The right temperature is crucial to hatching success. When I first began incubation, in my endeavour to get the "right" temperature, I must have bought over 2 dozens thermometers. I have most types available in the market - mercury, alcohol, digital to 2 decimal points, digital to one decimal point, those dial type with a probe that you stick into the incubator, you name it, I bought it. I ordered them from all over, from Japan, UK and from the States.
Most of the thermometers are on the shelf now as I have discovered that the kind of thermometer available for normal incubation use in the market is not accurate and that what is important is the thermometer's ability to read changes to the temperature accurately, not so much the right temperature. Let me explain. I have discovered that for each incubator, we must have one or two trial runs to arrive at the right temperature as measured by a particular thermometer. Say I have a new forced air incubator and a new thermometer. I would set it as recommended at say 99.5F. If the trial batch of eggs pips internally at Day 20, I will run another batch at say, 99.7F. If the second batch then pips internally at Day 18 or Day 19, I will then mark that thermometer and that incubator as 99.7F. Thus I will have an incubator which will read 99.5, another at 99.6 and yet another 99.9. But if I were to use a scientific thermometer, chances are all will be measuring 99.5F.
B. Introduction to the WLM
There is very little literature on the practical application of the Weight-Loss Method. Books generally agree that an egg must lose up to 13.5% of its weight optimally. From experience, I have found that the range of weight-loss is fairly wide, from 10.5% to 18%. In this range, if the temperature is right, the embryo will hatch. I have found that embryos can withstand too much weight-loss better than too little. Eggs losing about 9% and less will have a low rate of hatching. Eggs losing up to 25% of their weight have hatched successfully. This may account for the reported high rate of hatching using the Dry Method since the dry method would cause more weight-loss than otherwise. I weigh eggs on the day I set the egg, marking that day as Day 0. I target the egg to lose 13.5% of its weight by internal pip ("IP") day, which generally is Day 18 or Day 19.
C. Methods of Weighing
I have only used two methods. The first method is to weigh each individual egg. You do this for expensive or valuable eggs. I do it for serama eggs. I use a scale, which can measure up to one decimal point, and I prefer a metric scale. Since we are really measuring the weight of an egg against itself, no conversion of measurement is required. Thus a metric scale is preferable because of the smaller units available. You are keeping track of how many units of weight it has loss, so the smaller the units available the better. Some of my serama eggs weight a mere 12gms and losses of weight per day can be as little as 0.10gms. That's 0.42oz and 0.004oz respectively. A full size chicken egg of say, 48gms, losing about 0.36gms per day, is 1.69oz and 0.01oz respectively. You can see that to obtain a scale measuring in ounces to that accuracy will be expensive.
The second method that I have used is to measure in groups. I will place 10 eggs or 20 eggs and then average them out. Example, I will place 10 eggs measuring a total of 485.3 gms and work out the weight loss required per day which will be 3.6gms, or 17.12oz and 0.13oz respectively. Work based on the average for one egg ie, divide the preceding 485.3gms by 10 eggs to arrive at the average weight per egg. This way, when you toss out two or three eggs due to infertility, you just have to divide the total weight by the number of eggs left. If your scale cannot measure to that accuracy, you can increase the number of eggs to 15 or 20 numbers. Your scale need not be accurate in absolute terms. What is important is that it can measure changes to the weight of the eggs being measured. We are applying the same principle as that being applied to the thermometers in my opening paragraph. This is a pragmatic solution to the question of costs for small farmers. Note that the larger the number of eggs you measure at one go, the greater the chances of inaccuracies coming in, and the lower the rate of hatching.