Solid State Phenomena Vol. 164 (2010) pp 19-24 Online available since 2010/Jun/30 at www.scientific.net (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/ssp.164.19 High Precision Mass Measurement in Automation Tatjana Ivanova 1, Jānis Rudzītis 2 1 Riga Technical University, Ezermalas iela 6, Riga, Latvia 2 Riga Technical University, Ezermalas iela 6, Riga, Latvia puchkova@inbox.lv, aria@latnet.lv Keywords: mass comparator, high precision measurement accuracy, factors influencing accuracy Abstract. High-precision mass measurement equipment is required in some areas of science and technology. Physics, chemistry, pharmaceutics and high precision mechanics are common examples. In metrology, high-precision scales are used for verification and calibration of lower precision mass measurement equipment (weights and scales). Mass comparators are the most accurate mass measurement instruments available today. It is a special type of electronic scales designed to compare mass of two weights. They can be automatic or manual, with various measurement ranges and accuracy classes. This article discusses principles of operation of mass comparators and practice of high-precision mass measurement. There are special computer programs that can be used in conjunction with these instruments, which may significantly improve measurement accuracy (when mass comparator is controlled remotely) as well as simplify calculations and reporting procedures. This article describes one of these programs ScalesNet32 which can be used with mass comparators produced by Sartorius (Germany). Introduction Mass measurement is practiced widely both in industry and in trade and commerce. Mass is defined as the quantity of matter of an object. However, the measurement of mass is mostly carried out by weighing, using a variety of balances. Reference weights are classified into categories depending upon their precision, material and type of construction. There are seven classes defined in the OIML Recommendation 111, namely classes E1, E2, F1, F2, M1, M2 and M3. The weights are made from a metal or metal alloy. Generally platinum, iridium (E1), stainless steel (E2, F1, F2), brass or plated bronze (F2, M1), cast iron (M2, M3) are used. For example, maximum permissible error of 1 kg weight is 0.5 mg for class E1 and 500 mg for M3. Typical price of E1 weight set (1 mg 1 kg) is 3160, and only 690 for class F2. Today the high-precision weights (classes E1-F2) are calibrated mostly using mass comparators. These devices compare mass of reference weight (A) with mass of test weight (B). The construction of these balances is essentially based on the electromagnetic force restoration scales. Mass comparators are available with capacities in the range from micrograms to kilograms with excellent repeatability and linearity. Electromagnetic Force Restoration Scales The most accurate electronic balances are based on electromagnetic force restoration (EMFR), also called electromagnetic force compensation. These weigh scale sensors comprise a parallel guidance mechanism that ensures the accurate introduction of the object to be weighed, one or more levers, and an electromagnetic system (magnet and coil) that assumes the role of the weight in a two-pan scale balance. The triangular knife edges of a pan balance are replaced by flexible bearings. Changing the ratio of the levers allows forces smaller than 1 N to balance much bigger ones. Today it is common to have a system with one, two, or even three levers, depending on the load range. For greater convenience of use, modern EMFR balances integrate a Roberval mechanism like that in industrial scales. The balance must hold its parallelogram shape to maintain a state of equilibrium regardless of where the load is placed on the platform. The balance must retain a perfect All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 130.203.136.75, Pennsylvania State University, University Park, United States of America-15/06/14,07:15:53)
20 Mechatronic Systems and Materials: Mechatronic Systems and Robotics parallelogram shape so that the system remains in equilibrium no matter where the load is placed on the platform. Equilibrium is maintained by a control system incorporating an optical position sensor. When the coil force compensates the gravitational force exerted by an unknown mass, optical sensor detects a stable predefined zero position that indicates a state of equilibrium. The electric current that measures the applied mass is digitized, temperature compensated, filtered, and finally displayed. Weigh scale sensors based on the EMFR principle are typically 10 1000 times more accurate than strain-gauge-based industrial load cells and other devices based on the measurement of deformation. Signal strength also compares favorably with strain gauge output. An ideal EMFR load cell has no spring constant, shows no deformation under load, and self-compensates for the thermal expansion of the levers. In reality, some elastic deformation due to the load must reproduce within a few nanometers in order to guarantee the required precision, and the coil must typically be positioned to a tolerance within 10 nm (~3/1000 mil) to eliminate the effects of the unwanted but residual spring constant of the lever system. These sensors typically offer a displayed resolution of between 300000 and 3 million digits (1 million digits = 1 ppm), depending strongly on the application. In the analytical market segment, resolutions of up to 50 million digits are required as well as sub-ppm precision (e.g., 10 µg increments over a 200 g range). Some mass comparators, called window comparators, even have resolutions of 1 billion digits over a limited range. Mass Comparators Mass comparator is a special type of electronic scales used for mass measurement by comparison of two masses: known (reference weight A) and unknown (test weight B). Unknown mass of test weight B is being compared with known mass of reference weight A. Both weights are measured multiple times and then mass difference is calculated. Comparators are most often used in measuring laboratories for calibration of weights and masses classes E1, E2, F1, F2, M1 according to OIML R111. Main part of the comparator is high-precision measuring system which is separated from electronic circuits. Such solution guarantees elimination of temperature influence (warming of electronic elements) on the indications of comparator. Comparator is equipped with big graphic backlit display with user-friendly menu. In order to ensure proper accuracy of measurements, a half-automatic system of external calibration with calibration weight has been applied. Comparators have an interface for communication with computer. Mass comparators (m.c.) are usually based upon EMFR sensors (see above). This type of sensor provides the highest accuracy of measurement. Overall construction of m.c. must be as rigid as possible to avoid any mechanical instability. Comparator is highly influenced by external conditions such as temperature, breeze and vibrations. For the purpose of proper conditions, it is necessary to use the comparator in a room with very precise temperature control (laminar air-conditioning) and place of usage should bee from any vibrations. Electronic part of m.c. is separated from the mechanical (and electromagnetic) part. There are two types of m.c.: manual and automatic. In manual comparators, operator has to change weights (A and B) manually during measurement cycles. Automatic comparators have special device used to change weights. This greatly eliminates influence of operator and improves accuracy and repeatability. Also, automatic comparators may be controlled remotely using computer. For example, it is possible to place weights into comparator during working day, and perform measurement automatically at night when there is less interference (vibrations, etc).
