Humidity Control
Duane Degn
Posts: 10,588
I'm taking time out from robots and servo demos to work at my day job.
I need to run some experiments to test the effect of humidity on some samples I'm testing.
I don't think I'll need to lower the humidity in these experiments. It's winter here in Idaho and the air is pretty dry. I'm planning on running these experiments in a toaster oven. I can control the temperature of the toaster oven pretty well and I'd also like to be able to control the humidity.
I don't need temperatures that are very high in these experiments. 150 degrees Fahrenheit (~66C) will likely be the maximum temperature needed. I figure at these relatively low temperatures, I could place other equipment inside the oven.
One idea I have for controlling humidity if to use a container of water with a servo that can open and close its lid. When the humidity is too low, the servo would open the lid. Once the desired humidity is reached the lid would close.
I'm not sure if I'll need to add something to increase the surface area of water exposed to the air inside the oven. I've wondered about having a paper or cloth towel that is attached to the inside of the lid so as the lid is raised the towel is pulled up into the air.
Other ideas which I don't like as much include:
Dripping water on the oven's heating element.
Use some sort of misting spray bottle to spray a mist of water inside the oven.
Use an external tea pot and divert the steam into the oven.
As I think about these other ideas, the container of water with a servo controlled lid is looking better.
Anyone done something like this? Any other ideas?
I'll be use a Propeller as the brains in this project. I plan to log temperature and humidity to a SD card.
I'll be using a Sensirion Temperature/Humidity Sensor to monitor the humidity and temperature. I'll also be using a thermocouple to monitor (and control) temperature.
I also have a couple of these humidity sensors that I probably wont be using since the Sensirion is working so well and I don't want to spend the time required to write code to calibrate and monitor these other sensors (I'll likely do this in the near future though).
I need to run some experiments to test the effect of humidity on some samples I'm testing.
I don't think I'll need to lower the humidity in these experiments. It's winter here in Idaho and the air is pretty dry. I'm planning on running these experiments in a toaster oven. I can control the temperature of the toaster oven pretty well and I'd also like to be able to control the humidity.
I don't need temperatures that are very high in these experiments. 150 degrees Fahrenheit (~66C) will likely be the maximum temperature needed. I figure at these relatively low temperatures, I could place other equipment inside the oven.
One idea I have for controlling humidity if to use a container of water with a servo that can open and close its lid. When the humidity is too low, the servo would open the lid. Once the desired humidity is reached the lid would close.
I'm not sure if I'll need to add something to increase the surface area of water exposed to the air inside the oven. I've wondered about having a paper or cloth towel that is attached to the inside of the lid so as the lid is raised the towel is pulled up into the air.
Other ideas which I don't like as much include:
Dripping water on the oven's heating element.
Use some sort of misting spray bottle to spray a mist of water inside the oven.
Use an external tea pot and divert the steam into the oven.
As I think about these other ideas, the container of water with a servo controlled lid is looking better.
Anyone done something like this? Any other ideas?
I'll be use a Propeller as the brains in this project. I plan to log temperature and humidity to a SD card.
I'll be using a Sensirion Temperature/Humidity Sensor to monitor the humidity and temperature. I'll also be using a thermocouple to monitor (and control) temperature.
I also have a couple of these humidity sensors that I probably wont be using since the Sensirion is working so well and I don't want to spend the time required to write code to calibrate and monitor these other sensors (I'll likely do this in the near future though).
Comments
Classic!!
{raises hand as guilty to above charge}
Edit: I recall the "Chick-U-Bator" kid's incubator, which was a light bulb inside a plastic housing, above a water reservoir. So the light bulb heats the water, too. You might consider that, basically a puddle carburetor.
I have seen PM articles about making a low-temp heating chamber out of a styrofoam cooler using a light bulb. Don't exceed the critical temperature!
The oven is supposed to be a "convection" oven so it likely has some sort of fan to circulate the air. It's a pretty small oven so there's not a lot of air to circulate. I think the largest internal dimension is about 14 inches.
I can control the temperature pretty well with a thermocouple and relay (I just switched to a SSR, I'm going to miss the clicking).
It will be interesting to see how much lag there is to the humidity. I don't need high precision here. I'm trying to replicate a hot summer day with some possible green house effects from having the material in a closed environment. I'm trying to come up with a "worst case scenario" for material that may be problematic in hot humid places. I want to be able to provide my client with data to show how bad the conditions got (with the material hopefully maintaining its desired properties). Or at least let my client know what the conditions were when the material lost the "desired property".
Sorry, but I need to remain vague about what the "desired properties" are and what the material being tested is. Which is too bad because I'd really like some help coming up with a way to quantify the "desired property".
Where's the fun in that?
How about 2 independent heating elements & controls? One small element warms a small container of water to a reasonable temperature. Small opening on top minimizes random evaporation. Humidity sensor feedback controls power to that heating element raise your humidity; but the element is too small to heat the entire chamber. The main heating element is strictly temp-based and supplements the smaller heater to control overall temp.
