Trying to calcule dew point in a function note by grapping humidity and temp from two devices

GixxerPilot

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Sep 29, 2016, 2:42:13 PM9/29/16

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I want calculate dew point based on the following function:

function dCalc(h, t)

{

//iMV is the intermidate Value

var iMV = (Math.log(h/100)+((17.27* t)/(237.3 +t)))/17.27;

var dewPoint = (237.3 *iMV)/(1-iMV);

return dewPoint.toFixed(2);

}

I have two devices. One is getting the temp the other is getting a humidity reading. I have an idea that I should should use global context to store the two values and them use them in a function node that has the function declared. I am not sure how exactly I should go about this. The dew point should be displayed on a gauge and update live depending what values the two sensors are reading.

This is what I have been trying so far. The two devices are missing since I saw that when you export them they also give out your api keys.

this is my clipboard export:

Paul Reed

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Sep 29, 2016, 3:57:42 PM9/29/16

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I've done something similar, if it helps.

Paul

[{"id":"be728125.418d8","type":"function","z":"3f1992f2.c0e66e","name":"Parse METAR","func":"context.noaadata = context.noaadata || null;\ncontext.emontemp = context.emontemp || null;\n//using a switch statement to match payload to topic\nswitch (msg.topic) {\n case \"noaaTop\":\n context.noaadata = msg.payload;\n msg.payload = null;\n break;\n case \"emonTop\":\n context.emontemp = msg.payload;\n msg.payload = null;\n break;\n default:\n msg.payload = null;\n \tbreak;\n}\n//check if both data vaues have been received\nif(context.noaadata !== null && context.emontemp !== null) {\n//parse XML feed\nvar METARdata = context.noaadata.response.data[0].METAR[0];\nvar windkt = METARdata.wind_speed_kt[0];\nvar winddir = METARdata.wind_dir_degrees[0];\nvar windkph = (windkt*1.852);\nvar gusts = windkt; //if wind_gust_kt is not present, value defaults to wind_speed_kt\n if (typeof METARdata.wind_gust_kt != \"undefined\") {\n\t gusts = METARdata.wind_gust_kt[0];\n }\nvar altim = METARdata.altim_in_hg[0];\nvar pressure = Math.round(altim * 33.8637526); //convert hg to mb\nvar dew = METARdata.dewpoint_c[0];\ndew = Math.round(dew);\n\n//TempC obtained via emoncms\nvar tempC = context.emontemp;\nvar tempF = ((tempC*1.8)+32);\n\n//August-Roche-Magnus approximation to calculate rh_hum\nvar constA = 17.625;\nvar constB = 243.04;\nvar rh_numer = 10000.0*Math.exp((constA*eval(dew))/(eval(dew)+constB));\nvar rh_denom = Math.exp((constA*eval(tempC))/(eval(tempC)+constB));\nvar rh_hum = (rh_numer/rh_denom);\nvar humidityemon = Math.round(rh_hum);\nvar humidity = (humidityemon/100);\n\n//Wind Chill Index Calculations using the 'Joint Action Group for Temp Indices' (JAG TI) formula\nvar chill=(13.12+0.6215*tempC-11.37*Math.pow(windkph,0.16)+0.3965*tempC*Math.pow(windkph,0.16));\nif (windkph<4.83) {\nchill=tempC;\n}\n\n//Calculate Apparent Temperature\n//firstly, Calculate Vapor pressure in kPa\nvar vaporp = ((6.11*Math.pow(10,(7.5*dew/(237.3 + dew))))/10);\n//apply Steadman's Apparent Temperature formula\nvar apptemp = -2.7+(1.04*tempC)+(2*vaporp)-(windkt*0.3343886);\n \n//Calculation of 'feels like' temperature by using the 'chill factor' if\n//the temp is below 10degC, or 'apparent temp' if over 20degC, or if between\n//10 and 20degC using a linear interpolation of both.\nvar feels;\nif (tempC < 10.0) {\n\tfeels=chill;\n\t}\n\telse if (tempC > 20.0) {\n\tfeels=apptemp;\n\t}\n\t\telse {\n\t\tvar calcA=((tempC-10)/10);\n\t\tvar calcB=1-calcA;\n\t\tfeels=((apptemp*calcA)+(chill*calcB));\n\t\t}\n\t\t\tfeels=Math.round(feels*100);\n\t\t\nmsg.payload = ((windkt) + (\",\") + (gusts) + (\",\") + (humidityemon) + (\",\") + (pressure) + (\",\") + (feels) + (\",\") + (winddir));\n\n//reset switch filter\ncontext.noaadata = null;\n//uncomment next line to await for both new values to arrive\ncontext.emontemp = null;\n\nreturn msg;\n}","outputs":1,"noerr":0,"x":734,"y":443,"wires":[["2857ddc.fd7a822","5b713c72.a48ec4"]]}]

Dave C-J

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Sep 29, 2016, 7:44:48 PM9/29/16

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Hi,
What device nodes are they ? We need to "have a word"with the author about those credentials... They should not be exported...

