Humidity and Increasing Mass??

OK, this might be a weird question, and I don't know if anyone will know the answer.

Also, not sure this is the right area in the forum, but since it's something I've been doing while building, I decided to post it here.

So, I've become kind of obsessed with figuring out where the weight in my rockets is coming from. Partly because they've gotten heavier since I started. It's mostly paint, but I'm trying to see where else it comes from.

So, I weighed all parts of an Estes Hi Flier XL. Then I glued the fins on with wood glue and weighed after they had dried. The whole rocket gained about 0.5 gram, starting at 85.2 grams before glue, and up to 85.7 grams after.



I'll say that when I use my scale, which is graduated down to 0.1 gram, I do need to check the calibration with a 100g weight and sometimes re-tare, as it sometimes gains 0.1 to 0.2g when I first turn it on. Other than that, it's pretty accurate.

OK, so it's at 85.7 grams. Then I decide to weigh the fillets - three layers of Titebond No Run No Drip Molding and Trim Glue on all fins and the launch lug.

Once dry, the rocket weighed 89.5 grams, an increase of 3.8 grams.

A few hours later, I decided to weigh it again, in case the glue had lost some more water weight - though it seemed completely dry the first time.

In fact, the mass increased by 0.2 gram, to 89.7 grams.

OK, my scale can sometimes fluctuate a bit. So I re-tared several times, checked the calibration, and it stayed at 89.7 grams.

OK, so now I'm thinking that maybe my measurement was inaccurate the first time.

A few more hours later, and I check again. Now the rocket weighs 90.1 grams!

It's not that the measurement has gone up and down, as it might if the scale were merely acting up. And the 100 gram weight always weighed in at 100 grams. I haven't added a thing to this rocket since finishing the fillets - they've only been drying.

The only thing I can think of is that the humidity has climbed a bit today.

OK, I know this is kind of... obsessive. Maybe most people would say "hey, it's off by half a gram - who cares?"

But I want to know that my scale is accurate, and I'm trying to track these things for my own edification, and because I'm trying to see if I have some rocket blog posts I can get from the info I collect.

So, does it sound reasonable that the rocket - a paper model rocket kit with balsa fins, which has not yet been painted - might have taken on some ambient moisture and that's why it's gained a half a gram in the last few hours?

Or do I need a new scale?

(Also, why am I having trouble uploading pictures to this thread? Am I losing my marbles or did the "Go advanced" feature disappear?

Oh, there it is...

Let me post the pictures I was referring to...

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[Edit: I guess I haven't started a thread in so long, I forgot how to do this... These are all out of order, but at least you can see them.]

And now, an hour and a half later, the rocket has gained another 0.2 grams of mass, making it a total gain of nearly a gram - 0.8 grams - since this afternoon, with me doing nothing to the rocket.

I'm not a competition builder, but I guess this is frustrating because I just wanted to see how much mass each component added, which should be a simple matter. But since the fillets have dried, my rocket keeps magically gaining weight.

The forecast has the humidity taking a nose dive overnight. I guess I might know better tomorrow what the answer is. It is an interesting idea, however, that a rocket's mass can change with the weather. I knew that drag could go up or down with temperature (and I assume with humidity, but I haven't read about that). I have not heard of a paper rocket changing mass spontaneously in response to the weather.

The changes are small, to be sure. But, if one were doing competition rocketry, that might make some difference, I suppose. For my purposes, it's thwarting my ability to accurately track how much each bit is adding. Maybe I need a less sensitive scale.

I can't see any of the photos.

I would guess humidity is the problem.

Zeus-cat said:

I can't see any of the photos.

I would guess humidity is the problem.

Click to expand...

Yeah, I don't know why I was unable to attach them correctly.

It's up to 90.4 grams now BTW. That's nearly a whole gram increase.

Wood glue gets lighter as it dries, put exactly one wet gram on some tared foil to make sure. CA cures by water, so maybe it gets a tad heavier?

Perhaps your build materials were extremely dry and -those- are sucking water?

dhbarr said:

Wood glue gets lighter as it dries, put exactly one wet gram on some tared foil to make sure. CA cures by water, so maybe it gets a tad heavier?

Perhaps your build materials were extremely dry and -those- are sucking water?

Click to expand...

I suspect it's the building materials - the paper tube and balsa. There's very little CA on this rocket - just a ring around both ends of the tube - and it's already cured.

Humidity is down to about 48% this morning, and the rocket has already lost 0.4 gram - half the weight it mysteriously gained last night.

I know this is a very small amount of weight, and not a real concern, but I've never heard of model rockets fluctuating with weather like that.

Don't 'TARE' your scale to get it to '0'. Use the zero button to get a '0' weight.

'TARE' removes any added weight (like a bucket or container), and sets that as the 0 point. So, if you continually 'TARE' your scale, the 0 point is jumping around..

Most scales will fluctuate a little when weight is added, then removed; the hysteresis of the load cell / strain gauge isn't linear.. (It won't always return to the exact same '0' point, or the same 'weight' point..)

Depending on how you put the object on the scale can also influence the weight reading. One (or two) corners might read the weight differently than the others.. Put the test weight on each corner to see if the readings are all the same. You might find one corner is over / under what you expect. (We would always to a corner test when we calibrated our scales, and we had tricks to fix corner errors..)

Some quality scales will have a 'center of 0' feature to show you you're in the middle of '0'. Remember the graduations of the scale are set, and as long as the indication of the load cell in within that graduation interval, it'll show that number. So, if it's right at one end of the graduation interval, a hair will push it over to the next graduation; the hair can register as being .01g.. becausehte '0' reading was at eth upper end of the '0' graduation interval.

Some scales will 'drift'. The weight or the '0' point will drift over time, with heat, expansion, the lunar phase, etc..

Air pressure* can have an effect as well, as well as measuring under an air vent. Altitude as well. if you measure in the basement, then weight it on the 4th floor, you might see a difference of .01g

Put the scale on a flat surface, away from fans, vents, open doors, etc.. Measure at the same time of day, keep other environmental conditions the same. And for really precise work, consider a cover for the scale (put the item on the scale, then cover the whole things with, say, an aquarium, and let the item sit for a while to stabalize.)



* (I did a lot of work on the U-Scan self check out system. One store was constantly complaining that the 'security device' (a scale) was constantly in error. it turns out they installed the unit under an air conditioning vent, so the cold air bombarded the platter causing weight violations. it's now in their documentation not to install these things under vents. Another store also complained that the 'security device' was also constantly in error. it turns out the location (TX I believe) and the unit was installed right in front of the main door. So, when the door opened, you got a the gust of air. The sudden change in air pressure from the door opening, caused the air under the 'security device' to 'push' the platter up, giving weight violations. So, now in their documentations, they have to be a certain distance from doors..)

dr wogz said:

Don't 'TARE' your scale to get it to '0'. Use the zero button to get a '0' weight.

'TARE' removes any added weight (like a bucket or container), and sets that as the 0 point. So, if you continually 'TARE' your scale, the 0 point is jumping around..

Click to expand...

Well, I don't know if you can see the photos in the original post, but there is no zero on this scale. "Tare" is the only option, and I believe the instructions (which I can't find at the moment) say to use it if the screen isn't reading zero. In any case, it does re-zero the scale, whether it drifts upwards or downwards (showing a negative weight when there's nothing on it).