Thanks!
If it is not necessary to keep everything inside the toaster oven and providing you are willing to modify the toaster oven, I would pipe steam into the oven. For example, remove the whistle cap from a tea kettle and supply a suitable adapter for tubing and construct some type of butterfly valve. Boil the water to produce steam. If the humidity is suitable within the oven then route the steam to the atmosphere, otherwise route it into the oven.
Bruce
Here is another idea similar to erco's suggestion. Construct a hot plate from a Nichrome wire coil and a small aluminum plate with a perimeter. Make this hot plate small enough not to greatly affect overall temperature, but factor it in with your temperature control. Pipe water droplets to the hot plate to affect the humidity.
I might use a combination of erco's idea and your first idea by placing an electric tea pot (the bottom of the pot doesn't get hot) and route the steam into the oven. Rather than use a valve, I think it would be easier to just regulate the power going to the tea pot by turning on and off a relay. I've already done several AC relay projects so this method would probably be faster and easier for me.
BTW, your, right about my not minding modifying the oven. This is a "work" oven. No spousal aproval need to cut holes in it.
I'm marking this thread "Solved" for now and I'll let you all know what I end up doing and how it works out.
Thanks again for all the great ideas.
I have not used a Sensirion Temperature/Humidity Sensor but I would have to disagree with your comment:
Think of an air breather on a car. Within a lot of older vehicles there is a butterfly valve called a Thermac. I would construct something similar, but using a servo for valve control instead of temperature/vacuum. I would assume that the steam from a tea kettle should change the humidity very fast and the steam would have a force of it's own. So I would have two humidity sensors, one for high and one for low. When the humidity is low, crack the valve with the servo, and when it is high close it. I believe that regulating the power to the tea pot would be a waste of time. Just keep the power at a steady enough pace to produce steam and control that steam. Just my opinion.
Bruce
Whatever became of the update we were supposed to get?
Bruce
In all instances, to add humidity requires a reservoir of water. It can be either sprayed into the air under pressure or heated for dispersal.
To remove humidity, often we do something like toast socks on a fire, but that only works well on a specific object.
To remove humidity from a room, one can also drop the dew point via refrigeration and the water will drop out of the air. I run the A/C in my room at 27 degrees centigrate to keep the humidity at a low level AND to keep the temperature below the levels that slime mould and fungus seem to bloom. Adding heat to try to drive off humidity would be a disaster as the air outside is nearly saturated.
Fans may seem to be doing some dehumidifying, but again it is a localized removal of condensation. Generally, nothing is being pulled out of the air and actually more may be put into the air.
In the automotive contex, changes in pressure affect the dew point and the ablity of the air to hold moisture. Similarly compression and expansion situations should be observed closely.
Having said all than, it may help you decide the best approach to both sensors and control.
I got busy building robots.
Thanks for the reminder (I think).
This was a lot harder than I had anticipated. I ended up using the kitchen oven (with custom temperature control) and an electric tea kettle place directly in the oven.
I'd get the oven up to the desired temperature and then turn on the tea pot while monitoring the humidity. It actually takes a lot of water to raise the relative humidity in hot air. There's a logarithmic relationship between RH and temperature.
After adding enough steam to get the humidity up, the steam itself would warm the air past the desired temperature.
If I tried it the other way around and raised the humidity first, then the as soon as I started adding heat the humidity (RH) would plummet.
I was eventually able to get close to the desired RH and temperature by alternating between adding steam and adding heat. It didn't take much of a temperature drop (opening the oven) to cause it to rain in the oven.
I would have liked to automated the whole system but the guy paying me to do it didn't initially think the humidity mattered (it was the most important factor) and didn't want to pay me the price to do the experiment correctly.
I did learn a lot doing all of this and I have some ideas on how to do it better in the future if I need to control humidity again. One thing I think would be important would be to have some sort of airlock to insert and remove samples. I'd also want to use some big rubber gloves, with the sleeves sealed to the chamber's walls, to manipulate items inside the oven/humidity chamber.
I think I'd probably use the guts of a toaster oven for temperature control and use something like the teapot for humidity control. I'd want to use a clear plastic box as the chamber so I could see what's going on inside the chamber. I didn't need very high temperatures, but I'd want to research the best plastics to use at elevated temperatures (about 150 F) and made the chamber out of heat tolerate clear plastic.
Such a humidity control box would be a really fun project but I don't have a need to justify the expense.
There are actually dehumidifiers that use a water spray to lower humidity.
The water needs to be cold enough to lower the air temperature below the dew point which causes water in the air to join with the water spray.
I attempted using the water spray technique once, on a hot humid day when I was at college, by running my apartment's shower on the coldest setting. While the cold mist felt good when I was near the shower, it made the humidity problem worse. Apparently the water wasn't cold enough to draw moisture from the room.
I would have never thought that was true.... Interesting
That's funny