The glossary.

A number from 0 to 14. Lower numbers mean more acidic (like vinegar). Higher numbers mean more alkaline (like soap). Plants like the water in the middle, around 6.

pH is a logarithmic measure of hydrogen ion concentration. For leafy hydroponic crops, the optimal range is 5.8–6.8. Outside this band, certain nutrients become chemically unavailable to roots even if dissolved in the water — this is called "nutrient lockout." On the HarvEv-02 Mobile Unit schematic, the pH sensor is the inline probe that reads the reservoir continuously.

When you dissolve plant food in water, the water gets a little bit electric. EC measures how electric. More electric = more food = happier plants — up to a point.

EC measures dissolved salts (nutrients) by their ability to carry electrical current, in millisiemens per centimetre (mS/cm). For lettuce, target 1.0–1.8 mS/cm. Too low and plants are underfed; too high and roots are stressed by osmotic pressure. The HarvEv-02 schematic displays EC in microsiemens per centimetre (μS/cm) — multiply by 0.001 to convert.

TDS counts how much "stuff" is dissolved in the water — like counting grains of sugar in a glass. It's another way of saying the same thing as EC.

TDS is expressed in parts per million (ppm). It's mathematically related to EC by a conversion factor (typically 500 or 700 depending on the meter). Most 4-in-1 hydroponics meters display both. Use whichever one your reference materials use — pick one and stick to it.

Plant roots prefer water that's a comfortable temperature — not too cold, not too hot. Around 17–22°C is best.

Reservoir temperature affects oxygen solubility and root metabolism. Below 17°C, growth slows; above 22°C, dissolved oxygen drops and root rot organisms thrive. The HarvEv-02 schematic uses a slightly wider 18–22°C target band reflecting Tier 2 deployment conditions.

The bit at the bottom of your unit that holds the water. Your reservoir holds about 4 litres — roughly two big bottles of fizzy drink.

In a recirculating hydroponic system, the reservoir is the central holding tank. Water is pumped from here past the roots and returns by gravity. Reservoir health (pH, EC, temperature, oxygenation) directly determines plant health.

When the water level gets low because plants have drunk it. You add fresh water to bring it back up.

Plants take up water faster than nutrients, so over time the reservoir becomes more concentrated. Top up with plain water if EC is climbing, with nutrient solution if EC is falling. Always re-check pH and EC after a top-up.

A way of growing plants without any soil. The roots dangle in water that has plant food dissolved in it. Plants actually grow faster this way than in soil.

Hydroponics is soilless cultivation where roots access water and nutrients directly. Compared to soil, it uses ~90% less water, allows tighter control of plant nutrition, and eliminates many soil-borne diseases. Several variants exist (DWC, NFT, ebb-and-flow). Your unit uses a recirculating wick or shallow-flow system.

The amazing thing plants do — they take in water and a gas from the air (carbon dioxide), point themselves at light, and turn it all into sugar. That sugar is how the plant grows.

The biochemical conversion of CO₂ and water into glucose using light energy, releasing oxygen as a byproduct. Occurs in chloroplasts. The Mini Manual Unit makes photosynthesis visible — pupils can see leaves grow week by week, and link that growth to light hours and reservoir nutrient levels.

How long the lamp is on each day. Most leafy plants like a long day — 12 to 16 hours of light, then dark for the rest. The dark time matters too — that's when they rest.

The duration of light exposure in a 24-hour cycle. Different crops have different optima — lettuce thrives at 14–16 hours; flowering crops often need a shorter day to trigger maturation. Excessive photoperiod can stress plants and cause "tipburn" in lettuce.

Thriving = bright, springy, growing fast. Healthy = looks fine. Stressed = pale, slow, drooping a bit. Wilting = yellow, brown spots, struggling. Trust your eyes — you're a good observer.

These are deliberately broad observational categories that pupils can apply consistently. They map roughly to: turgor + colour + growth rate. Stressed/wilting plants almost always indicate a water-side problem (pH, EC, oxygen, temperature) — fix the reservoir and the plants follow.

When the lettuce is big enough to eat, you snip the outer leaves (or the whole plant). Then new leaves can grow back. This is harvest day.

For leafy crops, "cut and come again" harvesting (taking outer leaves while leaving the central crown) gives 4–6 weeks of continuous yield from a single planting. A tabletop unit can produce roughly a school lunchbox-portion of fresh greens per week once mature.

Big black panels that catch sunlight and turn it into electricity. Even on cloudy days, they make some electricity — just less.

