Calc dewpoint from BME280

[triphil]

with some help from the forum I am now quite happy with the output from my Wemos / Tasmota giving me data from the BME280. I have used global variables to store the temp and humidity so I can then calc the dew point in a seperate function node. Is that the best/easiest way ? Is there another way of doing it?

[krambriw]

@triphil

Is there another way of doing it?

I do not know a better way, I have not seen any node for that type of calculation but there are many flows and samples you may find on the net

I think I posted the code in here somewhere,,,how I do the calculation, I will have to search

Edit: I knew it, here it is

New bathroom ideas

You nailed it. This is the key point to keep track of. I have been monitoring this since years in certain areas in hour house (in the ground and in the attic) using sensors and related logic in Node-RED (earlier with EventGhost). I'm measuring the temperature & relative humidity, calculating the delta (difference from actual temperature to dew point). If delta goes negative there is a problem. Luckily we have never had any condensation so far, I have also verified this regularly with an instrum…

In this example I calculate the delta which is the interesting part, i.e. how many degrees you have left before condensation will happen

[Christian-Me]

Dew point

There is a simple approximation for humidity above 50%

[TotallyInformation]

I just put a function node after recieving the msg with the data and add the dew point and other standard data that way.

This is a bit long, sorry. All my hardware inputs go through this to get standardised data out. You will see where the dewpoint and heat index are added. At the end of the code are some standardised functions for calculating these things with some references:

/* Split the inbound message into separate topics in Homie-like format
//
// This is for input only and combined input/output devices that need Node-RED
// to output to homie style MQTT topics.
//
// Output only devices like Lightwave switches are handled separately.
*/

// msg.meta = {"TH1/0x7D01": {
//              location: "HOME/IN/00/LIVING",
//              type: "Sensor", 
//              description: "Oregon Scientific Temperature/Humidity Sensor"}
//            }

// Setup {
var topicRoot = 'homie/';
var topics = msg.topic.split('/');
var deviceID = topics[1];
var deviceTopicRoot = topicRoot + deviceID;
var sensorTopicRoot = deviceTopicRoot + '/sensors';
var sensorsType = '';
var sensorList = [];
var lastUpdate = new Date();
// }

// output to port 2 if payload is not an object as this is an unknown device type
//if ( (typeof msg.payload) !== 'object' ) {
//    return [null,msg]
//}

// Standard info for all input devices {
node.send([{ 'topic': deviceTopicRoot + '/$implementation', 'payload': 'nrlive/' + topics[0] }]);
node.send([{ 'topic': deviceTopicRoot + '/$name', 'payload': msg.meta.description }]);
node.send([{ 'topic': deviceTopicRoot + '/$location', 'payload': msg.meta.location }]);
node.send([{ 'topic': deviceTopicRoot + '/$type', 'payload': msg.meta.type }]);
node.send([{ 'topic': deviceTopicRoot + '/$source', 'payload': msg.src }]);
node.send([{ 'topic': deviceTopicRoot + '/$stats/$updated', 'payload': lastUpdate }]);
// }

// -- Device Specific -- // {
if ( msg.topic === 'WIFI/SP01' ) edimaxSP2101W();
// --------------------- // }

// -- rssi -- // {
var rssi = getProps(['rssi', 'status.rssi', 'payload.rssi', 'payload.status.rssi']);
if ( rssi !== '' ) {
    node.send( [{ 'topic': deviceTopicRoot + '/$stats/signal', 'payload': rssi }] );
}
// }

// -- battery -- // {
var battery = getProps(['battery', 'status.battery', 'payload.battery', 'payload.status.battery']);
if ( battery !== '' ) {
    node.send( [{ 'topic': deviceTopicRoot + '/$stats/battery', 'payload': battery }] );
}
// }

// ======= SENSOR DATA ======== //

// -- Temperature -- // {
var temperature = getProps(['payload.Temperature', 'payload.temperature.value', 'payload.temperature']);
if ( temperature !== '' ) {
    sensorList.push( 'temperature' );
    sensorsType = 'sensors';
    node.send( [{ 'topic': sensorTopicRoot + '/temperature', 'payload': temperature }] );
}
// }

// -- Humidity -- // {
var humidity = getProps(['payload.Humidity', 'payload.humidity.value', 'payload.humidity']);
if ( humidity !== '' ) {
    sensorList.push( 'humidity' );
    sensorsType = 'sensors';
    node.send( [{ 'topic': sensorTopicRoot + '/humidity', 'payload': humidity }] );
}
// }

