Weighing equipment

All scales must be kept clean and free from any buildup of polyurethane raw materials. The level and zero must be checked daily. A weight check also must be available in the normal weight range. The scales must be checked annually by a suitably qualified technician. 

The correct indication of temperature is important and the probes must be checked on a regular basis. Temperature indicators should have an internal check of the continuity of the probe s wires. Any breakage should be indicated on the display. The probe s sheath should be made from a material that is not attacked by any vapors that are present. Both RTD and K type probes have been shown to be satisfactory. 

Prepolymers heated in a microwave oven, even using a rotating table, will have uneven temperatures, and the material needs to be mixed well before taking a temperature reading. 

The location of the temperature probes is very important to obtain the correct temperature of the curing ovens. They must be in the air, away from direct heat from any elements. 

Measuring the temperature of cured polyurethanes requires a special technique. Polyurethanes are a poor conductor of heat and the probe will locally heat or cool the surface of the material. Once a temperature is obtained, the probe must be quickly moved to a second and third spot to obtain the correct temperature. The use of a noncontact thermometer (such as an infrared thermometer) can eliminate this problem. The instrument must be of the correct range and resolving power. 

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Automatic weighing has largely replaced manual weighing in the chemical-process industries because of the advent of larger-capacity processes and the need to economize on labor. Also, the dependability of weighing equipment has increased markedly, and investment cost has decreasea. Both batch and continuous weighing are used. 

The handling of eontaminated raw materials as deseribed previously may inerease the airborne contamination level, and if there is a eentral dispensing area preeautions m be neeessary to prevent airborne eross-eontamination, as well as that fiom infeeted measuring and weighing equipment This presents a risk for all materials but in partieular those stored in the hquid state where eontamination may result in the bulk being spoiled. 

Additional supporting documents are also included in the manufacturing records. These may include computerized print-outs from weighing equipment used to dispense chemical raw materials, or recorder charts obtained, e.g. from a freeze-drier upon completion of freeze-drying that batch of product. 

Fig. 2.24 The Roberval principle balance. From The National Bureau of Standards Handbook 94, Figure 5, The Examination of Weighing Equipment by M.W. Jensen and R.W. Smith.,U.S. Printing Office, 1965, p. 155. Reprinted courtesy of the national Institute of Standards and Technology, Technology Adminstration, U.S. Department of Commerce. Not copyrightable in the United States.

W.E. Kupper, Validation of High Accuracy Weighing Equipment, Proceedings of... 

Figure 2.2 Photograph illustrating typical laboratory balances and weighing equipment.

The method known as thermogravimetric analysis (TGA) requires little more than accurate and sensitive weighing equipment since the resulting information consists of the sample weight as a function of time with increasing temperature. The ideal but seldom realized goal is to achieve a uniform temperature increase of the test material. 

Feeding and Weighing Equipment for a Batch Mixing Process. 21-44... 

The schedule for checking weighing equipment for use in dispensing should include at minimum a check at values typical of the weights of material dispensed, on dates appropriate to the frequency of use. 

The standard weights used for checking weighing equipment should be stored in a suitably protective container or location and their calibration confirmed at appropriate intervals. 

Special attention is required in the use of the equipment for sampling and measuring raw materials — only completely dean and dry utensils should be used (e.g. scoops, spatulas, spoons, etc.). 4.2.4 With respect to weighing scales, there must be written specifications regarding to the accuracy of the scales. All weighing equipment must be inspected periodically. 

Accurately calibrated weighing equipment should be readily available and used for in-process monitoring of tablet weights. 

Measuring, weighing, recording and control equipment should be calibrated and checked at defined intervals by appropriate methods. More frequent verification of some weighing equipment may be advisable. Adequate records of such tests should be maintained. 

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