I wrote this for some one I managed to interest in waterbed hatching. I tried to keep it brief, I'm not good at that!
I have struggled to find a good set temperature for water bed hatching. The instructions they come with are just generic still air incubator instructions, but even in a still air incubator the air is heated and the egg is surrounded by it. In a waterbed incubator the water bag is heated the egg gets its heat from the contact with the water bag so the bottom of the egg is close to the temperature of the water bag but the temperature of the top of the egg kind of depends on the insulation of the incubator and the blankets over the eggs. The temperature difference between the top of the egg and the bottom is much greater than I ever thought. For this hatch I bought a Braun ear thermometer that shows it really well. For instance I'm still setting this little incubator, the temperature at the top of the egg is 37.2, the temperature at the bottom is 39.1. I don't know if it is accurate to assume that the actual temperature inside the egg is the average of those two(38.1)..but that is what I'm doing to establish a set temperature and what I'm finding is in line with the only research I can find on water bed hatching.
https://www.researchgate.net/public...Rural_Poultry_Farmers_in_Bauchi_State_Nigeria
They recommend a set temperature of 39 for days 1-3, a temperature of 38 for days 4-12, a temperature of 37.6 for days 13-16 and a final hatch temperature of 37.
My only addition to that would be to insert a temperature of 38.5 for days 3-6.
Anyways until now I've used a non contact forehead thermometer to monitor the eggs. It's easy and fast and gives a good idea of what is happening. At first the shell temperature cools very quickly when you open the incubator but as the days go by it cools slower, by about day 6 the circulation is established and the egg shell temperature starts to become closer to the same temperature top and bottom, by day ten the eggshell temperatures are really starting to increase and you can start to identify dead eggs because their low temperature. There is much information out there about making use of egg shell temperatures, these non contact forehead thermometers just are not accurate enough but they do show the trends and they can help you to know when it is time to adjust the incubator.
The reason I'm telling you all this is because after a week the incubator kind of goes from heating the eggs to cooling. In a circulated air incubator many people just set the temp at 37.5 and leave it at that set until hatch. They might find some improvement by making use of egg shell temperatures but the incubator itself compensates...on day one air blowing at 37.5 warms the eggs, by day ten when the egg shell temperature has increased to over 37.5 the air circulating at 37.5 is cooler than the eggs so it actually cools them. With a waterbed incubator you need to compensate for the increase in heat that the embryos are producing by removing a blanket and by lowering the waterbed temperature.
The directions that come with these waterbed incubators say to set the temperature at 38. With my last hatch my old thermostat had drifted to the point where I could not get the waterbed past 38. I thought "It'll be fine, that is right where they are telling me to set it" but it wasn't...only 3 out of 22 eggs hatched and they hatched over a day late!
Anyways I use two blankets to start with. I weigh all the eggs before I set them and mark the weight on the egg. All the waterbed hatches I have done have been dry hatch but still I check the combined weight loss of 5 or six random eggs at 7 days and again later if I have any doubts. I adjust the temperature as time goes by pretty much using the values in the African study. I remove one blanket, probably about day 12 or 14 when the egg shell temperatures are starting to get high. On day 18 I throw a carrot or two in, put the blanket over the eggs and wait until day 21, I take the chicks out if they climb out onto the blanket, otherwise I try to wait and not to peek!
I try to get all the air out of the waterbed and try to make a nice wrinkle free bed, I tape the thermostat probe and the thermometer probe to the bag...just a piece of tape behind the sensor since I don't want to have to deal with tape or adhesive on the sensor after the hatch. I tape the end of the water bag to the wall of the incubator...I'd suggest not to use duct tape...it can be hard to remove after 21 days in the incubator! I use a piece of drawer liner over the bag, I feel like it keeps the probes secure and like it is better for chicks than the plastic bag but it does affect the direct contact between the bag and the egg so I don't know if it is a good idea. I turn the eggs three times a day but that is just a number that works for me.
I found this article on CO2 very interesting because he summarizes how a broody hen operates so well. Also his graph showing "Attentiveness" validates how I use the blankets. It is a short easy read.
https://www.petersime.com/expertise/understanding-the-role-of-co2-in-commercial-incubation
https://www.petersime.com/expertise/understanding-the-role-of-co2-in-commercial-incubation
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I'm sorry for writing a book and I hope I haven't made it too confusing, but I know it is really difficult to find good information about water bed hatching, but I'm still learning and a lot of it is by trial and error!