GemPro 250 issue

[sjoliat]

Has anyone ever had an issue where the material being weighed caused discrepancies? As the title says, using a Gempro 250, been satisfied with it to this point though it does drift slightly during sessions. Have a filter on the AC cord, it's been on for days, calibrated and use the pad on a sturdy table with scale leveled.

Today I decided to weigh some bullets and now I am totally confused. I started with some 140 hybrids and they averaged 144.68. Finding this odd I started checking every flavor of bullet I have - the hybrids, Juggernauts, and several types of SMKs. Within a type, they were all very consistent, but all of them weighed around 3.4% heavy. From a single 52 gr SMK to multiple 200 gr SMKs at a time, 3.4% heavy. Where it gets weird is when I put the 20 gram test weight on it reads 308.6 - 308.64 gr which is right where it should be. No matter what combinations I try bullets are 3.4% heavy and the test weight is right on.

Any thoughts as to what is happening?

 

[jlow]

Have you ever done the two calibration required for precision/accuracy?

Think of a xy graph with one single point on it at 50 grams which is what the included test weight is. Now, if the linear calibration is perfect, any weight you put on the scale that is less than 50 grams will fall perfectly on a line that goes from 50 to 0. On the other hand, if the linear calibration is off, that line will not go through zero but either above or below it despite the fact that a 50 gram weigh will still give you 50 gram reading. This is why two calibration weights (50 and 20 gram) are required to do a linear calibration.

 

[sjoliat]

I re-calibrate every session. Mine only came with a 20 gram test weight, I am going to get a 50 when I can. What I just don't understand is how 308 grains(20grams) of test weight are right on yet the same amount of weight in bullets weighs heavy.

 

[jlow]

First of all, you are not going to get just the right amount of bullets to come to exactly 20 grams which is 308.65grs.

The problem with this is if you take a look at this simulation where the blue line simulates a perfect calibration but the red line a calibration that is off you will see that if you put a 20 gram (308.65gr) weigh on the balance and it was off, it will give the proper reading. However a lighter weight than 308.65gr will give you a heavier weight but a weight heavier that 308.65gr will give and under weight value.

BTW, the reason the 20 gram weight is always dead on is because you always do the single point calibration with it.

 

[snakepit]

However a lighter weight than 308.65gr will give you a heavier weight but a weight heavier that 308.65gr will give and under weight value.

Now I know what the problem with my GemPro was before I got rid of it. It was weighing light when it should have been weighing heavy and heavy when it should have been weighing light.

 

[M-61]

Have you ever done the two calibration required for precision/accuracy?

Think of a xy graph with one single point on it at 50 grams which is what the included test weight is. Now, if the linear calibration is perfect, any weight you put on the scale that is less than 50 grams will fall perfectly on a line that goes from 50 to 0. On the other hand, if the linear calibration is off, that line will not go through zero but either above or below it despite the fact that a 50 gram weigh will still give you 50 gram reading. This is why two calibration weights (50 and 20 gram) are required to do a linear calibration.

Not to get off the point but is this the reason the RCBS Chargemaster has TWO test weights and a specific order in which they are BOTH used to calibrate?
Always wondered why TWO?

 

[gambleone]

My gempro would drift & keep counting down. About every 10 - 15 secs
I called the company & they went through all the trouble shooting problems
& I even bought a ferrite bead. In the end after calling the company 3 times,
they told me the drifting was a common problem. I had to wait about 6 wks
for the warranty dept to mail a replacement.

 

[jlow]

M-61, that is exactly why Chargemaster has two and yes, you have to use them in a specific order since the machine expect the high and low standard in a specific order. The machine use the two to get that linear calibration.

Yes, this is where Gempro went wrong which is they should have included the 50 gram calibration weight. I brought mine for around $20 from Grainger.

gambleone - your problem is something else all together.

 

[snakepit]

In the end after calling the company 3 times,
they told me the drifting was a common problem. I had to wait about 6 wks
for the warranty dept to mail a replacement.