Solid State Phenomena Vol. 164 21 Fig. 1. Typical manual mass comparator Comparator is equipped with esthetic weighing chamber with glass anti-draught protection. All elements of the weighing chamber are manufactured from glass or metal which minimizes the influence of electrostatic charges on weighing result. High-precision mass measurement requires estimation of air density because buoyancy of weight may influence observed mass value. In fact, air density is part of definition of conventional mass. The most accurate mass comparators have vacuum camera in order to completely eliminate influence of air movements and density. These high-end devices can be found only in the highest-rated international laboratories. Mass comparators have very high resolution (millions and billions of discrete values). The very basic idea of comparator is that difference between masses of two items (DM=M1-M2) may be measured much better than their masses as such (M1, M2). If M1 is known, then M2=M1-DM. For example, measured masses of two weights are 975 and 977 mg. In reality, mass of first weight is 1001 mg (known from previous calibration). Unknown mass of second weight may be calculated: 1001+(977-975)=1003 mg. Of course, this is very simplified example. In real life, several measurement cycles are performed in order to estimate mass difference and assure its consistency. Usual measurement cycle is ABBA (where A is reference weight and B is test weight). Result of the first cycle is discarded. Results of three or more consecutive cycles are used for calculations. The whole procedure (four or more cycles) may take substantial time (40 minutes and more). Results of ABBA cycles must meet several requirements. For example, there must be no significant difference between results of separate cycles and previous calibration of comparator. Otherwise, result of the whole measurement (several cycles) is discarded and procedure must be started over again. Sometimes it takes many hours to obtain acceptable result that meets all the criteria. Result of successful measurement is always corrected using parameters such as air density, weight density, previous calibration and adjustment result, etc. The final result is conventional mass value of test weight. All these checks and calculations are performed automatically by the controller of mass comparator or computer running special software (see below). For example, typical mass comparator Sartorius CC310 has measurement range up to 200 g, resolution 0.01 mg, standard deviation is 0.01 mg. It allows calibration of weights up to class E1 (highest). Professional analytic scales Sartorius LA230S have range up to 230 g, resolution 0.1 mg, standard deviation 0.1 mg.
