Balances The balance is an instrument which measures the mass of a body or substance using the gravity force which acts on that body At present, there are two large groups of balances: mechanical and electronic balances.

Spring balance

Sliding weight balance.

Analytical balance.

Mechanical Balances 1.0 Spring balance Spring balance 2.0 Sliding weight balance.Sliding weight balance. 3.0 Analytical balanceAnalytical balance Its function is based on a mechanical property of springs as the force exercised on a spring This type of balance is equipped with two known weights which can be moved on setting scales

Operation Verification Verify that the balance is leveled. Test the zero mechanism. Verify and adjust the sensitivity. a) Lock the balance. b) Place a standard weight (equivalent to the optical scale range) on the plate. c) Position the micro setting to one (1). d) Release the balance. e) Adjust to the zero position.

Maintenance of the mechanical balance Daily Annually 1. Verify the level. 2. Verify the zero setting. 3. Verify the sensitivity adjustment. 4. Clean the weighing plate. 1. Calibrate the balance and document the process. 2. Disassemble and clean the internal components.

Electronic balances The weighing plate. A measuring device Electronic controlling parts and display

The weighing plate.

load cell

Theory of operation Electronic balances are mostly used two different technologies. Load cell / Strain gauge and electromagnetic force restoration. Load cell principle is used for all scales with higher limits and less accuracy like patient and baby scales, while high quality analytical balances are used electromagnetic force restoration principle.

Load cell / Strain gauge. A load on the pan creates as force and the load cell gets deformed. Length-sensitive resistors get deformed and change their resistance. Amplifier creates a voltage out of the current through the resistances and delivers a measurement voltage which corresponds to the weight. A Wheatstone bridge with three fixed resistors and a sensor resistor. When the bridge is in balance the output voltage is zero.

Electromagnetic force restoration The method give more precise measurements than the load cell principle. The coil is floating in a magnetic field which is created by an amplifier. The amplifier delivers always the right current to keep the lever in balance, regardless of the weight on the pan. A optical sensor detects the balance of the leaver. The needed current for this balancing is proportional to the weight on the pan, the current is also used to measure the weight. Therefore the current flows through a precision resistor which creates a measurement voltage. An A/D unit transforms the voltage into a digital signal. The following micro processor with an appropriate software delivers the measurement result which is finally show on the display. Additional sensors like a displacement detector and a temperature sensor are also controlling the amplifier and ensure the accuracy of the balance.

Microprocessor base Electronic Balance

Power Supply 1. The input socket with the plug from the external power supply. 2. The glass fuse. 3. A capacitor. Works together with the following coil as a filter for the DC input voltage. 4. Looks like a transformer, but is just a coil for filtering out any AC components of the DC voltage. 5. Three stabilizers for three different DC output voltages. 6. Capacitors for stabilizing the output voltages.

Maintenance Check list Clean the weighing plate, Remove any contaminations. Check door open and closed is smooth. Clean the weighing chamber, externally and internally. Check the “Zero point display ” is stable. Verify that the adjustment mechanisms on the front door of the weighing chamber works adequately clean, pre-weighed container for weighing (glass container or weighing paper if possible). Note that plastic can become electromagnetically charged and is not recommended for weighing powdered or granulated chemicals. Any spill must be cleaned immediately to avoid corrosion or contamination. Use 70% ethanol to disinfect the pan of the balance.

Basic fault Identification The weight reading is incorrect. The balance’s reader is unstable. Functional Error

1.0 The weight reading is incorrect. The balance was not adjusted to zero before the reading. The balance is incorrectly calibrated. The balance is not leveled. Place the balance on zero; repeat the measurement

2.0 The balance’s reader is unstable. The balance’s reader is unstable. The front door of the balance is open. Place the balance on a stable surface Close the front door to measure

Functional Error Readings not reproducible (hysteresis). Non-linear readings The screen indicates an overload or negative condition without a load being applied. The measurement cell is dirty. The measurement cell is badly assembled. Non-linear readings Mechanical system is in bad condition. Measuring cell damaged by overload. Measuring cell is inadequately assembled.