Pretty much what the title says. My scale is quite old. I’ve been weighing myself regularly, as I am on a diet. The results are getting weird, this morning I supposedly weighed 3.8(1.7 kg) pounds more then I did yesterday, although I’ve eaten very little. Should I look into getting a new scale?
YES! But also realize the trending of your weight is more important than any daily difference. A pint of pee (liquid) is around a pound.
From working 30 years in maintenance, I can assure you that any mechanical device will eventually wear out. Since you used 1/10 lbs I am assuming an electronic device. A scale in the bathroom is exposed to regular high moisture which attacks any connections. Corrosion on the boards. Electronics drift. Batteries die. Load cells do go bad. Anything on the floor also accumulates dust which gets into connections also. Plus dust will cover the board and decrease the efficiency of any heat sinks.
A fluctuation of 4 lbs in a day can be normal weight variation, due to water intake, bathroom habits, and sleep patterns. Also depending on the type of scale, the placement on the floor can change the reading by a few pounds. So, in general, trust the scale until you have a specific reason to distrust it. If you get on the scale 10 times in 10 minutes and it gives different readings then there’s something wrong with the scale. If it gives the same reading repeatedly then it’s more likely to be accurate.
Since you have precision of at least 0.2lbs, I’ll assume it’s digital and not a spring-based mechanical dial scale. Most electronic scales rely on load cells. These are chunks of metal that have foil strain gauges glued to them at highly engineered positions. The chunk of metal flexes when loaded and the electrical resistance of the gauge changes. The load cells are usually pretty conservative and doubling the rated load capacity is usually ok.
While it’s possible for metal fatigue to have occurred or the bonding compounds to have aged, I think you’d see much more dramatic errors. Weigh your self several times consecutively and make sure the thing repeats. Make sure the scale is on a firm surface and you stand in about the same position. You can also keep something fairly heavy but stable near the scale so you can weigh it as a calibration check. A 5 gallon bucket of sand or even water if sealed would work.
We had a mechanical scale back in 1976; I don’t know if it was some kind of “official” Bicentennial Scale, but the face was made up like the banner to, “Love, American Style.” This scale was not only off, it was progressively off–for a 50 lb child, it was off by 10 lbs. For a 100 lb person, it was off by 25 lbs. By the time a 200 lb person got on, it was off by +/- 75 lbs.
We have a digital scale and I am pretty sure that it has been programmed to give the same reading if the weight is +/- 1.5 pounds from the previous reading (within a time limit maybe?).
It measures to 2 tenths of a pound, but if you weight yourself (and it says 140.0 lbs), get off, then get on it again holding a one pound weight, it will say the same (140.0 lbs). However, if you then cycle it with a small weight, like 30 lbs, and then weight yourself again with the one pound weight, it will measure you accurately (141.0 lbs).
I assume if they did not do this, then slight variations in the measurement (due to floor level, people leaning to one side or the other) would make it give slightly different readings if people measured themselves twice in a row. That would make the scale seem “inaccurate” to the user, so they programmed it to give the same value if close in weight and time.
You could get a beam scale like the one in your doctor’s office. Pretty sure the one we used to have was accurate down to ounces.
One of my favorite interactions with the medical profession was a time I had a very early morning (like, 7 am or something obnoxious) appointment to see my g.p. Even though it was just him and a receptionist at that hour, we did the whole doctor visit procedure complete with a weigh-in, using his old beam scale.
I got on, and he started adjusting. And adjusting. And adjusting. He finally told me to get off, and he got on, and tried to weigh himself. After a few seconds of further wordless arguing with the thing, he stepped off and mumbled something to the effect of “that thing’s broken, let’s move on.”
They’ve used digital ever since they replaced it. 
Whose doctor’s office?
Mine has been digital at least since I started going there 15 years ago.
Digital (and non-digital) scales are only good if you calibrate them. I always wonder how often the scale at the doctor’s office is maintained, considering all the use it gets.
the older I get, the harder it is to find a scale that works correctly.
Yeah, I guess you are right. My doctor’s as well. But I think there is a beam scale in the locker room of the health club I use.
I guess I coulda just said “… a beam scale …”
My scales are inaccurate, the same way my metronomes fail to keep accurate time! 
For how most people use bathroom scales, the calibration, assuming it isn’t too far off, in some ways doesn’t matter. Say it was reading 1% low so that a 200lb person measured 198.5lb. As long as the scale repeats and is linear, most people (like OP) are interested in trends of weight change. A 2lb change from yesterday is the figure they’re after and you wouldn’t be able to see that 1% error. The absolute accurate weight isn’t usually too important for health and fitness. It would be different if you were, say, weighing a patient to determine very sensitive drug dosages.
I tried out a scale at Bed Bath & Beyond once, a seemingly simple mechanical scale, and every time I stepped on it and stepped off, it would “leave” five more pounds on the scale. So it started at 0lb, get on, get off, 5lb, get on, get off, 10lb, get on, get off, 15lb. It just wouldn’t zero out and kept ratcheting up every time. I tried another one sitting next to it, and the same thing happened. What a piece of garbage. It reminded me of the mechanical lamp timer that had little plastic pins you’d put around the dial to switch it on and off. That’s all well and good except the pins were so cheap and poorly secured to the dial that instead of the pin flipping the switch, the force of the pin against the switch would push the pin out of the dial. “You had ONE job!”
Nitpick - if it gives the same reading repeatedly then it is **precise **. You can’t determine accuracy unless you compare it to another known accurate scale.
I know that mine has broken. It consistently says that I am overweight when in my own mind, I am fine. Cheap garbage scale.
By fun coincidence, just today I saw a meme on Facebook that said something like “I moved my scale to the corner of the bathroom. And that’s where it’s going to stay until it apologizes to me.”