I went through 2 GemPro 250's, my original one and the first replacement one. I could never get a reply from my phone call messages, emails or letters. Customer service at Old Will Knott Scales told me that was a common complaint about My Weigh and since I was over 30 days Old Will could not take it back. They said to just send it to My Weigh and they would send me a new one. After almost 3 months from when I started the return process a new one showed up. After a few months of good service this one began to act up so again with them not replying to any of my correspondence I sent it back. After about 2 months of hearing nothing I bought a FX120i. Latter on a new one did show up. I hope their customer service has improved, you said you talked to them after 3 times trying to call them and got a replacement after 6 weeks so you did better than I did. My experience with them was from over a year ago.

 

[sjoliat]

First of all, you are not going to get just the right amount of bullets to come to exactly 20 grams which is 308.65grs.

The problem with this is if you take a look at this simulation where the blue line simulates a perfect calibration but the red line a calibration that is off you will see that if you put a 20 gram (308.65gr) weigh on the balance and it was off, it will give the proper reading. However a lighter weight than 308.65gr will give you a heavier weight but a weight heavier that 308.65gr will give and under weight value.

BTW, the reason the 20 gram weight is always dead on is because you always do the single point calibration with it.

Referencing the graph, the situation I have is not a line with a different slope and intersect at 308.64 grains. I have a parallel line that is off by 3.3% except for the test weight which is dead on.

52gr SMK = 53.68 =+3.2%
2 - 155gr SMK palmas = 320.18 =+3.2%
4- 200gr SMKs = 826.94=+3.3%

So, lighter or heavier than test weight, by any amount, always reads ~3.3% heavy.
I really prefer digital to analog, but this makes me want to bang my head against a wall.

 

[Wow]

I have a gem pro 250 I also have a 20 & 50 gram weight in which order do I need to calibrate the heavy first or light weight first or does it matter. Thanks for any help. I've always just used the 20 gram weight and it doesn't drift to often.

 

[jlow]

The last time I put this on the board was 2 years ago so it is probably gone so here it is again. Follow the instructions to the letter and you will be OK.

Turn on the scale
Let it equilibrate for at least 30 min

To do the linear calibration do the following:

Hit the “Tare” button to zero the scale
Hit “Light”, then “Units” button each once in that order – the scale should now read “LinE”
Hit the “Tare” button again, the scale will now blink “0000” for a few seconds while it zeros itself
The display will change to “20.000” – this is your signal to put on the 20 gram weight
Leave it on until the display says “50.000” – this is your signal to take the 20 off and gently put on the 50
When the calibration is done, the display will show a steady "50.000" – your linear calibration is done.
Remove the 50 gram weight.

Follow this for the single point calibration do the following:

Hit the “Tare” button again to zero the scale
Hit the “Light” button once
Hit the “Tare” button once and the display will say “20.000”
Hit the “Tare” button one more time the display will say “0000” for a few seconds then “20.000”
This is your signal to put on the 20 gram weight
Wait a few seconds until the display stop blinking and say “20.000”
This means it has been calibrated.
Remove the 20 gram weight.

Now since the scale automatically change the units to grams to do calibration, you hit the “Units” button 4 times and it will change it back to grains – you are completely done.

 

[glockaholic]

Good information and calibration instructions.... thanks. Don't forget that calibration weights come in different classes as defined by ASTM. Cheaper class 5, 6 and 7 weights can be off by .001 to .007 grams (0.1 GN). While not a ton or even 1%, it could throw things off.

Aren't we really just looking for a relative weight, as long as it is repeatable. Sort bullets by weight. Sort cases by weight. .... all relative. Measure powder, repeatably. Even is its off by 3% it's OK IF it repeatable and the same balance is used from OCW to end of barrel life.

 

[sjoliat]

Aren't we really just looking for a relative weight, as long as it is repeatable. Sort bullets by weight. Sort cases by weight. .... all relative. Measure powder, repeatably. Even is its off by 3% it's OK IF it repeatable and the same balance is used from OCW to end of barrel life.

This is something I have thought about, living in my own isolated world using numbers that only make sense to me. All the numbers I get show good consistency, just heavy. Oh, except for the @%$^#@*! test weight that reads dead on. A friend of mine has a Gempro also so we are going to do a little comparing when the opportunity arises.

Ignorance would be bliss had I never decided to sort bullets.

 

[Nomad47]

Maybe your test weight is actually 3.3% light. That would make everything else read 3.3% heavy.

 

[Wow]

Jlow thanks for sharing your info really helpful I plan on trying that this weekend.