I am building a controller for a salt water reef aquarium that will turn on and off heaters, lights, fans, etc. based on the temperature of the water. I'm using DS18B20 sensors and I have three of them. In bench testing all three of them vary by up to 1 degree. In addition they all show higher than my standard thermometer on the wall although I can't really vouch for it.My question is: How do people "calibrate" the sensors? I can vary the output in code to agree with the wall thermometer. In practical use the temp should only vary by 3 or 4 degrees. Is there a practical "known accurate" thermometer to be used for calibration?
Boiling water (100C or 212F)
That would be at sea level only. As your altitude increases, the boiling temperature is lower.
My question is: How do people "calibrate" the sensors?
If you really want accuracy, you need a thermocouple.
RTDs vs ThermocouplesThe two most common ways of measuring industrial temperatures are with resistance temperature detectors (RTDs) and thermocouples. Choice between them is usually determined by four factors.What are the temperature requirements? If process temperatures are between -200 to 500 °C (-328 to 932 °F), an industrial RTD is the preferred option. Thermocouples have a range of -180 to 2,320 °C (-292 to 4,208 °F), so for temperatures above 500 °C (932 °F) they are the only contact temperature measurement device.What are the time-response requirements? If the process requires a very fast response to temperature changes--fractions of a second as opposed to seconds (e.g. 2.5 to 10 s)--then a thermocouple is the best choice. Time response is measured by immersing the sensor in water moving at 1 m/s (3 ft/s) with a 63.2% step change.What are the size requirements? A standard RTD sheath is 3.175 to 6.35 mm (0.1250 to 0.250 in) in diameter; sheath diameters for thermocouples can be less than 1.6 mm (0.063 in).What are the accuracy and stability requirements? If a tolerance of 2 °C is acceptable and the highest level of repeatability is not required, a thermocouple will serve. RTDs are capable of higher accuracy and can maintain stability for many years, while thermocouples can drift within the first few hours of use.
QuoteThat would be at sea level only. As your altitude increases, the boiling temperature is lower.And that's only for pure water - what's your point?
My point is, that it would be incorrect to try to calibrate a sensor to 100c using boiling water at 1000m altitude - kind of like the phrase "the blind leading the blind".
We've been using a Thermopen, which will set you back about $200, but so far it's the cheapest reference quality solution I've found.