Athena vs Pulse vs AROYA VPD Charts: Why They Disagree

Three charts can be useful and still disagree

Put the same temperature and RH into different VPD resources and the result may not match. That does not automatically mean one vendor has bad math. It often means the chart is answering a different question.

The important distinction is whether a number represents air VPD, leaf-to-air VPD, or a leaf-aware calculation that assumes a fixed difference between leaf and air temperature. Those are related values, but they are not interchangeable. A chart can also be stage-specific, while another tool may show broad operating bands or only a clone-room procedure.

For a commercial facility, the useful question is not which logo has the one correct chart. It is: What does this number represent, what assumptions produced it, and can the team use that same convention consistently?

This comparison audits what Athena, Pulse, and AROYA publicly publish. It is not a replacement for the fundamentals in our complete cannabis VPD guide, nor is it a new stage-target chart. The goal here is to make vendor numbers legible before they get written into room SOPs.

The comparison rule: name the temperature reference

Air VPD is the difference between saturation vapor pressure at air temperature and the actual vapor pressure in the air. A common air-VPD expression is:

Air VPD = saturation vapor pressure at air temperature − actual air vapor pressure

Leaf-to-air VPD uses the saturation vapor pressure at leaf temperature instead, while still using the vapor pressure of the surrounding air. The e-GRO university extension brief recommends calling this value VPDleaf or leaf-to-air vapor pressure difference so it is not confused with air VPD.[1]

That naming matters. At 20°C (68°F) and 60% RH, e-GRO calculates air VPD at 0.94 kPa. With the same air conditions, a leaf at 18°C (64°F) produces 0.66 kPa leaf VPD, while a leaf at 22°C (72°F) produces 1.24 kPa.[1] Nothing about the room RH changed. Only the leaf-temperature reference changed.

For a fuller explanation of the distinction, see Leaf VPD vs Air VPD: Why Your Sensor Readings Lie. The practical takeaway is simpler: whenever a chart says “VPD,” check whether it states air temperature, leaf temperature, or a leaf offset.

What Athena actually publishes

The verified current Athena source for this comparison is its VPDome procedure. It is a cloning and acclimation instruction, not a general veg and flower VPD chart.[2]

After cuttings harden off, Athena says to regulate the room containing the VPDome to 70 to 80°F (21 to 26°C) and 65 to 75% room humidity to achieve about 0.8 to 1.0 kPa.[2] Those are room conditions attached to a specific propagation workflow. The source does not publish a leaf-temperature offset, state that leaf temperature was measured, provide a leaf-VPD formula, or establish stage-by-stage recommendations for vegetative and flowering rooms.

That is a limitation of the available source, not a criticism of Athena. The recommendation can be useful for the clone application it describes. It should not be stretched into a claim that Athena has a general air-VPD rule, a general leaf-VPD rule, or a complete commercial-room chart. The stated temperature and RH ranges are also broad, so not every possible pairing inside them should be treated as exactly 0.8 to 1.0 kPa.

For the broader propagation process around that handoff, including the transition from rooted material into veg, see our commercial clone and seedling environment guide.

What Pulse charts assume

Pulse is the clearest of the three sources about a fixed leaf-temperature assumption on its static charts. Its separate clone, veg, and flower charts are captioned with a leaf temperature of 2°F below room temperature in Fahrenheit and 1°C below in Celsius.[3] That means these static visuals are not zero-offset air-VPD charts.

Pulse’s VPD guide provides both versions of the calculation. For air VPD, it uses saturation vapor pressure at air temperature and RH. For leaf VPD, it calculates saturation vapor pressure at leaf temperature, then subtracts the actual vapor pressure of the air.[4] The same guide says leaves are typically 1 to 3°C, or 2 to 5°F, cooler than air, but “typically” is not a universal room correction.[4]

There is an important page-specific caveat: the sample flower chart in that guide is described as using a 0°F leaf-temperature offset, while the dedicated stage-chart page uses a minus-2°F/minus-1°C assumption.[3][4] A grower can therefore encounter two different Pulse values without either page necessarily being a calculation error. The assumed offset changed.

Pulse also documents a user-entered offset workflow in its app. It instructs growers to take four to six canopy-leaf readings and four to six non-plant readings, average each group, then subtract the non-plant average from the canopy average. That result is entered as the leaf-temperature offset. Pulse gives minus 5°F as an example and recommends checking the offset monthly.[5]

This is a meaningful distinction. Pulse supports a leaf-aware calculation with an offset that a grower measures and enters. Its public support article does not establish that a standard Pulse climate sensor continuously measures canopy leaf temperature. The IR reading process is a periodic measurement workflow, not evidence of continuous direct leaf sensing.

