Precision Balances Pty Ltd was established, after eight years of in-house and field experience through other outlets, in July 1993 to provide a service for clientele which use weighing systems within their company structure.
The objective of Precision Balances Pty Ltd is to provide quality products and services which consistently meet our customers needs and expectations in the most effective manner.
To achieve this objective Precision Balances Pty Ltd is committed to the implementation and maintenance of a NATA Laboratory accreditation in accordance with the National Association of Testing Authorities in Australia (NATA) to International Standards AS ISO/IEC 17025.
Our aim is to further our activities with the aid of a NATA controlled structure and maintain to be a competitive company. Education and training in NATA Assurance is provided where necessary to all employee's to supplement testing skills.
Precision Balances Pty Ltd is a sales and service
agent for Ohaus
Analytical and Toploading Balances and we are
also authorised to service Sartorius
Weighing Products.
Evaluating Laboratory Balance Performance
The purpose of this article is to help laboratory balance users evaluate their equipment to determine whether it requires repair, on-site adjustment, or a change in how the equipment is used.
Functional problems fall into either of two categories:
1) Problems that are a matter of adjustment and can be corrected on-site by a field service technician.
2) Problems caused by defective components within the equipment and which indicate that the instrument need to be repaired.
Other
problems are the result of environmental factors (see
preliminaries) and/or the way the instrument is operated. The end user can correct these.
Listed
below are tests and indicators that you can use to evaluate your laboratory balances:
Drift
Drift refers to weight readings that do not stabilize, or unstable readings with or without a weight applied. All analytical balances show some uncertainty, some do so more than others. The clearest indication of malfunction is a change in the amount of instability.
Two
environmental factors affect the instrument’s stability dramatically being
temperature and static charge.
Temperature control is imperative, which includes both control of the
weighing room and maintaining the internal temperature of the instrument. For best stability a change within two
degrees of the room is required (day and night). Always leave the instrument plugged in and
turned ON. Static charge will cause
erratic readings; hence controlling the humidity (65%) is important. Instruments should be operated on a static
dissipating surface (anti-static mat).
Operators should stand on anti-static floor covering. Avoid plastic containers for items being
weighed in. Never replace broken glass
doors on instruments with plastic ones.
Eliminate sources of floor vibration and air currents. On analytical balances with glass doors, be
sure the doors are closed fully. Instruments that have served
satisfactorily but suddenly become unstable are likely to need repair.
Repeatability
Repeatability
refers to an instrument’s ability to consistently deliver the same weight
reading for a given object, and to return to a zero reading after each weighing
cycle. Test this by repeatedly weighing
the same object. The best test is a
weight intended for that purpose. It
should match the maximum weighing capacity of each range of the
instrument. When a test weight is not
available, an alternative object that is solid, non-porous, dirt free,
non-magnetic, and non-static container can be used. Water in a container is definitely not
suitable due to evaporation. Poor
repeatability is often caused by damage to the weighing cell.
Inoperative instruments
Inoperative
instruments are those that are either damaged, or clearly not producing
weighing results. Examples are bent,
broken, or damaged parts of numerical readouts that are blank, frozen, or
nonsensical. Some instruments are
damaged by mishandling or by contamination from liquids or chemicals. These problems clearly require component
level repair.
Cornerload
Cornerload refers to the ability of an instrument to
deliver the same weight reading for a given object anywhere on the weighing
pan. Position an object of half the
maximum weighing capacity on the weighing pan.
The readings should be the same, within a few digits, at all
positions. Cornerload
errors are adjusted during a field service.
Linearity
Linearity refers to the quality of delivering identical sensitivity throughout the weighing capacity. Test this characteristic by weighing two stable objects separately, each of approximately one half the weighing capacity.It is imperative that these two weights not be interchanged within this procedure. Refer to the individual weights as "weight A" and "weight B."
Tare the display. Place "A" on the pan (at the centre), and record the reading.
Remove "A" and place "B" on the pan next to the centre. Tare the display.
Again place "A" on the pan. Record the reading.
Calculate the difference between the two (0-50 and 50-100) readings.
The difference should be less than the advertised tolerance for linearity or accuracy.
Linearity is adjusted during on-site service.
Special Note:
A common error in linearity (accuracy) testing is to
simply place test weights on the weighing pan and observe the difference
between the indicated weight and the nominal value of the test weight.
