How often should you be calibrating?
Once the 577 is turned on it should be warmed up for about ½ hour. The reason we recommend this is because as the tungsten bulb warms up the internal parts of the search unit the reading may vary by up to 10% to 15% in the first 15 minutes. This is due to the temperature sensitivity of the photo sensor. Once the unit is warmed up then one can expect less than 1 to 2 units change in a 24 hour period given that the unit is undisturbed. If the unit is physically knocked or banged about, this can affect the tungsten filament or bulb placement such that the reading can change by 10 or more units. Once the unit is turned on the calibration values of the plaque that the customer has entered into the unit will stay in memory and all that needs to be done to calibrate is to put the unit on the plaque and hit “Change” and the “STD” button twice. This takes no more than 10 seconds to complete. If the unit has been turned off then the values for the calibrator have to be entered in to the unit. This might take a minute.
In our lab calibration is done frequently (every ½ hour or just before a series of measurements) since it takes very little time, however, out if the field customer may desire a less frequent calibration. The frequency depends on what shifts in the calibration are allowable. Within one hour the unit typically shifts no more than 0.1 to 0.5 units if left undisturbed. If one wants to calibrate infrequently and be able to detect changes in calibration then the calibration plaque can be measured on occasion and verified that it measures within +/- x units of the stated plaque value. If it is outside that value then the calibration can be performed simply by pressing the “change” button and then the “STD” button twice. One very important thing to note is that if the sample is being measured with the search unit on top of the sample then the calibration with the plaque should also be performed with the search unit on top of the plaque (rather than the plaque being placed on the search unit. For the black zero standard it does not matter which direction the zero calibration is done.
In summary, once the instrument is warmed up, I recommend a calibration or a verification every hour or sooner. The zero calibration could be done every 4 hours since it does not change that much.