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Left panel shows photograph of part of the apparatus inside the large insulated cardboard box. Visible are several of the jars including the one currently containing the six sensors.
A different salt solution is in each jar and the lid can simply be swapped onto each in turn. The samples, electronics and fans are all in the top half of the box.
A grille is just visible in this image. The heater is beneath that, in the lower half of the box. Right panel is an overview schematic of the whole apparatus.
Apologies for my art work. I am much better a data plots. I have not invented this technique. It is a well established method of controlling humidity, used in many fields.
An extensive literature on this topic allows you to select a particular salt to generate any humidity to match your requirements.
My literature survey yielded in excess of 60 chemicals previously used for this purpose and we need to select a suitable subset for this experiment.
The humidity above solutions is to some degree temperature dependent because the salt's solubility is temperature dependent. Some chemicals are of course poisonous, flammable, reactive, unstable or dangerous in a variety of ways.
I can then interpolate and get a precise value for the temperature at which I obtained the reading. I rejected those which showed strong thermal coefficients.
By contrast, solubility of sodium chloride NaCl, common kitchen salt is barely effected over that temperature range and the expected RH varies only from Despite being so commonplace, sodium chloride is an outstanding candidate for this experiment.
Except for the very most extreme cases e. All were checked against databases of NFPA and EU Dangerous Substances Directive classifications to select only those I considered 'relatively safe to handle in a domestic setting' and did not pose significant disposal challenges.
Many of the compounds selected are common garden fertilizers which makes disposal at the end of the experiment relatively easy. Another interesting case is potassium chloride KCl.
This is approved as a low sodium table salt substitute in food, is commonly used medicinally and certainly seems to fulfill my 'relatively safe to handle' requirement.
It is also used as the lethal injection for executions in the USA. Obviously you were not actually going to inject yourself with any of these chemicals, but I mention all this as a caution to think carefully about what 'relatively safe to handle' means xkcd Context and sensible precautions are everything.
Sodium hydroxide NaOH is an everyday household chemical drain cleaner but is extremely corrosive and should be treated with utmost respect; goggles and gloves at all times.
I read manufacturers' MSDS documents for specific handling guidelines and rejected anything vaguely scary or photo-sensitive.
By now the list was down to about twenty compounds, about half of which proved difficult to buy cheaply in small quantities, so that defined my sample.
In reality I was primarily constrained by safety and availability. Final sample is shown in figure 2 , right panel. Where data for a particular salt are available from multiple authors I have taken a simple average rather than choosing any one as absolutely correct.
The literature contains humidity measurements for a considerable selection of chemicals. The selected sample of chemicals actually used in the current experiment.
Chemicals were bought where I could find them fairly cheaply. Several were only "agricultural grade" and probably not very pure which could affect accuracy of my results.
Mixtures of different salts will generally yield a lower humidity than either of the two pure components. The NaCl was "food grade"; simply non-iodized table salt.
In Figure 2, Ammonium nitrate stands out as being the only one with a significant temperature coefficient. Calcium chloride also has a strong temperature coefficient and was used for a while.
It appears in some plots but was abandoned since it has no obvious advantage over magnesium chloride and the published reference values seemed ill determined.
All solutions were made with distilled water though I doubt tap water would have any real effect for the precision achieved here. This data point will be of varying use depending on individual devices.
Their internal firmware caps all output at Since most of my sensors seem to read marginally high, they hit One of my sensors B reads slightly low at very high humidity so that does generate a usable value for pure water.
As I say above, for most of my devices it was worthless anyway since they already read I experimented with dry calcium chloride dihydride for zero humidity.
This will not generate a true zero. Using anhydrous calcium chloride ought to improve this to 0. I also tried silica gel beads but found the small quantity I had to hand was ineffective.
Again taking values from "Drying Agents" Merck, , under ideal conditions silica gel could achieve 0.
In practice the sensor's firmware appears to be capped at a minimum output value of 1. I returned to calcium choride at the end.
Instead of using the CaCl as a reference, I used the calibrated sensors to measure the air above dry, crystaline CaCl.
I do think that tells us much about CaCl since I have no way to assess how well dehydrated the CaCl really was. Given that I did not achieve the 1.
It takes time for the vapour to equilibrate. The sensors were sealed in the test vessel with each chemical for between three hours and three days.
Data for my analysis are typically averages of 12—24 hours of logged values. The smaller the test vessel air volume and the larger the surface area of the solution, the quicker equilibrium is likely to be achieved.
My experience is that vapour equilibrium was generally established in just a couple of hours as long as I had the fan running.
Achieving stable thermal equilibrium was much more difficult and that is also easier for a smaller, simpler test vessel.
The single biggest source of error I was able to identify was temperature and temperature stability. There are at least three different, competing, temperature dependent effects.
As shown above Figure 2 , the expected RH depends weakly on temperature. The salts used were selected to exhibit smaller than average temperature coefficients.
My hypothesis had been that choosing solutions which were not temperature sensitive would minimize my results' sensitivity to temperature variation.
In practice this proved fairly unimportant because it is easy to calibrate out and other sources of error dominate. Including a temperature sensitive salt does however provide a good illustration that the experimental procedure works as expected.
Figure 3 shows the measured and predicted humidity as a function of temperature for three solutions; potassium iodide, ammonium nitrate and sodium bromide.
Ammonium nitrate is expected to have a much stronger thermal coefficient and the effect is obvious in this plot. All three data sets are slightly offset from the reference curves and the slope of the ammonium nitrate data does not perfectly match expectation, but it is clear that the hygrometer is responding to genuine, small humidity changes as the ammonium nitrate solubility changes with temperature.
This plot is very important and greatly increases my confidence that the test cells are properly equilibrating at the set temperature.
Lines are the reference curves from Figure 2. Ammonium nitrate exhibits the expected strong temperature effect.
More dramatic is the way the atmosphere as opposed to the salt solution responds to temperature change. Relative humidity is a strong function of air temperature.
For a given absolute humidity i. Initially one might expect the error to correlate to temperature, but for a sealed, air-tight test vessels this particular source of error correlates to rate of change of temperature or temperature gradients within the test cell.
We are using the natural equilibrium which the solution, vapour and air are continuously trying to achieve.
If the temperature is rising then the air is continuously being able to support more vapour, the solution will be evaporating to provide that and the measured humidity will always be lower than expected because it is always trying to catch up.
Similarly when the temperature is falling, the vapour will be condensing back into the solution but is likely to always be lagging the equilibrium target and the measured RH will be higher than the calibration value we are trying to achieve.
Only when the temperature remains stable for a period of time will the equilibrium be established. This effect was clearly visible in my logged data for the version 1 test cell which did not have active thermal control Figure 4.
Data from the version 1, non-thermally-regulated test cell. Over-plotted is the air temperature. When the air is cooling the sensors all give higher than average values.
When warming up, the RH measurements are all low. Note the correlation is with temperature change, not temperature. Just after both the maximum and minimum temperatures, the RH measurements are very nearly the same.
These are the times when the test cell has dwelled at a near constant temperature. Serendipitously this provided a criterion to determine when the system has equilibrated.
A measurement was only taken when two consecutive maxima and minima yielded consistent results; typically 36 hours to four days after sealing the vessel.
This problem has largely been eliminated through active thermal control and better air circulation. From Wikipedia, the free encyclopedia.
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Nu är det bara tre blommor. Hoppas att ni alla har underbara dagar när till slut vintern släppte taget. Man kan ju bara hoppas.
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Här är första sittlappen virkad och nästa steg är Jag väntar till jag har ett par stycken. Mannen min lovprisar sin sittlapp som han har i skogen.