What Does My UKAS Calibration Certificate Tell Me?
If you have been looking at your UKAS Calibration Certificate and are unsure or curious about the specifics of any of the tests undertaken, where any of the information was obtained or how the results are achieved hopefully the following breakdown may be useful:
Calibration Certificate Balance Details
These are the various uniquely identifying details of your balance. Make, Model and Serial Number are obvious to most, but are usually found on a small tag/label on the side, rear or underside of the main housing. System describes the type of Weighing Cell used in operation of the balance. Also displayed will be Capacity and Readability, which respectively indicate the maximum readable load the smallest increment of the display (the increment by which the display will change).
Location describes the position of the balance on the customer site, as moving most balances will necessitate them being recalibrated to account for any changes in environment and level. We monitor temperature during all calibrations to ensure that the weights are not adversely affected by too much temperature variation. We also monitor Humidity and Pressure for balances high resolution balances as these can affect Air Buoyancy and a relevant correction factor must be applied.
These are the unique Identification Numbers and Certificate Numbers specific to each set of weights and the relevant calibration certificate to that set. These certificates give the true weight value of each individual weight and can be requested from Precisa UK for customer reference if required. The accuracy of the true values of the weights are always reported to a greater accuracy than the accuracy of the balances they are used to certify (usually by at least 2 decimal places).
As Found/Post Service
There are usually at least two sets of results reported on your UKAS Calibration Certificate, these are labelled as “As Found” and “Post Service”. As Found results indicate the behaviour of the balance before any Servicing or Adjustment has taken place. This is usually a limited Linearity test (for a POST SERVICE CALIBRATION) but can encompass full Linearity, Eccentricity & Repeatability (for a FULL AS FOUND/POST SERVICE CALIBRATION). Post Service results indicated the behaviour of the balance once all necessary Servicing and Adjustment has been completed.
This is a measure of how the balance performs during sequential weighing at increasing increments throughout its range (from the Zero-Point to the Maximum Capacity). This will show you how you balance reads when a weight is placed at one of several critical points and how far from the “True Weight” your balance is reading around that point.
This is a measure of how changing the position of the weight (usually 1/3-1/2 capacity) under analysis from a central position to a placement halfway between the centre and the corner-points (3/4 corners). This will reveal any errors that might be the result of changing the position of the object or substance under analysis. The number of eccentricity results that are reported is affected by the shape of the pan (found in balance details). For example, Round or Square pans will have 6 points (2 central, 1 Rear Left, 1 Rear Right, 1 Front Left & 1 Front Right) Triangular pans will have 5 points (2 central, 3 outer) but some other types (some Suspended or Bowl/Scoop pans etc) can sometimes be impossible to assess for eccentricity .
This is a measure of how the balance performs during sequential weighing of the same increments near the maximum capacity of the balance without the use of the Zero/Tare function (with the automatic tracking of the Zero-Point deactivated or circumvented). This should give a good indication of how consistent the balance is reading and how the mechanical stresses of the weighing sell are affecting the precision of analysis.
Some balances have an Internal Calibration Weight that can be used to adjust the Span (values between 0-point and a set point within the range) of the balance. An Internal Weight can be useful to keep a balances accuracy in check, but problems with the weight or mechanism can cause result in improper calibration/adjustment. Some Internal Weights can be adjusted and if possible they will be redefined as part of the Adjustment. The value of the internal weight will be reported on the UKAS Calibration Certificate if present.
Broadly speaking the Uncertainty value reported at the end of the Calibration Certificate is a measure of the cumulative estimated possibility of error covering all factors involved in the assessment of the balance. This includes deviation from the true value of the weight, any physical variation for the weights (through wear or temperature, pressure and humidity), accuracy of the balance, type of weigh cell, position and placement on the pan, variation through repeated weighings, mechanical Flexion/Stiction (loss/gain of weight after a weight has been removed).
There are some balances that do not operate only in one range (capacity and readability), but in two or more. A Dual Range balance is a balance with two ranges where the different ranges must be manually selected. The Zero-point for each range will be specific to the readability of the range. A Floating Range will transition between ranges as the capacity of one range is exceeded the readability will change. When weight is removed the balance will return to the Zero-point of the lowest readability once stability is reached. Poly Range balances have 3 or more ranges (of increasing capacity and decreasing accuracy) and usually function in the same way as Floating Range units.
Precisa UK Scale Calibration Certificate and Services
If you are looking for scale calibration services, we’d be happy to help you with your requirements. We are fully UKAS accredited and able to deliver calibration services. You can find out more about our calibration services on our website. To talk to one of our advisors, simply contact us.
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