Walter Kraembring

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Sep 30, 2016, 1:41:18 AM9/30/16

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I have "traditional" sensors from Oregon for humidity & temperature readings and a RFXtrx transceiver. I calculate the dew point delta (means how far away we are from dew point with the actual temperature) for the attic and the ground of our house. I use the flow below. The two formulas give approximately the same result. In daily operation I decided for the first one since it is a little bit "tougher", it tends to give lower values for the delta calculation means it has an additional safety margin

[{"id":"84c48a06.db3f68","type":"rfx-sensor","z":"4668031a.72819c","name":"","port":"14ce85ac.bb2a6a","topicSource":"single","topic":"TH1","x":380,"y":240,"wires":[["eada6dfd.7069c","6e7815f4.b6c9bc"]]},{"id":"731c1b31.351334","type":"debug","z":"4668031a.72819c","name":"","active":true,"console":"false","complete":"payload","x":790,"y":280,"wires":[]},{"id":"eada6dfd.7069c","type":"function","z":"4668031a.72819c","name":"","func":"// initialise the variables if they doesn't exist already\ncontext.set('dpAttic',context.get('dpAttic')||0);    \ncontext.set('dpGround',context.get('dpGround')||0);    \n\nfunction updateNodeStatus(txt) {\n    node.status({\n    \ttext : txt\n    });\n}\n\nfunction computeDew(temp, rh)\n{\n    if ( (temp === null || temp.length === 0) ||\n        (rh === null || rh.length === 0) ) {\n        return;\n    }\n    if (rh >= 85) {\n        dewp = temp-0.133 * ( 100.0-rh );\n    }    \n    if (rh < 85) {\n        dewp = temp-0.2 * ( 95.0-rh );\n    }    \n    return (temp - dewp).toPrecision(4);\n}\n\ntemp = msg.payload.temperature.value;\nrh = msg.payload.humidity.value;\n\nif (msg.topic === 'TH1/0x0704') {\n    context.set('dpGround',computeDew(temp, rh));\n    msg.payload = 'Ground: '+context.get('dpGround');\n}\nif (msg.topic === 'TH1/0x2002') {\n    context.set('dpAttic',computeDew(temp, rh));\n    msg.payload = 'Attic: '+context.get('dpAttic');\n}\ntxt = 'Ground: '+context.get('dpGround') +' Attic: '+context.get('dpAttic');\nupdateNodeStatus(txt);\n\nreturn msg;","outputs":1,"noerr":0,"x":590,"y":200,"wires":[["558410a5.ef78f"]]},{"id":"6e7815f4.b6c9bc","type":"function","z":"4668031a.72819c","name":"","func":"// initialise the variables if they doesn't exist already\ncontext.set('dpAttic',context.get('dpAttic')||0);    \ncontext.set('dpGround',context.get('dpGround')||0);    \n\nfunction updateNodeStatus(txt) {\n    node.status({\n    \ttext : txt\n    });\n}\n\nfunction computeDew(temp, rh) {\n    if ( (temp === null || temp.length === 0) ||\n        (rh === null || rh.length === 0) ) {\n        return;\n    }\n    tem2 = temp;\n    tm = -1.0*temp;\n    es = 6.112*Math.exp(-1.0*17.67*tm/(243.5 - tm));\n    ed = rh/100.0*es;\n    eln = Math.log(ed/6.112);\n    dewp = -243.5*eln/(eln - 17.67 );\n    return (temp - dewp).toPrecision(4);\n}\n\ntemp = msg.payload.temperature.value;\nrh = msg.payload.humidity.value;\n\nif (msg.topic === 'TH1/0x0704') {\n    context.set('dpGround',computeDew(temp, rh));\n    msg.payload = 'Ground: '+context.get('dpGround');\n}\nif (msg.topic === 'TH1/0x2002') {\n    context.set('dpAttic',computeDew(temp, rh));\n    msg.payload = 'Attic: '+context.get('dpAttic');\n}\ntxt = 'Ground: '+context.get('dpGround') +' Attic: '+context.get('dpAttic');\nupdateNodeStatus(txt);\n\nreturn msg;","outputs":"1","noerr":0,"x":590,"y":280,"wires":[["731c1b31.351334"]]},{"id":"558410a5.ef78f","type":"debug","z":"4668031a.72819c","name":"","active":true,"console":"false","complete":"false","x":790,"y":200,"wires":[]},{"id":"14ce85ac.bb2a6a","type":"rfxtrx-port","z":"","port":"/dev/ttyUSB0"}]