How to use a balance beam scale

how to use a balance beam scale

The extent to which a given measurement agrees with the standard value for that measurement ¹ The ability of a scale to provide a result that is as close as possible to the actual value. Example, if a known calibration standard weight of 100.000 grams was placed on the Scientech SP250 and the display shows 100.002 grams we could say the accuracy of the balance is 0.002 grams or 2 milligrams. Accuracy tells how close a balance gets to the real value. The accuracy of the scale is very sensitive to the calibration process. It is recommended to perform a calibration at the end user facility. If calibration is well performed, we can usually say the accuracy of the scale should be within +/- one display resolution with most scales.

Calibration:

To determine, check, or rectify the graduations of (any instrument giving quantitative measurements). ¹ Calibration is the comparison between the output of a scale or balance against a standard value. Calibration requires a standard weight and the balance to be set in the "calibration mode."

Calibration technically means to determine the difference between the balance/scale readout and the actual weight on the weighing platform to determine accuracy. Adjustment means to bring a balance/scale into the state of accuracy required for its use. Therefore, 'calibration," actually means "adjustment."

Some instruments contain internal calibration weights that are quite accurate, and can be used by the operator routinely, but we recommend external calibration. The readability of the scale will determine which class calibration mass (Class 1. Class 2. ASTM 6. Class F. etc) will be appropriate for calibrating your balance. Check your operator’s guide since most balances must be calibrated with a specific mass value.

Example: the Ohaus CS200 requires a 200 gram ASTM Class 6 mass. You can not calibrate the CS200 if you have a 100 gram mass. It must be a 200 gram mass. The scale is programmed and the software within the scale is configured only to accept a 200 gram calibration value to store the calibration data within the EPROM on the scale’s motherboard.

Calibration Certificate :

There is no certificate of calibration included with any balance we sell since this can only occur at the place of installation. A calibration certificate can only be attained at the place of installation for a balance. One can't calibrate a balance here in Massachusetts and ship it to another destination with the calibration certificate that would be valid.

The reason for this included:

  • Any location in the world is positioned differently from "magnetic north", thus effecting the acceleration of gravity, which effect the force a cal mass exerts on the balance's weighing sensor.
  • Barometric Pressure makes a difference and if the balance is calibrated in the hills of the rocky mountains and you're at sea level say in the California Bay Area your elevation is different effecting the barometric pressure.

You can purchase optional calibration weights for most balances, but the question then becomes do you needs weights with a "certificate of accuracy", "calibration report" or "mass value certificate". Read more about calibrations weights and the huge selection we offer at http://calibration.balances.com/ .

Basically local scale companies are in business just for this reason of calibrating and providing the documentation requirements. You might want to call a local scale company to see what they charge. Regardless, calibration certificates is something neither Precision Weighing Balances nor the scale manufacturer can offer unless we come on site to install the balance. Calibration is an additional service and this is always best done exactly at where the balance is in operation.

Another option to attain a certificate of calibration is to do it yourself with a certified calibration mass which we sell. You can basically copy the calibration instructions in the manual and create a QAD (Quality Assurance Directive), QAP (Quality Assurance Procedure) or SOP. You can also create your own calibration procedure from scratch like this one here.

People sometimes call us saying a competitor or catalog company offered to ship their balance with a calibration certificate if they purchase the balance from them. Companies offering balance(s) with a calibration certificate are providing false documents unless the

calibration is performed on site. Sure, companies can sell a oscilloscope or power supply with a certificate of calibration when the power supply is calibrated halfway across the country since a power supply isn't effected by the different longitude and latitude of magnetic north like a balance.

Calibration error

The difference between what a weight of near the full capacity of the instrument reads on the digital display and its true mass.

Capacity

The actual or potential ability to perform, yield or withstand. ¹ The largest weight the balance is capable of weighing.

Capacitance Loadcell

The fundamental design of a capacitance loadcell is that of the electrical capacitor. The loadcell contains two closely spaced, parallel, electrically-isolated metallic surface, one of which is essentially a diaphragm capable of slight flexing when pressure is applied. When pressure is applied to the capacitance loadcell a minute change occurs in distance between the plates. The varying gap between the plates creates in effect a variable capacitor. The resulting capacitance is detected send to a linear comparator and amplifier which is then processed by a microprocessor and displayed on the LCD. Dendritics. and many of the Tanita Pocket and Foods Scales use a capacitance loadcell.

Cornerload

Cornerload refers to the ability of an instrument to deliver the same weight reading for a given object anywhere on the weighing pan. (Of course, an instrument that does not perform acceptably with regard to drift and repeatability cannot possibly deliver acceptable cornerload performance.) Test this characteristic using the same test weight that was used to test repeatability. Position the object at various locations on the weighing pan. The reading should be the same, within a few digits, at all positions. A good example and test for cornerload can be found here regarding the Ashiba line of pocket scales.

Cornerload error

Refers to variations in the displayed weight as the object being weighed is moved to various positions on the weighing pan.

Count

The smallest increment of weight which the digital display resolves. Also called "division.".

Digit

The smallest increment of weight that the digital display resolves.

Divisions

The amount of increments a scale offers. The amount of divisions can be determined by taking the scale's capacity divided by the scales readability (the smallest number a scale can display. Example the Ohaus SP401 features 4,000 divisions. The capacity is 400 grams and the scale's readability or another way to say it is the numbers on the display increase in 0.1 gram intervals. Therefore 400 / 0.1 = 4,000 divisions. Another example would be the Ohaus SC2020 features 20,000 divisions. The capacity for the SC2020 is 200 grams and the scale's readability is 0.01 gram. Therefore 200 / 0.01 = 20,000 divisions. It is the divisions which determines the cost of a scale - not the capacity or readability, but instead the combination of both the capacity and readability to determine the amount of divisions. The more divisions the better the quality of the weighing sensor and larger the A/D converter needed to resolve the analog output from the weigh sensor to a binary number for the digital display.

Drift is a progressive (continuously upward or continuously downward) change in the number displayed on the digital readout. The weight readings does not stabilize, or unstable readings with no weight applied. All analytical balances show some uncertainty. Some do so more than others.

Two environmental factors affect the instrument’s stability dramatically—temperature and static electricity. Temperature control is imperative. This includes both control of the room temperature and maintaining the internal temperature of the instrument. For best stability, maintain the room temperature within two degrees constantly (day and night). Leave the instrument plugged in and turned ON. Static discharge can also be accomplished by putting some ionizing devices around the weighing pan.

Drift may be related to RFI (radio frequency interference). There is not a lot you can do is RFI other than move the balance to a different area where the RFI is less. Leveling of the balance can also result in drift.

Flexible Bearings - Precision components in the measuring cell (force motor) which allow the force coil to move without friction.

Source: balances.com

Category: Forex

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