// -- Light -- // {
var light = getProps(['payload.Light', 'payload.light.value', 'payload.light', 'LDR']);
if ( light !== '' ) {
    sensorList.push( 'light' );
    sensorsType = 'sensors';
    node.send( [{ 'topic': sensorTopicRoot + '/light', 'payload': light }] );
}
// }

// -- Pressure -- // {
var pressure = getProps(['payload.Pressure', 'payload.pressure.value', 'payload.pressure']);
if ( pressure !== '' ) {
    var sealevel = getProps(['payload.Pressure_Sealevel', 'payload.PressureSealevel', 'payload.pressureSealevel', 'payload.pressure_sealevel']);
    if ( sealevel === '' ) {
        var A = 201; //Altitude of sensor  <<-- TODO: replace with lookup from master device object (HOME/deviceLocations)
        var P = pressure;
    	sealevel = round( P/Math.pow( (1-(A/44330.0)), 5.255 ), 2 );        
    }
    sensorList.push( 'pressure' );
    sensorsType = 'sensors';
    node.send( [{ 'topic': sensorTopicRoot + '/pressure', 'payload': sealevel }] );
    node.send( [{ 'topic': sensorTopicRoot + '/pressure_abs', 'payload': pressure }] );
}
// }

// -- Add extra data on combinations -- //

// Calculate dewpoint and heatindex
if ( doesContain(['temperature','humidity'], sensorList) ) {
    heatindex(temperature, humidity);
    dewpoint(temperature, humidity);
}

// -- Output sensor list and type (or debug msg if none) -- // {
    if ( sensorsType !== '' ) {
        node.send( [{ 'topic': sensorTopicRoot + '/$type', 'payload': sensorsType }] );
        node.send( [{ 'topic': sensorTopicRoot + '/$properties', 'payload': sensorList.join(',') }] );
    }
// }

// ====== COMMANDS ====== // {
if ( (msg.src === 'RFX/Lights') | msg.hasOwnProperty('unit') | msg.payload.hasOwnProperty('command') ) {

    // Work out the command and optionally, the unit {
    var command = '', unit = '';
    // msg.command/.unit?
    if ( msg.hasOwnProperty('command') ) {
        command = msg.command;
    }
    if ( msg.hasOwnProperty('unit') ) {
        unit = msg.unit;
    }
    // payload.command/.unit takes preference
    if ( typeof msg.payload === 'object' ) {
        if ( msg.payload.hasOwnProperty('command') ) {
            command = msg.command;
        }
        if ( msg.payload.hasOwnProperty('unit') ) {
            unit = msg.unit;
        }
    } else {
        command = msg.payload;
    }
    // }
    
    // If the command is 1/0 or true/false, convert to standard On/Off {
    if ( (command === 1) || (command === true) ) {
        command = 'On';
    }
    if ( (command === 0) || (command === false) ) {
        command = 'Off';
    }
    // }
    
    if ( command !== '' ) {

        // Record unit command
        if ( unit !== '' ) { // For unit based commands only
            const strUtopic = deviceTopicRoot + '/unit/' + unit;
            // What command recieved
            node.send( [{ 'topic': strUtopic + '/command', 'payload': command }] );
            // When command recieved
            node.send( [{ 'topic': strUtopic + '/timestamp', 'payload': lastUpdate }] );
            // Record last time this command recieved
            node.send( [{ 'topic': strUtopic + '/last' + command, 'payload': lastUpdate }] );
            
            // Save unit command times (in ms since 1970/1/1) {
            const unitCmdTimes = global.get('unitCmdTimes') || {};
            if ( ! unitCmdTimes.hasOwnProperty(strUtopic) ) {
                unitCmdTimes[strUtopic] = {'lastCmd': null, 'lastOn': null, 'lastOff': null};
            }
            unitCmdTimes[strUtopic]['last' + command] = lastUpdate.getTime();
            // }