Photovoltaic (PV) panels convert solar radiation directly into DC electricity via the photovoltaic effect in silicon semiconductors. Tier 2 and Tier 3 systems use rooftop or wall-mounted PV to charge battery storage during daylight, freeing the unit from grid dependency. The HarvEv-02 schematic displays current generation in watts.

Solar panels make electricity, but the amount changes with the weather. The MPPT is a small computer that constantly works out the best way to take that electricity and feed it into the battery — like a sail-trimmer on a boat.

A Maximum Power Point Tracking controller is a DC-DC converter that continuously calculates the optimal voltage/current operating point of the solar array as conditions change, then steps that into the correct charging profile for the battery. The HarvEv-02 uses a Victron BlueSolar MPPT 100|30 rated for 30A output.

A kind of rechargeable battery — the same kind that lives in most hybrid cars. They can be charged thousands of times. The HarvEv-02 uses these batteries rescued from scrapped cars.

Nickel Metal Hydride is a robust rechargeable chemistry historically used in hybrid vehicles. Compared to lithium-ion, NiMH is safer (no thermal runaway risk), tolerates wide temperature ranges, and has predictable degradation. After ~10 years in a vehicle, NiMH packs typically retain 70%+ capacity — ideal for stationary second-life applications. HarvEv-02 uses repurposed Hybrid EV battery packs (28kg post-casing, 13.1 kWh, 201.6V nominal).

When a car is scrapped, its battery often still works fine — it just isn't powerful enough for fast driving anymore. RecyclaCell rescues those batteries and uses them to power things like the bigger HarvEv units.

When EV/hybrid batteries fall below ~70% of original capacity, vehicles retire them, but they remain perfectly viable for less-demanding stationary applications (school power, building backup, telecoms). Second-life deployment displaces both new battery manufacturing emissions and end-of-life recycling burden.

A row of eight little switches that the system controls electronically — turning the grow light on and off, running the water pump, and so on. Each switch is called a "channel."

A Modbus-controlled relay module with eight independently-switchable channels. On HarvEv-02, channels are assigned to the grow light, water pumps, fans, and other AC/DC loads. The schematic displays which channels are currently energised.

The wall-mounted box that holds all the clever electronics — the MPPT, the small computer, the relay. It's sealed against water and dust so it can live anywhere.

The RecyclaCell CU-01 is an IP65-rated wall-mount enclosure housing the MPPT controller, BOSGAME P4 Plus mini PC (AMD Ryzen 7 5825U), Mean Well DDR-120B-12 power supply, and Waveshare 8-channel Modbus POE relay. Communicates with sensors and external systems via Modbus RTU/TCP and MQTT.

The temperature of the air outside the unit — in the room, or outdoors if it lives outside. Different from the water temperature inside the reservoir.

Ambient air temperature near the unit. On Tier 2 deployments, this is read from an external sensor and helps separate "the room is hot" from "the reservoir is hot" — useful when diagnosing temperature drift.

Different gases warm the planet by different amounts. CO₂e is a way of adding them all up as if they were just carbon dioxide. Saving 1 tonne of CO₂e is roughly the same as not driving a car for 4,000 miles.

A standard unit (tonnes CO₂e or tCO₂e) used in greenhouse gas accounting. Each Tier 2 unit is independently calculated to avoid 0.80 tCO₂e annually — comprised of avoided new-battery manufacturing emissions plus displaced grid electricity. Used in SECR reporting and the Trust's annual Climate Action Plan progress.

Like your toaster or your kettle. The Mini Manual Unit plugs into a normal wall socket. The bigger HarvEv units run on solar and batteries instead.

Tier 1 Mini Manual Units run on standard 240V AC, drawing through a low-voltage transformer for the LED light and pump. This makes deployment simple — no electrical works needed. Tier 2 and Tier 3 units add solar + battery to provide grid-independent operation.

Tier 1 is your tabletop unit. Tier 2 is the big tower at Elmhurst with proper sensors and solar power. Tier 3 is the future "Grow Hut" — a whole growing room. Schools start at Tier 1 and earn the next.

The Trust's phased deployment model. Every school starts at Tier 1 (Mini Manual Unit). Engagement, evidenced by consistent dashboard submissions, qualifies a school for Tier 2 (HarvEv-02 / Mobile Unit) — a full RecyclaCell-powered tower with automated sensors. Tier 3 is the Grid to Grow Hut, an outdoor scaffold-built sustainability environment.

