> On Sun, Jul 25, 2010 at 10:39 AM, Vasile Surducan wr= ote: > > Hi VG, one question please: how do you will control accurately both > > temperature and humidity in the same time? VG said > I have algorithms that can manage that pretty well. Also, tolerance is > wide enough. What Vasile is saying is "Y' canna break the laws of Physics" (as Scotty noted from time to time). In a completely closed system you CANNOT control both temperature and RH at once. Read on for what you can do, and how. For practical purposes, in a closed system there is a completely constrained relationship in a closed system between temperature, pressure, volume and Relative Humidity for a given chamber size, amount of air, and amount of water. Assuming that your other contents do not alter (and what eg plants do MAY or may not violate that constraint) then messuring eg just mean temperature will give you mean RH and controlling just temperature will control RH - not necessarioly in a manner that is desired. IF the system under consideration is not closed then you CAN control both temperature and RH. If it IS closed then you cannot. eg you may have a growing space in a semi sealed container inside a larger lab. The space can be ventilated with a fan from the lab and can be heated. Air circulation is provided by an internal fan. If the test box is too be held warmer than the outside lab then it can be heated and controlled. Heating the air will reduce the RH. If the RH in the box is to be lower than the lab then you can probably juggle in blown air and heating to achieve a desired RH and temperature mix (don't take that as Gospel.) You can reduce RH in the box with eg a Peltier cooler that reduces air below its dew point so that it loses water content. If you reintroduce the coiled dier air directly into the box then net temperature and RH will fall. If you reheat the air before reintroduction you can reduce box RH and lower or raise temperature depending on degree of reheat. If there IS no box and the lab is your whole environment then the above method works if you drain the condensed water to outside the lab. If instead you want to increase RH you can use steam or eg an ultrasonic fogger and water supply. Overall the heater, cooler, water drain and fogger will give you complete control. The system is not closed as you allow water to enter and leave and energy for heating and cooling. For a true otherwise sealed system you will need to vent your Peltier hot side to outside the system in some cases or at all times + more heating. BUT if you do not have these cross boundary linkages OR a system within a system (eg box inside lab) you cannot control both temperature and RH at once. But you can have years of harmless fun trying. _________ Have you found a source of 1-2577 in small quantity yet ? :-) (I have a 1 litre bottle thereof but am 'a bit far away'). Russell McMahon --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .