Does humidity make powder heavier?

[Saulnyiya]

Hello to all , just curious I was loading some 9mm rounds and I noticed my throws were consistently heavier than other times. Same powder, same powder dispenser and rotor " RCBS little dandy". With it being a little humid today? Any thought? Thanks all !

[JMP]

It is possible that some H2O can bond to the powder, but I highly doubt it would be the result of loading in one humid day. More likely than not, it is your metering. If you are using a spherical propellant, the weights should be very close.

[ExtremeX]

If you didn't touch the thrower... you could be seeing your scale drift. What type of scale are you using?

If it is thermal drift... its good to have a set of check weights like this laying around... even a cheap-o set like these are better than nothing.

https://www.amazon.com/American-Weigh-Scales-Calibration-WGHTKIT/dp/B003STEJAC/ref=sr_1_2?ie=UTF8&qid=1467192665&sr=8-2&keywords=check+weights

In MS Excel I created a conversion table from grams to grains, and the OIML Class M2 tolerances for those weights in grains. This is a good quick spot check to make sure your scales are consistent from day to day.

If you have a friend with an Analytical balance, you can check the check weights... but that's a whole new level of OCD.

[TKM]

Many manufacturers store their powder underwater. Humidity shouldn't have a noticeable effect.

[Saulnyiya]

I'm using 505 Rcbs the was not adjusted and it did look like drifting. I'm pretty new to reloading,this was the first time this happened and on a humid day. The other times it was more steady beam and about three tenths less in weight. Thanks

[Saulnyiya]

Oh, I use HP38

[Pardini]

It can. So can temp extremes.

[ExtremeX]

I'm using 505 Rcbs the was not adjusted and it did look like drifting. I'm pretty new to reloading,this was the first time this happened and on a humid day. The other times it was more steady beam and about three tenths less in weight. Thanks

The 505 is a good scale but without a set of check weights I mentioned above you have no baseline for comparison...

It could have simply been user error or some dust or debris on the scale. They are sensitive to position and leveling.

[ar15barrels]

Hello to all , just curious I was loading some 9mm rounds and I noticed my throws were consistently heavier than other times. Same powder, same powder dispenser and rotor " RCBS little dandy". With it being a little humid today? Any thought? Thanks all !

Single base powders are hygroscopic.
That means they attract water and would get heavier in higher humidity when measured by volume.

HP38 should be a double base powder though.