In fact I ordered this item but have not received it yet. So on what base am I writing this review? Well, first I read other reviews and noticed the problem that the top tier is too cool. Then, I thought about how cool it can be based on some thermal physics: Heat transfer through conduction (meaning by contact, imagine putting your finger on a hot pot), radiation (without contact, imagine what you feel when putting your finger 1 inch away from a hot pot but do not touch it), and convection (heat carried by wind). To solve the problem I looked up heat conductivity coefficients and the relevant material is glass (as in the jar), air, milk (or water), and plastic (as in the yogurt maker cover). Air is pretty low, 0.024W/K/m, plastic is between 0.2 and 0.4, water or milk is 0.5, and glass is 1.0. I also needed the power of the yogurt maker which is 13 watts for YM80, and I know this power is primarily used to overcome heat loss. A little calculation tells me that heat loss by radiation is very little, at most a couple of watts from the heated base which has an area of about 0.04m^2, so most of the heat is lost due to conduction, i.e. contact between the yogurt maker's plastic cover and the ambient air in the kitchen. Further calculation of the temperature gradient tells me that the air inside the maker and just below the plastic cover can't get very hot, probably only 1 F below the ambient temperature. So the average temperature of the air below the cover is about half way between the temperature of the heated bottom and the temperature of the kitchen. Assuming the bottom is kept at 110F, the average temperature of the air inside the maker is about 95F if you keep the kitchen at 80F, or 90F if the kitchen is at 70F, and so on. When you use only the 7 jars in the bottom tier, the milk inside is heated by conduction from contacting both the heated bottom and the air. Since the heated bottom is at 110F and higher than the air, the bottom tier's milk will no doubt be kept at 110F. However, for a 2nd tier, the glass jars are not in contact with the heated bottom and thus are heated only by conduction from the air, thus eventually will reach the average air temperature inside the maker. As calculated above, depending on how cool the kitchen is, the top tier could reach the incubation temperature (I read 95F is the lower limit), or could be below 90F if your kitchen is cold, so won't be good enough for making yogurt. An additional factor is how warm the milk is when you put it in. If it was slightly above 110F, it will slowly cool to 95F (or 90F, or whatever) and may provide long enough time for the yogurt to develop. But if you start from milk cooler than 95F and hope the maker will heat it up, then you probably will find complete failure in the top tier. Of course, the above calculation is very rough but might already be good enough to explain why some people had great success and some had complete failure with the top tier. Assuming the calculation is correct, how to avoid the problem? Below are just a few thoughts: Method 1) Start from the warmest temperature possible for the milk, so the milk will cool slowly but may allow enough time for the culture to multiply; Method 2) Covering the maker with a towel: the idea is to reduce the thermal conductivity of the plastic cover, hence reduce the heat loss and keep the inside temperature warm. But one has to use a pretty good towel/wrapper, since you have to reduce the thermal conductivity of the plastic cover (0.2-0.4) to a value close to air (0.024) to increase the air temperature inside. Perhaps making a yogurt maker "cozy" with quilted material will work; Method 3) switching top and bottom jars mid-way. Well, basically you are changing the temperature of the two tiers. This might work, but is very subtle and the taste of the two tiers may still be different depending on how culture develops at low temperature in the top tier. Method 4) if you are only interested in making more yogurt at one time, cutting out the middle of this top tier and fit tall mason jars might be for you (as described by another reviewer). In this case, all tall mason jars will be in direct contact with the heated bottom so there won't be problem of uneven temperature. Of course, cutting the tier with a saw may not be what you want to do. Maybe put tall mason jars on the heated bottom and just cover the whole thing with a cardboard box, an upside-down thermal cooler, or whatever (this is how many people make yogurt without yogurt makers, just put the mixed milk next to a bottle of hot water and put the whole thing in a thermal cooler). - I am going to try this first. Hope this helps. Will post updates if I find something wrong with my own analysis. Update: Got the tier yesterday and tried overnight with a 7-cup glass container on the bottom level and 3x 7oz jars on the top. Measured the milk temperature 4 hours into incubation: the bottom milk was at 105F and the top was about 97F. The top milk seems to have set well this morning. So the estimation seems to be accurate and the tier worked for me. Will have to see what happens in the winter when the kitchen is much cooler.
