[{"data":1,"prerenderedAt":246},["ShallowReactive",2],{"doc-\u002Ftroubleshooting\u002Fprobe-physics":3},{"id":4,"title":5,"body":6,"description":236,"edit":237,"extension":238,"meta":239,"navigation":240,"path":241,"seo":242,"stem":243,"vertical":237,"weight":244,"__hash__":245},"content\u002Ftroubleshooting\u002Fprobe-physics.md","Probe physics",{"type":7,"value":8,"toc":224},"minimark",[9,19,24,32,35,43,47,54,57,64,68,71,74,77,81,88,91,98,101,105,116,119,122,139,143,146,149,153,221],[10,11,12,13,18],"p",{},"A complement to ",[14,15,17],"a",{"href":16},"\u002Ftroubleshooting\u002Fwrong-readings","wrong readings"," —\nwhere that page is about what the user sees, this one is about why.\nEach section is a physics phenomenon that shows up in the data\nlooking like a software bug.",[20,21,23],"h2",{"id":22},"_1-self-heating","1 · Self-heating",[10,25,26,27,31],{},"A WiFi sensor in transmit mode draws ~150 mA at 3.3 V → ~0.5 W. That\nhalf watt is dissipated inside the same plastic shell as the\ntemperature probe. In a sealed enclosure with no airflow, the sensor\nreads 1.0–1.5 °C ",[28,29,30],"strong",{},"warmer"," than the ambient air.",[10,33,34],{},"Symptom: a sealed-box installation reads consistently 1 °C higher\nthan a separately-mounted reference probe.",[10,36,37,38,42],{},"Fix: ventilate the enclosure (drill two holes 5 mm in diameter, top\nand bottom — convection chimney) or accept the offset and configure\nit as the channel's ",[39,40,41],"code",{},"offset",". We see this most often when operators\nput a Shelly inside a small NEMA box for \"weather protection\"; the\nNEMA box is also a thermos.",[20,44,46],{"id":45},"_2-thermal-mass","2 · Thermal mass",[10,48,49,50,53],{},"A probe glued to a metal plate sees the ",[28,51,52],{},"plate"," temperature, which\nlags ambient air. In a fridge that holds 4 °C ± 1 °C, a plate-mounted\nprobe will read 4 °C ± 0.3 °C — flatter, slower to respond, and\ndelayed by 60–180 seconds against the air.",[10,55,56],{},"Symptom: a door-open event spike in the air is missed by the probe;\nthe chart looks fine while a separate inspection finds the food was\nbriefly warm.",[10,58,59,60,63],{},"Fix: mount probes ",[28,61,62],{},"in air",", not in contact with metal. If the only\nmount point is metal, use a thermal standoff (3 mm of plastic between\nsensor and metal).",[20,65,67],{"id":66},"_3-evaporator-drip","3 · Evaporator drip",[10,69,70],{},"Walk-in fridges defrost by warming the evaporator coil. Liquid water\ndrips. A sensor mounted under the evaporator gets dripped on,\nperiodically. The wet sensor reads colder (evaporative cooling) by\n1–3 °C until it dries.",[10,72,73],{},"Symptom: regular sub-zero excursions on a fridge set to +4 °C,\nclustering at the same hour each day (the scheduled defrost).",[10,75,76],{},"Fix: move the probe away from the drip path. Or shroud the probe\nwith a perforated cover that lets air through but blocks splashes.",[20,78,80],{"id":79},"_4-door-opening-transient","4 · Door-opening transient",[10,82,83,84,87],{},"When the fridge door opens, a thin layer of room air pours over the\nsill and falls down (cold air weighs more, but the ",[28,85,86],{},"temperature\ngradient"," moves room-air down too once equilibrium starts). A\nsensor in this stream sees 12–15 °C briefly before the door closes\nand the compressor catches up.",[10,89,90],{},"Symptom: short (5–15 s) spikes during business hours, regular\npattern (every 3–10 minutes during service).",[10,92,93,94,97],{},"This is not a bug; it is the ",[28,95,96],{},"real"," temperature next to the door.\nBut it is not the temperature of the food in the back.",[10,99,100],{},"Fix: do not mount probes near the door if you are monitoring the\nfood. Move to the back. Or accept the spikes and set the grace\nperiod to 60 seconds.",[20,102,104],{"id":103},"_5-stratification","5 · Stratification",[10,106,107,108,111,112,115],{},"A walk-in fridge is ",[28,109,110],{},"not"," at one temperature. The air at the top\ncan be 6 °C; the air at the floor can be 1 °C. Two probes at\nopposite vertical extremes can read 5 °C apart and ",[28,113,114],{},"both be\ncorrect",".",[10,117,118],{},"Symptom: two sensors in \"the same fridge\" disagree by 4 °C\npersistently.",[10,120,121],{},"Fix: this is the design of the fridge, not a sensor problem. Either:",[123,124,125,133],"ul",{},[126,127,128,129,132],"li",{},"Mount ",[28,130,131],{},"both probes at the same height"," so they are measuring the\nsame air mass. Or:",[126,134,128,135,138],{},[28,136,137],{},"one probe at the warmest stored item"," and use it as the\ncompliance reference. The other probe is informational.",[20,140,142],{"id":141},"_6-window-draft-bonus","6 · Window draft (bonus)",[10,144,145],{},"For indoor climate sensors in offices and museums: a temperature\nsensor near a window reads ambient (cold in winter, hot in summer)\nnot room average. A humidity sensor near a window reads condensation\nor evaporation depending on the season.",[10,147,148],{},"Fix: mount climate sensors away from windows, away from HVAC\ndiffusers, away from radiators, on internal walls when possible.",[20,150,152],{"id":151},"summary","Summary",[154,155,156,169],"table",{},[157,158,159],"thead",{},[160,161,162,166],"tr",{},[163,164,165],"th",{},"Symptom",[163,167,168],{},"Probable physics",[170,171,172,181,189,197,205,213],"tbody",{},[160,173,174,178],{},[175,176,177],"td",{},"Sensor reads 1 °C warmer than reference",[175,179,180],{},"Self-heating in enclosure",[160,182,183,186],{},[175,184,185],{},"Lag and flat chart",[175,187,188],{},"Thermal mass \u002F metal contact",[160,190,191,194],{},[175,192,193],{},"Daily cold spikes at fixed hour",[175,195,196],{},"Evaporator defrost drip",[160,198,199,202],{},[175,200,201],{},"Short hot spikes during service hours",[175,203,204],{},"Door-opening transient",[160,206,207,210],{},[175,208,209],{},"Two probes in same fridge differ",[175,211,212],{},"Vertical stratification",[160,214,215,218],{},[175,216,217],{},"Cold in winter, hot in summer indoors",[175,219,220],{},"Window draft",[10,222,223],{},"If you can match your symptom to a row, you do not have a software\nbug. Move or shroud the probe, then look at the data again before\nopening a ticket.",{"title":225,"searchDepth":226,"depth":226,"links":227},"",3,[228,230,231,232,233,234,235],{"id":22,"depth":229,"text":23},2,{"id":45,"depth":229,"text":46},{"id":66,"depth":229,"text":67},{"id":79,"depth":229,"text":80},{"id":103,"depth":229,"text":104},{"id":141,"depth":229,"text":142},{"id":151,"depth":229,"text":152},"The five physical mistakes that masquerade as software bugs",null,"md",{},true,"\u002Ftroubleshooting\u002Fprobe-physics",{"title":5,"description":236},"troubleshooting\u002Fprobe-physics",480,"KNLgPmNF5ampegm2NTkfte7WBDr-v-moJNXC4ONMBSc",1779022955834]