Posted September 2, 20213 yr comment_82436 The acceptability range for our both of our scale balances (200 g and 500 g) when weighed during day of use scale QC is ±5 g. I didn't set this range, and I'm not sure how it was determined, but one of the techs would really like the range readjusted. I agree with them that it's probably best practice to create a more stringent range, but I'm not sure where to start. The scale we're using only measures in one gram intervals with a maximum range of 1 kg (it's an Ohaus CR2200), so a lot of what I've read about weight range determination will be difficult to implement. It also seems like a good bit of the advice pertains more to weight calibration, which our facilities outsources. I hope y'all have some insight for me! Maybe I'm overthinking this whole situation.
September 3, 20213 yr comment_82437 I assume that as there are no decimal points involved there is little, if any, variability on day to day readings. Remember the purpose of daily checking the scale is to ensure it is fit for YOUR purpose at the max acceptable ranges in variability, which is why information is harder to find, each lab has to determine what is appropriate for them. Therefore if your inspectors have no problem with the range why would you. An additional advantage of the wide range is that you will/may see a drift in readings if the scale starts to malfunction and so have time to call someone in to fix it before you can no longer use it. I personally see no reason to make things harder without good reason or just because you can.
September 5, 20213 yr comment_82441 I'm just curious but what do you weigh on this scale that would make a range of +/- 5 gms unacceptable?
September 6, 20213 yr comment_82443 So many questions... Like @John C. Staley said, what is the scale used for? We have several in our lab. Some are for weighing whole blood or platelets, some are for making reagents. Different tolerances could be allowed based on the risk. What does the manufacturer claim it can perform at? We do daily QC on our scales. Four points, 20g, 100g, 200g, and 500g. We expect the 20 to be +/- 0.1g and the others to be +/- 1g.
December 15, 20213 yr Author comment_82757 Sorry for the late response! The scale is used solely for weighing the output of therapeutic phlebotomies, in which blood is wasted immediately following the procedure. The only claim the manufacturer makes for readings is that the repeatability (or standard deviation) should be 1g, otherwise no guidance is provided. Based on y'all's responses, it doesn't seem necessary to tighten the range at all. I think it makes sense to keep the range wider to watch for drift, especially because therapeutic phlebotomies don't require much precision.
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