry or in RO water shortens its life. Replace the probe when it can no longer be adjusted to match buffer, or when its slope stays below about 90–95 % after cleaning.

Sources

NC State University / NC Department of Agriculture and Consumer Services. The pour-through extraction procedure: a nutrient management tool for nursery crops. ncagr.gov/ncsu-pour-through-extraction-procedure/download

Prasad L, Meehan M. Using electrical conductivity and total dissolved solids meters to field test water quality. NDSU Extension WQ1923 (rev. 2025). ndsu.edu/agriculture/extension/publications/using-electrical

Greenspan L, 1977. Humidity fixed points of binary saturated aqueous solutions. Journal of Research of the National Bureau of Standards 81A(1):89–96. doi.org/10.6028/jres.081A.011

Rodriguez-Morrison V, Llewellyn D, Zheng Y, 2021. Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science 12:646020. doi.org/10.3389/fpls.2021.646020