On Fri, 2022-07-15 at 18:07 -0700, Russell Senior wrote: > On Fri, Jul 15, 2022 at 5:17 PM Tomas Kuchta > <[email protected]> wrote: > > > > On Fri, Jul 15, 2022, 17:04 Russell Senior <[email protected]> > > wrote: > > > > > I have used a bunch of these: > > > > > > https://www.amazon.com/gp/product/B00T0K8NXC/ > > > and > > > https://www.amazon.com/gp/product/B01G7BE9WK/ > > > > > > with https://github.com/merbanan/rtl_433 and an rtlsdr (like this: > > > https://www.amazon.com/gp/product/B00VZ1AWQA/) to decode. > > > > > > For improved accuracy, I calibrated the sensors in an icebath (in > > > several layers of ziplock bags and desiccant) for 0 degrees offset. > > > . > > > > > > These are good sensors with great battery life. I also have ThermPro > > sensors, they work, but the ones Russell lists are better, smaller and much > > longer battery life. > > > > I do not calibrate them. I care about being comfortable and saving power > > rather than worry about whether the temperature reading is 0.2-0.3 degrees > > different from absolutely correct value. > > Fwiw, I didn't calibrate mine for a long time, but mine aren't so much > for automated control, they were for understanding the temperature > environments, and in particular differences and/or gradients in > various microclimates. I noticed that a particular outside location > seemed to get down to freezing before the other sensors. The > significance of small temperature differences increases the closer you > are to freezing, for example. Eventually, I just wanted to understand > whether the difference was due to the sensor or the microenvironment > it happened to be in. Unless vigorously stirred, there can be > significant temperature differences over very short distances, due to > heat sources, stratification, illumination, etc. > > I'd really like to have a lab grade temperature sensor, accurate to > 0.01°C, to actually calibrate against. I encountered sensors when I > worked in Oceanography with that kind of precision, but they were > designed for water temperature and also were several thousand dollars. > I wouldn't like to have one *that* much. Most consumer grade sensors > only claim ±1°C. >
I was going to comment about - what atmospheric pressure did you calibrate your sensors at - then I had second thought thinking about relatively high energy needed/stored in H2O phase change which is connected to low dV (V-volume) on the opposite sides of liquid/solid phase. H2O is very interesting substance indeed. Anyway, I checked the theory, so I would not fool myself with the other smart people here: This is because of the Clausius-Clapeyron equation dlogT / dlogP = (P dV) / L where T is the temperature of the phase transition, dV is the change in volume, and L is the latent heat. The water/gas transition has an enormous dV because gas is much less dense than water, so dT/dP is large. The water/ice transition has a dV about 10^-3, so dT/dP is small. There is some 'cost' L to be paid doing the phase transition, most of it is paid by thermal energy. If the volume changes during the transition, the P dV work can help lowering the necessary temperature. So, it makes sense that dT/dP depends on the ratio of these two contributions. https://en.wikipedia.org/wiki/File:Phase_diagram_of_water.svg So, as a consequence - water boils at low temperature on Mt. Everest or in space, but ice cream is about as difficult to make at low/high pressures (unless going above 1GPa+ (maybe there is a lot of ice(cream) in the middle of sun/black-hole!) Happy weekend, it should be ideal for temperature observations (with ice-cream) -T
