Intonation. How is it possible that over time, while playing, my tuning goes sharp rather than flat?

[deeptubes]

Are you initially tuning with the bass in your lap? If you are tuning with your bass with it sitting flat in your lap, it puts a slight amount of pressure on the neck, adding a just a touch of bow. When you remove the bass from the flat laying position that smidgen of bow is released, adding just a touch of tension to the strings and making them go slightly sharp. Always tune in the playing position.

When I was a newbie, I would often tune my bass while it was sitting in my lap. Would play, and things just sounded a little off. Checked my tuning, and it was sharp. Drove me nuts! Took me a little while to figure out what was going on.

 

[Rockin Mike]

I don't get the nut binding idea. If the string sticks at the nut, then later works through, that would cause the string to go flat, not sharp. Maybe I'm missing the point.

In any case, stringed instruments do not stay perfectly in tune. The laws of physics make sure of that. Temperature, humidity, and friction all cause variation in tuning. Just be glad you don't play harp.

Could work either way.

If you start with the pitch too high, and the tension equal on both sides of the nut,
then you tune down until the string is in tune on the fretboard side of the nut,
but there was binding at the nut, that would leave extra tension on the headstock side of the nut.
Later when you hear that little ping and the string goes sharp, that was the nut bind coming free and equalizing the tension - a little lower on the headstock side (which we don't care about) and a little higher on the fretboard side (leading us to wonder - how did it slip sharp?)

If you start with the pitch low and tune up, then a nut bind will cause it to slip flat when it comes free.

As has been noted elsewhere, this is a lot more common on guitars than basses.

 

[paparoof]

I know in my house it's a temperature thing. I play for a while, get up and go talk to the wife or take a leak or something, then go back and my tuning is sharp.

Deeptubes point above made me think about another tuning technique issue I've noticed (no idea if this applies to the OP or not). When I upgraded to the Turbo Tuner - it's so damned accurate - that I started noticing how just the slightest bit of pressure from my left hand (the one on the tuning peg) in any direction - up, down, sideways - flexes the neck and totally affects the tuning. You can watch it happening. So now when I'm tuning, I'm very consciously trying to make sure the only pressure from my left hand is turning the tuning peg and not flexing the neck in any direction.

This could be something for the OP to at least rule out - because if you tune in a certain position, bass in lap (like Deeptubes was talking about) or leaning your arm against the neck while you're tuning without noticing that you're doing it, your bass could look like it's in tune even though it's not. Then the next time you click the tuner on, you're sitting in a different position with different pressures in different directions and you notice your tuning is out.

 

[RED J]

Heat,man heat ! Play, play, play, neck warms up expands, skreeeeeek sharp note.

 

[mikeswals]

Heat,man heat ! Play, play, play, neck warms up expands, skreeeeeek sharp note.

Yup. Walk into a cold or unused practice room, get the body heat and energy going and the instrument expands...you find your tuning go sharp.
I grew up as a horn player in school. You bet all those outdoor performances give suprises once cold instruments warm up.