Even with the same scale, you may not get an accurate accounting of your weight loss. I agree that sticking to the same scale is your best bet to see progress, but you are trying to measure a system that is constantly changing - both your body as well as how the scale measures it. Changing the time of day that you weigh, the way you distribute your weight on the scale, limitations of the accuracy of the scale (especially if you change weight dramatically) and a thousand other variables determine what number shows up.
At any rate, there is quite a bit of information on this site (that I highly recommend you read) that explains why the scale might not be the best measure of success. In my mind, the biggest reason to avoid using the scale is that you have no direct control over it.
You may find much greater success in focusing on the things you can control (calories and exercise) while using several measurements to determine success.
My personal measure of success is calorie differential over a one-week period. It is something I can control directly through what I eat and how much exercise I do. When I stay focused on maintaining my target differential, the weight (averaged over time) as well as other indicators of overall health does what I expect. The nice thing about spreading my goal across a week is that it lets me lose a daily battle or two without losing the war. I can shrug it off and hit the exercise a little harder the next day. Even if I have a bad week, I can look at my differential over a month and see progress.
| Pounds lost: 53.6