You! If you take readings, water the plants, log harvests, or just notice things — you're a Climate Champion. The Trust wants more of you.

A pupil (and teacher) leadership role within the Trust's Climate Action Plan. Climate Champions take ownership of the unit, advocate for sustainability across the school, and represent their school in inter-school events. Acts as a low-cost intervention with measurable engagement outcomes.

A 30-pod tower at Elmhurst with lots of plants and lots of sensors. It does what your unit does, but the readings happen automatically.

Tier 2 of the programme. A YUZINO 30-pod vertical hydroponic tower with mother.life full-spectrum lighting, Atlas Scientific sensor array (pH, EC, water temp, water level), Modbus relay control, and a dedicated RecyclaCell power system (solar PV charging repurposed Hybrid EV vehicle battery packs). Currently deployed at Elmhurst as the lead pilot site.

The dream. A small building, outside, where lots of different plants grow all year round. The Trust wants every school to have one, eventually.

Tier 3 of the programme. A scaffold-built, all-weather growing environment with multiple integrated grow systems, comprehensive sensor networks, rainwater capture and nutrient recycling, and a touchscreen dashboard for live curriculum integration. Designed for outdoor school estates and aimed at engaged Tier 2 schools.

Your school is part of the New Vision Trust, along with four other schools in East London: Downshall, Elmhurst, Gallions, Nelson, and Vicarage. You're all in this together.

A multi-academy trust of five East London primary schools. The Trust's Climate Action Plan 2025–2050 sets out the sustainability strategy of which the HarvEv programme is the flagship pupil-facing pillar.

The people who design and build the HarvEv units, and rescue old EV batteries to power them. They believe waste batteries can have a second life.

The parent organisation behind the HarvEv programme. Specialises in second-life EV battery applications, with a focus on educational and public-sector deployments. Operates in partnership with HRM Global Solutions, which provides the technology platform.

A computer that can do clever things — like understanding what you say, recognising pictures of plants, or writing a sentence. It's not actually thinking like you do, but it can be very good at copying the patterns.

AI is an umbrella term for software systems that perform tasks traditionally requiring human intelligence — recognition, prediction, language. Modern AI is mostly built using machine learning. In this programme, AI is a tool used by humans to build the dashboard and toolkit, not an autonomous agent making decisions about plants.

Imagine showing a computer a million pictures of cats and dogs and saying which is which. After a while, the computer learns to spot a cat or dog by itself. That's machine learning. It's how spam filters, voice assistants, and AI farming tools all work.

Machine learning (ML) is a class of techniques where systems learn patterns from large datasets rather than following hard-coded rules. Most "AI" you encounter — chatbots, recommendation engines, image classifiers — is ML under the hood. In agricultural contexts, ML models are trained on historic crop data, sensor readings, and image libraries to predict outcomes or detect anomalies.

Big farms are starting to use AI to spot sick plants from photos taken by drones, predict how much harvest they'll get, and decide exactly when to water crops. Your unit is much smaller, but it works on the same kind of data — pH, EC, light hours — that AI farming uses too.

Commercial precision agriculture increasingly uses AI for yield prediction, disease detection from drone or greenhouse imagery, automated nutrient dosing, and optimal harvest timing. The same input variables your pupils measure manually (pH, EC, water temp, photoperiod) are the variables professional AI systems analyse at scale. The Mini Manual Unit makes the underlying science tangible — your pupils are learning the data literacy that AI farming relies on.

The page where everyone can see how all five schools' units are doing — like a leaderboard, but for plants.

The public-facing data view at /mini-units/dashboard.php. Renders all five schools' latest readings, recent activity, photos, and pH trend lines. Updates in real time as teachers submit. Designed to be linkable and shareable with parents, governors, and inspectors.

Numbers tell us what changed. The logbook tells us why. "We harvested 6 lettuce." "Year 4 visited." "We added more nutrients." Stories matter.

The qualitative companion to numerical readings. Captures discrete events (harvest, top-up, repair, visit) with optional quantity and free-text narrative. Provides essential context when interpreting data trends — a sudden EC drop is meaningless without "topped up with 2L of plain water" alongside it.

Green means good. Amber means watch out. Red means do something! The dashboard colours every reading automatically.

Each numerical reading is auto-categorised by the dashboard into one of three bands. Healthy (green) = within optimal range. Watch (amber) = within tolerable range but trending. Alert (red) = outside acceptable bounds, intervention needed. Bands are defined in config.php and adjustable centrally.