22 Mechatronic Systems and Materials: Mechatronic Systems and Robotics Fig. 2. Typical automatic mass comparator Comparators are used mostly for high-precision measurement. Their users are national and international mass measurement laboratories and other similar organizations. This is a very small market for such complex devices. Therefore, prices are very high. Typical comparator may cost more than a new car. Two main vendors of comparators are "Sartorius" (Germany) and "Mettler Toledo" (Switzerland). For example, Sartorius is making comparators at their headquarters in Göttingen, Germany. There is small factory where complex equipment is manufactured piece-bypiece. Software Comparators may be connected to computer using various interfaces (RS232, RS485, USB). Special software expands capabilities of comparators. It allows remote control of comparator as well as simplified calculations, reporting and database features. For example, exact mass values of reference weight may be stored in database. Operator only selects required reference weight from the list. All tests on measurement results are performed entirely by software, making impossible reporting of invalid results. Air measurement instruments (thermometer, barometer, hygrometer) may be connected to the same computer for measurement of air density (this is needed for calculation of the conventional mass). For example, comparators made by Sartorius can be used in conjunction with ScalesNet32 software developed by "Maro Elektronik" (Germany). This software provides the following capabilities: calibration of customer weights, dissemination of Mass Scale, Calibration of reference weights, calibration and adjustment of weighing instruments (comparators), recording and graphic plotting of external parameters (air temperature, etc). When the weight is calibrated, the data that must be recorded includes the work order or customer, as well as weight-specific data such as serial number, manufacturer, design, characteristics, etc., to ensure unambiguous identification of the weight. This identification data is stored in a database, which makes it possible to generate a history of the weight at any time. The mass comparators used in calibration of weights are monitored by ScalesNet32, and must be calibrated and adjusted at specified intervals. The results of these procedures are also stored in a database. Furthermore, ScalesNet32 uses connected sensors to monitor reference weights and climate stations. The user is
Solid State Phenomena Vol. 164 23 informed when a particular reference weight, climate station or mass comparator is due for calibration; if calibration is not performed within a defined period, the particular device is blocked by ScalesNet32 from further use. The software consists of several components using the client-server model. ScalesServer is server part, using SQL database (MS SQL server) for centralized data storage. ScalesDesk and ScalesMass are clients. ScalesMass must be connected directly to comparators. All parts communicate over IP network. Calibration protocol is the output of ScalesNet32. This document contains all essential information regarding reference and test weights, measurement cycles, result testing and calculations: class, nominal mass, conventional mass and uncertainty, vendor, ID, density of reference weight. This weight must be traceable up to international prototype of kilogram; the same information for test weight (the weight being calibrated); results of three (or more) separate ABBA measurement cycles; results of conventional mass value calculation of test weight and its uncertainty. The procedure is performed automatically (controlled from laptop running ScalesMass component of ScalesNet32). At the same time, measurement of air density is performed. After that, the program calculates conventional and true mass values of "B", as well as its uncertainty. All intermediate results are included into protocol. Calculation algorithms comply with OIML Recommendation 111. These are explained in ScalesNet32 manual too. Unfortunately, ScalesNet32 has no means to evaluate long-term instability of scales and weight. Probably this is because the program has been designed for near-to-ideal conditions of use. In real life, long-term instability of measurement process may substantially increase overall uncertainty of results. However, existing recommendations and methods pay little attention to this fact. For example, calibration of the same set of weights every month over one year period may produce significant dispersion of results even if these weights and scales were handled very carefully and all their calibrations were successful. This is unclear issue which has to be investigated further. Usage of Mass Comparators Mass comparators are extremely sensitive to both electrical and mechanical environmental conditions. This is a price for improved convenience and accuracy. This fact is often overlooked by users and vendors of mass comparators. Expected accuracy of comparator may only be reached in nearly ideal conditions. For example, it is very hard to achieve class E1 performance even if comparator has been designed for this. Mechanical influences are important for any type of mass measurement system (mechanical or electronic). The laboratory must be built on stable ground. The building must be massive and stable (thick walls). Mass measurement must be carried out on the ground floor. There must be no large moving masses around (like trucks and trains). High-quality air conditioning is essential. Mass comparators are very susceptible to electromagnetic and electric influence. Strong magnetic fields may directly interact with electromagnetic system of the comparator. Poor quality of electrical power (harmonics, voltage instability, etc) degrades accuracy of the whole EMFR part of device. According to operation manuals, Sartorius mass comparators may be operated over wide range of acceptable voltage and frequency of grid power. However, we have discovered that stability of measurements is tightly correlated with stability of grid voltage. In fact, it can be predicted using the cheapest digital multimeter bought at marketplace. If voltage is jumping between 215-230 volts, don't start measurement. It will fail. Wait until voltage becomes more stable. Of course, this effect may be easily eliminated using UPS with double conversion (also called "true on-line"). These UPS provide the best quality of output voltage regardless what they receive from grid. Other laboratories also found that proper grounding of mass comparators are very important.
24 Mechatronic Systems and Materials: Mechatronic Systems and Robotics Summary Use of comparators for high-accuracy mass measurement and specific requirements for their accurate work are described. Influence of environmental factors on measurement accuracy is emphasized. References [1] International Recommendation OIML R 111-1 Edition 2004 (E), Weights of Classes E1, E2, F1, F2, M1, M1-2, M2, M2-3 and M3, Part 1: Metrological and technical requirements, International Organization of Legal Metrology, Paris (2004) [2] Traceability of Measurements, ILAC G2:1994, International Laboratory Accreditation Cooperation (ILAC) (1996) [3] Premium Accuracy for Weighing and Mass Comparison, Publication W 1064-e04116, Sartorius mechatronics, year of publication unknown, available online at http://www.dataweigh.com/. [4] The Latvian law: On uniformity of measurements ( Par mērījumu vienotību ), initial edition 27 February 1997, current edition 21 July (2006) [5] Kochsiek M., Gläser M., Comprehensive Mass metrology, Mass Comparators, Physikalisch- Technische Bundesanstalt, Braunschweig (Germany) (1984)
Mechatronic Systems and Materials: Mechatronic Systems and Robotics 10.4028/www.scientific.net/SSP.164 High Precision Mass Measurement in Automation 10.4028/www.scientific.net/SSP.164.19