What AROYA’s chart and guide say

AROYA’s public VPD chart is interactive. It exposes controls for air RH, air temperature, leaf temperature, and VPD, and it publishes broad bands: under 0.4 kPa as under-transpiration danger territory; 0.4 to 0.8 for early vegetative growth or propagation; 0.8 to 1.2 for late veg and early flower; 1.2 to 1.6 for mid and late flower; and above 1.6 as over-transpiration danger territory.[6]

Unlike Pulse’s static stage charts, the public AROYA tool does not state a fixed leaf offset. A user can supply a leaf-temperature value. On the observed initial chart state, leaf temperature equaled air temperature, which is effectively a zero-offset starting condition until the input changes.[6]

The chart page says air temperature and RH drive the calculation, with leaf temperature relevant when evaluating leaf-to-air VPD.[6] That makes the calculator leaf-aware, but it does not prove that every value shown is a measured-leaf value or that AROYA’s standard climate station directly and continuously measures leaf temperature.

AROYA’s educational guidance adds an operational nuance that is especially relevant in large rooms. Tyler Simmons writes that he does not recommend using one infrared leaf-surface-temperature reading, or one ambient reading, to calculate VPD for an entire room because leaf temperatures vary across the canopy. He favors ambient air-temperature sensors for more reliable room-level results, then recommends setting phase targets, reverse-engineering RH, monitoring climate data, and adjusting from there.[7]

That is not a conflict with the interactive leaf-temperature field. A calculator can accommodate leaf temperature, while a room-control SOP can still avoid treating one leaf reading as the crop-wide truth. AROYA’s broader hardware and crop-steering context is covered separately in AROYA Alternatives: Crop Steering Without Proprietary Hardware.

Side-by-side: what the public sources support

Why a fixed offset breaks down in a commercial room

A fixed leaf offset is a practical shortcut, not a plant constant. Leaf temperature can differ by several degrees from air temperature because of net radiation, air movement, stomatal conductance, and transpiration.[1] A scientific review similarly describes canopy temperature as the outcome of energy exchange involving radiation, ambient heat, reflected light, and water loss through transpiration.[8]

In a commercial canopy, that can mean top leaves under intense fixtures, edge plants near supply air, interior leaves with less air movement, and shaded lower leaves do not share one temperature. The same room sensor can therefore support a useful air-VPD control number while any single IR leaf measurement remains local to the leaf and moment sampled.

That does not make offsets useless. It means the offset must be treated as an assumption with a location, timing, and method behind it. A fixed minus-2°F chart may be a reasonable facility convention if the team understands it is a convention. It becomes a problem when it is compared directly with a zero-offset air-VPD number or entered into a controller as though it were a universal physical constant.

It also explains why changing vendors, charts, or dashboards mid-run can create false alarms. The displayed kPa may move even when the room has not. The definition behind the display changed.

A practical facility standard

Commercial operators do not need to settle the physiology debate before setting a usable SOP. They need a convention that survives shift changes, sensor replacements, and retrospective batch review.

Use this checklist when standardizing VPD reporting:

1. Choose the reported value. Decide whether the room log and targets use air VPD or leaf-to-air VPD. Put that exact term in the SOP and on the dashboard export.
2. Record the calculation inputs. Save air-sensor location, height, sampling interval, temperature unit, RH source, and any leaf-temperature or offset assumption.
3. If using an offset, name it. Record whether it is a fixed chart assumption, a periodic IR-derived value, or a manually entered operating convention. Include the date and measurement method.
4. Do not promote one leaf to a whole-room sensor. If IR readings are used, sample multiple canopy and non-plant locations, as Pulse’s workflow describes, and document what was averaged.[5] Treat the result as an estimate, not a continuous crop-wide measurement.
5. Keep day and night records distinct. Day and night conditions have different plant and equipment behavior. Do not assume a daytime chart target transfers unchanged into the dark period.[4]
6. Avoid switching conventions mid-run. If a change is necessary, retain the old and new definitions in the historical record so apparent performance changes can be interpreted correctly.
7. Review crop response beside the kPa value. Use plant response, disease pressure, irrigation behavior, and run outcomes to test whether the facility standard is serving the crop. A vendor band is a starting framework, not a guarantee.

Existing sensor exports can support historical comparisons, but retain the source convention and sensor metadata with each record. A bare kPa column without its definition is difficult to compare across rooms or runs.

For quick calculation checks, use the Growgoyle VPD calculator. Just make sure the temperature you enter matches the question you are trying to answer.

References

1. Kubota, C. (2023). VPDair vs. VPDleaf: Two different ways to determine VPD. e-GRO Alert 12(42). Ohio State University and e-GRO extension network.
2. Athena Ag. VPDome Procedure.
3. Pulse Grow. VPD Charts in Fahrenheit and Celsius With Leaf Temperature.
4. Pulse Grow. The Ultimate Vapor Pressure Deficit (VPD) Guide.
5. Pulse Grow Support. How to Measure and Set the VPD Leaf Temperature Offset.
6. AROYA. AROYA VPD Chart.
7. AROYA. Getting into the weeds of vapor pressure deficit.
8. Grossiord, C. et al. (2020). Plant responses to rising vapor pressure deficit. Annual Review of Plant Biology, 71, 59-84.