Performing calculations in Node-RED

Third party integrations Node-RED
1

Hello,

I would like to perform the following calculation in Node-RED:

A value should be calculated from two input values ​​(e.g. temperature and relative humidity) using a formula. The result of the formula should then be displayed in the dashboard as a numerical value in a simple field.

Which elements are used in Node-RED?

Thank you!
Michael

2

are you asking about how retrieve from and push data back to HA or about a node to perform calculations?

3

Simple calculations can be done in almost any node using jsonata. More complex may be better suited for the function node. Here is an example for air VPD

//get HA values
const temp = global.get('homeassistant.homeAssistant.states["sensor.2b_air_sensor_01_temperature"].state');
const hum = global.get('homeassistant.homeAssistant.states["sensor.2b_air_sensor_01_humidity"].state');
// F to C
const tCelsius = (5/9)*temp -32;
// VPD calc
const saturation = 0.61078 * Math.pow(Math.E, (17.2694 * tCelsius) / (tCelsius + 238.3));
const actual = ((100 - hum) / 100) * saturation;

// empty message
msg = {};
// round to 2
msg.payload = actual.toFixed(2);

return msg;

You would then send that value to a sensor node, which will create an entity in HA you can then display on a dash. You will need the nodered companion app in hacs for this.

If this is for vpd it maybe easier/more straight forward just creating this sensor in HA.