This process fails to account for the fact that test weights are imperfect and
that the difference between the nominal value and the actual weight might be
significant. This is especially true with analytical balances, where the
balance may be more accurate than any standard test weight. The above procedure nullifies this problem by comparing the weight
readings of the same object, both with and without a preload. The accuracy of
the test weight is thus immaterial.
Calibration
Calibration
refers to the actual accuracy of an instrument.
To test this yourself you need a test weight with approximately the same
or better accuracy as your instrument.
Consider buying more than one, e.g. half and maximum of the weighing
capacity of the instrument, in order to test linearity. Many instruments contain internal calibration
weights that are quite accurate, and can be used by the operator
routinely. Check your installation
manual. Calibration is verified and
adjusted during on-site service by the field technician with very accurate test
weights.
On-site service
On site service is available throughout Western Australia
and The Northern Territory from Precision Balances Pty Ltd by an experienced
field technician. Contact us with any questions
about how and where to send your instrument for evaluation and repair.
Principle and Construction of a Electronic Balance
- The load on pan (21) is conveyed to the amplifying
lever through the parallel guide mechanism consisting of upper (22) and
lower (24) parallel spring beds and moving column (25), and is balanced with
the electromagnetic force.
The position detection signal being generated by displacement of the amplifying lever is converted to the current through PID control unit (2) and is supplied to the coil in electromagnetic force generation unit (26).
The current, which is supplied to the coil, is converted to the voltage by output resistor (3) and is conveyed to analog switch (7) after passed through low-pass filter (4).
The signal of temperature sensor (5), which is located in the electromagnetic force generation unit, is amplified by amplifier (6), and is supplied to analog switch (7) and reference voltage unit (8). The reference voltage unit compensates the reference voltage by the temperature signal.
The weighing and temperature signals, which are supplied to the analog switch, are converted into the digital value by dual-slope A/D converted unit (9).
The CPU (central processor unit) fetches the digitally-converted superimposed data and temperature data. After executing the compensating process, the weighing value is displayed in display unit (11).
Handling
test weights
The weights
used to test laboratory balances are precision devices and need to be handled
accordingly. When handling weights, avoid direct hand contact with weights
by using clean gloves or special lifting tools with non
metal tips. Also avoid sliding weights across any surface,
especially across the stainless steel weighing pan of the balance under test.
To improve maintaining the actual measured value of calibrated weights it is
good practice to place a small piece of paper on the pan first and zero the
balance. Remember external calibration is important to keep your readings
accurate, so the weights are to be handled correctly.
Environment
In order to pass any test of reproducibility an
instrument must be operating in an acceptable environment. A poor
environment will degrade the results of a standard deviation (STD) test and
falsely suggest that the performance is substandard. There are several
aspects of the environment that impact the performance of a laboratory balance,
e.g. gravitational acceleration hence regular calibration is
required.
Temperature
The accuracy and overall performance of any laboratory
balance is affected by the room temperature. For best stability and
performance the room temperature should be regulated to within one degree
Celsius without interruption. The instrument should remain with power ON
continuously.
Air Drafts
In the cases of measurements with resolution of 1
milligram and less the force exerted by moving air is readily detectable. A
shroud or enclosure around the weighing pan will shield the pan from these
effects. Avoid plastic materials for draft shields.
Static
Electricity
Static electricity exerts a mechanical force which is
readily detectable by analytical and microbalances. An example of static
electricity exerting a mechanical force would be lint sticky to clothing.
Static will be a problem when it exists on the object being weighed, on the
person using the balance, on draft shields, or on weighing vessels.
Sources of static are carpets, Vibrum shoe soles,
plastic draft shields, plastic weighing vessels, and melamine (Formica)
tabletops. Low ambient humidity exacerbates static problems.
You can test for a static problem easily. On an
analytical balance place a metal enclosure (a coffee can works well) over the
weighing pan, so that the pan is enclosed by the can but NOT touched by
it. If the weight readings stabilize with the can in place, then static
may be the cause of the instability. Notice that the coffee can provides an effective draft shield too.
Floor vibration / Table Instability
Many laboratory balances are
extremely sensitive to vibration or movement. If the weight readings
change as you walk around the instrument, or if the readings change as you lean
on the table or move objects on the table, then the table and floor are
affecting weight readings. You can minimize these effects by using
an especially sturdy table and minimizing movement. Users of
microbalances often need specially built marble tables on their own foundation.
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