            // Send elapsed on/off times {
            // Don't update if new cmd is same as old.
            if ( command !== unitCmdTimes[strUtopic].lastCmd ) {
                if ( command === 'On' ) {  // How long was it off? msOn-msOff
                    const msOn = lastUpdate.getTime();
                    const msOff = unitCmdTimes[strUtopic].lastOff || null;
                    if ( msOff ) {
                        node.send( [{ 'topic': strUtopic + '/lastOffFor', 'payload': toHHMMSS( (msOn - msOff) ) }] );
                    }
                }
                if ( command === 'Off' ) {  // How long was it off? mmOff-msOn
                    const msOn  = unitCmdTimes[strUtopic].lastOn || null;
                    const msOff = lastUpdate.getTime();
                    if ( msOn ) {
                        node.send( [{ 'topic': strUtopic + '/lastOnFor', 'payload': toHHMMSS( (msOff - msOn) ) }] );
                    }
                }
            }
            // }

            unitCmdTimes[strUtopic].lastCmd = command;
            
            global.set('unitCmdTimes', unitCmdTimes);
        }

        // Record latest command {
        node.send( [{ 'topic': deviceTopicRoot + '/command/cmd', 'payload': command }] );
        node.send( [{ 'topic': deviceTopicRoot + '/command/unit', 'payload': unit }] );
        node.send( [{ 'topic': deviceTopicRoot + '/command/timestamp', 'payload': lastUpdate }] );
        node.send( [{ 'topic': deviceTopicRoot + '/command', 'payload': { 'cmd': command, 'unit': unit } }] );
        node.send( [{ 'topic': deviceTopicRoot + '/command/last' + command, 'payload': lastUpdate }] );
        // }
    }
}
// --- End of Command output --- // }

//return msg;
// =========== DEVICE SPECIFIC FUNCTIONS =========== // {

function edimaxSP2101W() {
    // TODO: Change fixed IP to dynamic (from ARP data)
    
    const sp01 = global.get('sp01') || {'state': null, 'ip': '192.168.1.103', 'lastOn': null, 'lastOff':null, 'lastCmd': null};
    node.send([null,{global: { 'sp01': sp01 }}]);
    
    const info = getProps('payload.deviceinfo');
    node.send([{ 'topic': deviceTopicRoot + '/$online', 'payload': true }]);
    node.send([{ 'topic': deviceTopicRoot + '/$mac', 'payload': translateMac(info.mac) }]);
    node.send([{ 'topic': deviceTopicRoot + '/$localip', 'payload': sp01.ip }]);
    node.send([{ 'topic': deviceTopicRoot + '/$fw/name', 'payload': info.model }]);
    node.send([{ 'topic': deviceTopicRoot + '/$fw/version', 'payload': info.fwVersion }]);
    
    const status = getProps('payload.status');
    sensorsType = 'sensors';
    sensorList.push( 'power' );
    node.send( [{ 'topic': sensorTopicRoot + '/power', 'payload': status.nowPower }] );
    sensorList.push( 'current' );
    node.send( [{ 'topic': sensorTopicRoot + '/current', 'payload': status.nowCurrent }] );
    sensorList.push( 'stats' );
    node.send( [{ 'topic': sensorTopicRoot + '/stats', 'payload': { 'power':{'day': status.day, 'week': status.week, 'month': status.month}, 'cost':status.cost} }] );

    // Update command status if needed
    if ( sp01.state !== status.state ) {
        sp01.state = status.state;
        const cmd = status.state ? 'On' : 'Off';
        node.send( [{ 'topic': deviceTopicRoot + '/command', 'payload': { 'cmd': cmd } }] );
        node.send( [{ 'topic': deviceTopicRoot + '/command/cmd', 'payload': cmd }] );
        node.send( [{ 'topic': deviceTopicRoot + '/command/timestamp', 'payload': status.lastToggleTime }] );
        node.send( [{ 'topic': deviceTopicRoot + '/command/last' + cmd, 'payload': status.lastToggleTime }] );
        // Also sent pseudo "unit" entries to match LightwaveRF, etc
        node.send( [{ 'topic': deviceTopicRoot + '/command/unit', 'payload': 0 }] );
        node.send( [{ 'topic': deviceTopicRoot + '/unit/0/command', 'payload': cmd }] );
        node.send( [{ 'topic': deviceTopicRoot + '/unit/0/timestamp', 'payload': status.lastToggleTime }] );
        node.send( [{ 'topic': deviceTopicRoot + '/unit/0/last' + cmd, 'payload': status.lastToggleTime }] );

        // Save unit command times (in ms since 1970/1/1) 
        const lastToggle = new Date(status.lastToggleTime)
        sp01['last' + cmd] = lastToggle.getTime();

