Pan masses

Be sure the pans are clean and that the instrument has been tared for pan mass and calibrated using a standard mass in the range of the sample weight. 

FIGURE 2.8 Comparison of several DSC heat flow curves for a potential drug candidate. No reference pan reference pan matched to sample pan weight matched reference pan mass plus an additional aluminum lid matched reference pan plus two additional aluminum lids. All runs are at 10°C/min. Nitrogen purge gas at 10 ml/min. 

Before the DSC experiment is started, the two calorimeters (i.e., the sample and reference pods, since they are separate calorimeters with one heater) are in equilibrium, they are at the same temperature Tu = T = where is the block temperature, is the sample temperature and T, is the reference temperature. When the operator starts the heating experiment, the block will be heated at a linear rate therefore the sample and the reference calorimeters will also be heated. They will lag behind the block temperature, but to a different extent since the heat capacity of the sample calorimeter is higher because of the additional mass of the sample as compared with an empty pan for the reference. The sample temperature will lag behind T,. Assuming the pan masses are identical, the Th - and Th - temperature differences will be proportional to the heat capacity of the sample and reference calorimeters, respectively. The temperatures Th, T, and T, are measured by thermocouples. 

The T4P mode, which also includes a correction for the differences in mass of the sample and reference pans, and a correction for thermal resistance of the pan material (aluminum, copper, etc.) in addition to the terms listed for the T4. One should correct for the difference in pan masses to account for their effects on the heat capacities of the sample and reference calorimeters. Only the QIOOO and now the Q2000 are capable of operating in the T4P mode. 

The additional P correction is done using a model based on the pan type and consists of summations of the thermal resistances between the sample and the sensors. This value is then substituted back into Eq. (2.6) to correct for smearing of the heat flow due to the additional thermal resistances. Also allowed in the T4P mode is the correction for the difference between the sample and reference pan masses to account for slight differences in the pan heat capacities if the pan masses were not matched. 

If possible, all the pan masses (i.e., sample pan, reference pan, and the empty pan in the sample position for the baseline run) should be identical in heat... 

Using an equal-arm balance (see Fig. 1), the unknown mass of an object can be deterrnined by placing it ia one pan, and adding test weights to the other until the beam balances. The result wHl be the same regardless of location because the object and the test weights ate subject to the same value of. Any scales that measure an unknown mass by comparing it with a known mass (with or without a lever system), wHl be unaffected by variations in g. 

Buoyant Effect of Air. Weighing operations performed m vacuo are not affected by buoyancy forces. An object in air, however, is subject to a buoyancy force that is equal and opposite to the gravitational force on the mass of air the object displaces (10). If the equal arm balance of Figure 1 is in balance with a test weight of mass, in one pan, and material of mass, m, in the other, m = m if they have the same density. If the densities are different, then the buoyancy forces acting on each pan affect the result. Taking moments about the center pivot point gives... 

Humidification. For wiater operation, or for special process requirements, humidification maybe required (see Simultaneous HEAT and mass transfer). Humidification can be effected by an air washer which employs direct water sprays (see Evaporation). Regulation is maintained by cycling the water sprays or by temperature control of the air or water. Where a large humidification capacity is required, an ejector which direcdy mixes air and water in a no22le may be employed. Steam may be used to power the no22le. Live low pressure steam can also be released directly into the air stream. Capillary-type humidifiers employ wetted porous media to provide extended air and water contact. Pan-type humidifiers are employed where the required capacity is small. A water filled pan is located on one side of the air duct. The water is heated electrically or by steam. The use of steam, however, necessitates additional boiler feed water treatment and may add odors to the air stream. Direct use of steam for humidification also requires careful attention to indoor air quahty. 

G. Astarita, Mass Transfer With Chemical Reaction, Elsevier Publishing Co., Amsterdam, the Netherlands, 1967 C. Y. Wen and L. T. Pan, Models for... 

When the bulk containing the binder is uniform, it is compressed on pneumatic, hydraulic, or ram-type presses. Compression can be carried out in presses provided with suitably designed cavities or in metallic pans. The pans ate filled with the powder mass, and a plunger with a cross-sectional shape similar to that of the pan is used to compress the tablet. The resulting tablets ate commonly used with powder puffs or cosmetic bmshes. 

Tiated and untinted soft Hpstick masses are distributed ia pans as Hp glosses. These are appHed with the fingers or with Hpstick bmshes. Hard, tinted, pencil-type sticks have been marketed as Hp liners. Chap sticks are unpigmented Hpsticks iatended to alleviate scaling and to prevent cracked Hps. Compositions of Hpsticks, Hp glosses, and chap sticks have been pubHshed (90). 

The ebonite compound before cure is a rather soft plastic mass which may be extruded, calendered and moulded on the simple equipment of the type that has been in use in the rubber industry for the last century. In the case of extruded and calendered products vulcanisation is carried out in an air or steam pan. There has been a progressive reduction in the cure times for ebonite mixes over the years from 4-5 hours down to 7-8 minutes. This has been brought about by considerable dilution of the reactive rubber and sulphur by inert fillers, by use of accelerators and an increase in cure temperatures up to 170-180°C. The valuable effect of ebonite dust in reducing the exotherm is shown graphically in Figure 30.3. 

The speed of rotation establishes the cycle time and this, being an important design parameter, is limited mechanically by the mass inertia of the swinging pan and its wet cake load at the point... 

Copolymers of AN with ISP, containing more than 25% of monomeric units of ISP with a molecular mass of 50000 to 60000, obtained in emulsion at pH 3, in distinction to PAN, are capable of passing into the state of viscous flow without destruction and cyclization and are processed into fibres at 180—220 °C. When copolymers of higher molecular mass are used it is necessary to raise the temperature of processing. This leads to an intensive crosslinking and to cyclization, due to which it becomes impossible to obtain fibres from them. 

Many of the earlier studies of mass transfer involved measuring the rate of vaporisation of liquids by passing a turbulent air stream over a liquid surface. In addition, some investigations have been carried out in the absence of air flow, under what have been termed still air conditions. Most of these experiments have been carried out in some form of wind tunnel where the rate of flow of air and its temperature and humidity could be controlled and measured. In these experiments it was found to be important to keep the surface of the liquid level with the rim of the pan in order to avoid the generation of eddies at the leading edge. 

Hinchley and Himus(40) measured the rate of evaporation from heated rectangular pans fitted flush with the floor of a wind tunnel (0.46m wide by 0.23m high), and showed that the rate of vaporisation was proportional to the difference between the saturation vapour pressure of the water Ps and the partial pressure of water in the air Plv. The results for the mass rate of evaporation W were represented by an empirical equation of the form ... 

More often than not, measurements are accompanied by two kinds of error. A systematic error is an error present in every one of a series of repeated measurements. An example is the effect of a speck of dust on a pan, which distorts the mass... 

Self-Test B.1B A miner panning for gold in an Alaskan creek collects 12.3 g of the fine pieces of gold known as gold dust. The mass of one gold atom is 3.27 X 10-25 kg. How many gold atoms has the miner collected ... 

E.3 In your new nanotechnology lab you have the capability to manipulate individual atoms. The atoms on the left are bromine atoms (molar mass 80 g-mol L), those on the right are atoms of calcium (molar mass 40 g-mol" 1). How many calcium atoms would have to be added to the pan on the right for the masses on the two pans to be equal ... 

Lee PC, Tseng FC, Pan C (2004) Bubble dynamics in microchannels. Part 1 single microchannel. Int J Heat Mass Transfer 47 5575-5589... 

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