Does anyone know what “Repeatability” means in the context of balance specifications? For example, in this specification XPR105DR a repeatability value of 0.007 mg indicates that the balance is expected to provide results within 0.007 mg of each other when weighing the same sample repeatedly. In other words, if you weigh the same item several times using this balance, you can expect the measurements to vary by no more than 0.007 mg from the average result.
Repeatability is essentially the precision of the balance. If you have a calibration weight and put it on there, that’s the deviation from the true value for the environmental conditions.
From Determining if a Lab Balance Is Accurate Enough | Blog | LabBalances.net
- Repeatability: This tells you the ability of the balance to repeat the same result for a given load under the same conditions. The repeatability value is often the same as the readability, but may be different in some cases. Repeatability can change depending on the load, so may be provided for different masses in the manufacturer specifications. This value is calculated as a standard deviation of multiple tests (usually 10 or more), and is denoted as Rstd.
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This si what tech support from mettler told me:
“Thank you for your inquiry. “Typical” values shown in our specification sheets are not guaranteed but represent the typical (average) behavior of the balance. The values are indicated as absolute values for a specific mass value and/or with formulas, covering the whole weighing range. Typical values may be used for a first estimation of a typical measurement uncertainty of a balance that can be potentially expected, however cannot replace calibration of the device at its final installation location. A calibration must include actually measurements during testing and must not be based on typical (theoretical) values of a specific model. Furthermore, a calibration also takes into account the measurement uncertainty of the references, in our case, the reference weights used to calibrate the balance, and some further factors as e.g. buoyancy, weight drift over time and convection (change of the apparent mass of the reference due to convection of air, based on a temperature difference between weights and environment). Therefore, an estimation of a potential measurement uncertainty based on typical balance specifications should the user only give a feeling on the uncertainty that can be expected when the balance is calibrated at its final installation location, and cannot be considered the actual value until tested. In other words, you can use this as a guide, but you would use the true repeatability tolerance shown on a calibration certificate in actual use of the balance.”
Another resource: Here is a whitepaper from Sartorius summarizing ISO 8655-6 for gravimetric calibration of pipettes. On page three, there is a short summary of how repeatability is used and what limits are required (per the standard) for calibration measurements.
Apologies if I’m jumping to conclusions by providing this link, but I assume the core of this question is related to calibrating or measuring liquids dispensed by a pipette.