        // Save elapsed on/off times {
        // Don't update if new cmd is same as old.
        if ( command !== sp01.lastCmd ) {
            if ( cmd === 'On' ) {  // How long was it off? msOn-msOff
                const msOn = lastToggle.getTime();
                const msOff = sp01.lastOff;
                if ( msOff ) {
                    node.send( [{ 'topic': deviceTopicRoot + '/unit/0/lastOffFor', 'payload': toHHMMSS( (msOn - msOff) ) }] );
                }
            }
            if ( cmd === 'Off' ) {  // How long was it off? mmOff-msOn
                const msOn  = sp01.lastOn;
                const msOff = lastToggle.getTime();
                if ( msOn ) {
                    node.send( [{ 'topic': deviceTopicRoot + '/unit/0/lastOnFor', 'payload': toHHMMSS( (msOff - msOn) ) }] );
                }
            }
        }
        // }
        
        sp01.lastCmd = cmd;
    }

    global.set('sp01', sp01);
}

// =========== UTILITY FUNCTIONS =========== //

function getProps(props) {
    if ( (typeof props) === 'string' ) {
        props = [props];
    }
    if ( ! Array.isArray(props) ) {
        return undefined;
    }
    let ans = '';
    for (var i = 0; i < props.length; i++) {
        try { // breaks if an intermediate property doesn't exist
            ans = RED.util.getMessageProperty(msg, props[i]);
            if ( typeof ans !== 'undefined' ) {
                break;
            }
        } catch(e) { 
            // Do nothing
        }
    }
    return ans || '';
}

// Is arrCheck a subset of arrSuperSet?
function doesContain(arrCheck,arrSuperSet) {
    return arrCheck.every(elem => arrSuperSet.indexOf(elem) > -1);
}

// Convert ms to hh:mm:ss (ES2015+)
function toHHMMSS(mSecs) {
    const secs = mSecs / 1000;
    
    const sec_num = parseInt(secs, 10);
    const hours   = Math.floor(sec_num / 3600) % 24;
    const minutes = Math.floor(sec_num / 60) % 60;
    const seconds = sec_num % 60;
    
    return [hours,minutes,seconds]
        .map(v => v < 10 ? "0" + v : v)
        .join(":");
        // Put the following between map and join if you want to remove zero hours/minutes
        //.filter((v,i) => v !== "00" || i > 0)
}

// translate mac address string into : separated mac address string
function translateMac(mac) {
    const s = mac.split('');
    return s[0] + s[1] + ':' + s[2] + s[3] + ':' + s[4] + s[5] + ':' + s[6] + s[7] + ':' + s[8] + s[9] + ':' + s[10] + s[11];
}

// Output dewpoint
function dewpoint(temperature, humidity){
    if ( typeof temperature === 'string' ) temperature = parseFloat(temperature)
    if ( typeof humidity === 'string' ) humidity = parseFloat(humidity)
    sensorList.push( 'dewpoint' )
    sensorsType = 'sensors'
    if ( !('dewpoint' in msg.payload) ) {
        node.send( [{ 'topic': sensorTopicRoot + '/dewpoint', 'payload': dewPointFast(temperature, humidity) }] )
    } else {
        node.send( [{ 'topic': sensorTopicRoot + '/dewpoint', 'payload': msg.payload.dewpoint }] )
    }
}

// Output heatindex
function heatindex(temperature, humidity){
    if ( typeof temperature === 'string' ) temperature = parseFloat(temperature)
    if ( typeof humidity === 'string' ) humidity = parseFloat(humidity)
    sensorList.push( 'heatindex' )
    sensorsType = 'sensors'
    if ( !('heatindex' in msg.payload) ) {
        node.send( [{ 'topic': sensorTopicRoot + '/heatindex', 'payload': computeHeatIndex(temperature, humidity, false) }] )
    } else {
        node.send( [{ 'topic': sensorTopicRoot + '/heatindex', 'payload': msg.payload.heatindex }] )
    }
}

function round(mynum, decimals) {
    if ( typeof mynum === 'string' ) mynum = parseFloat(mynum)
    var n = Math.pow(10, decimals);
    return Math.round( (n * mynum).toFixed(decimals) )  / n;
}

function convertCtoF(c) {
    if ( typeof c === 'string' ) c = parseFloat(c)
    return c * 1.8 + 32;
}
function convertFtoC(f) {
    if ( typeof f === 'string' ) f = parseFloat(f)
    return (f - 32) * 0.55555;
}

