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Chapter 11

Calculated Values

HydroNode can work out more from the readings a sensor already sends: dew point and VPD from temperature and humidity, wind chill from temperature and wind, rain of the last hour from the rain gauge. Turn a value on and it is calculated from every new reading and stored like a measured one, so it shows up in charts, widgets, Home Assistant, limits and alerts. No new hardware, no change on the device.

01Values

What HydroNode can calculate

ValueNeedsUnitGood to know
Pressure at sea levelAir pressure, the altitude you enterhPaComparable with weather maps. Only the display changes, readings stay as measured.
Dew pointTemperature, humidity°CAbove about 18 °C the air feels muggy. Below the surface temperature of a window, it fogs up.
Absolute humidityTemperature, humidityg/m³Water in the air, independent of the temperature. Good to compare inside and outside before airing.
VPDTemperature, humiditykPaHow strongly the air pulls water from leaves. Most plants grow best between 0.8 and 1.2 kPa.
Leaf VPDLeaf temperature, temperature, humiditykPaVPD at the leaf, from an infrared leaf sensor (type LEAF_TEMPERATURE). More exact under lamps.
Heat indexTemperature, humidity°CHow hot it feels in humid air. Equal to the temperature below 21 °C.
Wind chillTemperature, wind speed°CHow cold it feels in the wind. Equal to the temperature above 10 °C or in calm air.
Feels likeTemperature, humidity, wind speed°CApparent temperature in the shade, from all three at once.
Rain last hourRainfallmmSum of the last 60 minutes, updated with every rain reading.
Rain last 24 hoursRainfallmmSum of the last 24 hours, updated with every rain reading.

Pressure at sea level is different from the rest: it is not a new value but another way to show the pressure, so its switch moves every chart, the history included. It sits at the top of the page with the altitude field. Everything below is calculated from each new reading and stored as a value of its own.

A value counts as available when the sensor sends every reading it needs. Which sensor sends what does not matter: a BME280 over WiFi, a LoRaWAN node or a weather station all work the same.

02Set it up

Turn a value on

  1. 1
    Open the settings
    On the sensor page press Settings at the top right, then Calculated values under Sensor on the left. The entry shows how many are on. Air pressure at sea level comes first, then the two lists.
  2. 2
    Read the two lists
    Can be calculated holds every value the sensor has the readings for, with the readings it is made from. Not possible yet holds the rest, with a tick on each reading the sensor sends and a dashed mark on the one it does not.
  3. 3
    Switch it on and save
    Switch on as many as you like, then press Save changes in the bar at the bottom. HydroNode then calculates each new value for the last 30 days from the readings already stored, one day after the other, and shows it as it goes. You can close that window: the calculation goes on. After that every new reading brings a new value.
Calculated values of a sensor: dew point and VPD switched on, absolute humidity and heat index available, wind chill and rain sums listed with what they needCalculated values of a sensor: dew point and VPD switched on, absolute humidity and heat index available, wind chill and rain sums listed with what they need
Settings, Calculated values: what the sensor can get on top, what it cannot yet below.
The window The last 30 days are calculated: 16,758 values, one band of day marks each for dew point and VPD, from 1 October to todayThe window The last 30 days are calculated: 16,758 values, one band of day marks each for dew point and VPD, from 1 October to today
After saving: every value gets a band of days, filled from today back.

The owner and people the sensor is shared with at Can manage switch values on and off. Everyone else who can see the sensor sees the list and the values.

03Where it shows

Like a measured value

A calculated value gets its own line on the sensor, just like a reading the device sends. That is why everything else in HydroNode simply works with it:

  • The chart on the sensor page and the live readings on the Dashboard.
  • The widgets on the phone, with the other values of the sensor.
  • Home Assistant: a new entity appears with the next poll.
  • Limits and targets, for example a VPD band of 0.8 to 1.2 kPa with an alert outside it.
  • Automations: a rule on the dew point can switch a fan before the glass fogs up.
  • Weekly report and CSV export.
Rain sums, wind chill and feels like jump with every shower and gust, like rain and wind themselves. Anomaly detection does not judge them statistically; your own limits on them still apply.
04How it works

When a value is calculated

  • Every time a reading the value needs is stored, HydroNode takes the newest reading of each other input, measured at most 2 minutes before. Readings of one round arrive together, so they always match. A temperature from the round before is never paired with a new humidity.
  • The calculated value gets the time of the reading it came from. Both readings of a round lead to the same value, and it is stored once.
  • Rain of the last hour and of the last 24 hours add up all rainfall readings of that time, ending at the new reading.
  • Readings a device stored and sent later (a buffer, a cellular board that sends once an hour) are calculated with the time they were measured.
  • Switching a value on calculates the last 30 days too, never further back than the sensor keeps its readings (Settings, General, Keep readings). Each value takes a few seconds, one day at a time, so the rest of HydroNode stays fast. Nothing about the past raises an alert or runs an automation.
  • Turning a value off stops new ones. What was calculated so far stays in the chart.
05Limits

What to keep in mind

CaseWhat happens
The device sends the value itselfIt is listed as Not possible yet with "This sensor measures it itself". HydroNode does not calculate a second, slightly different series.
Two temperature sensors on one deviceThe first one the sensor ever sent is used, and stays the same when you add another. The row names the readings it uses.
Value slotsEvery calculated value takes one of the 30 value slots of a sensor. The page shows how many are in use.
The appsDew point, absolute humidity and heat index are known to every app version. VPD, leaf VPD, wind chill, feels like and the rain sums show their full name from the next app update on.
06Reference

Formulas

Inputs in °C, % relative humidity, m/s and mm. Results are rounded to two decimals, like the readings a device sends.

ValueFormulaSource
Pressure at sea levelp · (1 − 0.0065·h / (T + 273.15 + 0.0065·h))^−5.257 (15 °C without temperature)ISA standard atmosphere
Dew pointγ = ln(RH/100) + 17.62·T/(243.12+T); Td = 243.12·γ/(17.62−γ)Magnus, Sonntag 1990
Absolute humidity216.7 · 6.112·e^(17.67·T/(T+243.5)) · RH/100 / (273.15+T)Vapour pressure and ideal gas law
VPDes(T) · (1 − RH/100), es(T) = 0.6108·e^(17.27·T/(T+237.3))Tetens, FAO 56
Leaf VPDes(T_leaf) − es(T_air) · RH/100, at least 0Tetens, FAO 56
Heat indexRothfusz regression in °F with both NOAA adjustments, Steadman below 80 °FNOAA National Weather Service
Wind chill13.12 + 0.6215·T − 11.37·V^0.16 + 0.3965·T·V^0.16 (V in km/h)Environment Canada
Feels likeT + 0.33·e − 0.70·ws − 4.00, e = RH/100 · 6.105·e^(17.27·T/(237.7+T))Steadman 1994, Bureau of Meteorology
Rain sumsΣ rainfall over (t − 1 h, t] and (t − 24 h, t]Sum of the readings

What the values mean for plants, windows and walks in the rain: Calculate dew point and VPD from temperature and humidity.