// See: http://arduinotronics.blogspot.co.uk/2013/12/temp-humidity-w-dew-point-calcualtions.html {
// delta max = 0.6544 wrt dewPoint()
// 6.9 x faster than dewPoint()
// reference: http://en.wikipedia.org/wiki/Dew_point }
function dewPointFast(celsius, humidity) {
    if ( typeof celsius === 'string' ) celsius = parseFloat(celsius)
    if ( typeof humidity === 'string' ) humidity = parseFloat(humidity)
    var a = 17.271;
    var b = 237.7;
    var temp = Math.log(humidity * 0.01) + ((a * celsius) / (b + celsius));
    var Td = (b * temp) / (a - temp);
    return round( Td, 2 );
}
/*
function dCalc(humidity, celcius) {
	//iMV is the intermidate Value 
	var iMV = (Math.log(humidity / 100)+((17.27 * celcius)/(237.3 + celcius)))/17.27;
	var dewPoint = (237.3 *iMV)/(1-iMV);
	return dewPoint.toFixed(2);
}
function computeDew1(temp, rh) {
    if ( (temp === null || temp.length === 0) ||
        (rh === null || rh.length === 0) ) {
        return;
    }
    if (rh >= 85) {
        dewp = temp-0.133 * ( 100.0-rh );
    }    
    if (rh < 85) {
        dewp = temp-0.2 * ( 95.0-rh );
    }    
    return (temp - dewp).toPrecision(4);
}
function computeDew2(temp, rh) {
    if ( (temp === null || temp.length === 0) ||
        (rh === null || rh.length === 0) ) {
        return;
    }
    tem2 = temp;
    tm = -1.0*temp;
    es = 6.112*Math.exp(-1.0*17.67*tm/(243.5 - tm));
    ed = rh/100.0*es;
    eln = Math.log(ed/6.112);
    dewp = -243.5*eln/(eln - 17.67 );
    return (temp - dewp).toPrecision(4);
}
*/

//boolean isFahrenheit: True == Fahrenheit; False == Celcius
function computeHeatIndex(temperature, percentHumidity, isFahrenheit) {
    if ( typeof temperature === 'string' ) temperature = parseFloat(temperature)
    if ( typeof percentHumidity === 'string' ) percentHumidity = parseFloat(percentHumidity)
    
    // Using both Rothfusz and Steadman's equations
    // http://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml
    var hi;

    if (!isFahrenheit) temperature = convertCtoF(temperature);

    hi = 0.5 * (temperature + 61.0 + ((temperature - 68.0) * 1.2) + (percentHumidity * 0.094));

  if (hi > 79) {
    hi = -42.379 +
             2.04901523 * temperature +
            10.14333127 * percentHumidity +
            -0.22475541 * temperature * percentHumidity +
            -0.00683783 * Math.pow(temperature, 2) +
            -0.05481717 * Math.pow(percentHumidity, 2) +
             0.00122874 * Math.pow(temperature, 2) * percentHumidity +
             0.00085282 * temperature * Math.pow(percentHumidity, 2) +
            -0.00000199 * Math.pow(temperature, 2) * Math.pow(percentHumidity, 2);

    if((percentHumidity < 13) && (temperature >= 80.0) && (temperature <= 112.0))
      hi -= ((13.0 - percentHumidity) * 0.25) * Math.sqrt((17.0 - Math.abs(temperature - 95.0)) * 0.05882);

    else if((percentHumidity > 85.0) && (temperature >= 80.0) && (temperature <= 87.0))
      hi += ((percentHumidity - 85.0) * 0.1) * ((87.0 - temperature) * 0.2);
  }

  return isFahrenheit ? round( hi, 2 ) : round( convertFtoC(hi), 2 );
}

/*
function heatIndex(tempC, humidity) {
    var tempF = convertCtoF(tempC);
    var c1 = -42.38, c2 = 2.049, c3 = 10.14, c4 = -0.2248, c5= -6.838e-3, c6=-5.482e-2, c7=1.228e-3, c8=8.528e-4, c9=-1.99e-6  ;
    var T = tempF;
    var R = humidity;
    
    var A = (( c5 * T) + c2) * T + c1;
    var B = ((c7 * T) + c4) * T + c3;
    var C = ((c9 * T) + c8) * T + c6;
    
    var rv = (C * R + B) * R + A;
    return round( rv, 1 );
}
*/
// EOF }