Oven temperature, variations

As practiced by the UL, the procedure for selecting an RTI from Arrhenius plots usually involves making comparisons to a control standard material and other such steps to correct for random variations, oven temperature variations, condition of the specimens, and others. The stress-strain and impact and electrical properties frequently do not degrade at the same rate, each having their own separate RTIs. Also, since thicker specimens usually take longer to fail, each thickness will require a separate RTI. 

A variation on the method of thermal modulation is the use of a length of eapillary eolumn eoated with a thiek film of stationary phase. At ambient oven temperatures, this results in a retention of semivolatile analytes, whieh may be subsequently released to the seeondary eolumn onee the trap is heated. The rapid eyeling time possible with this methodology has resulted in its eommon applieation as the intermediate trap in eomprehensive GC. 

A number of variations of this test exist. An oven temperature is increased linearly. Continuous monitoring of the temperature and pressure outputs from a sample tube in the oven provides qualitative information about the thermal characteristics of the sample. In many cases, the pressure data can also yield valuable information. Any discontinuity in a plot of In P against 1/T indicates noncondensable gas generation. (The plot is often an essentially straight line if the pressure increase is due solely to the vapor pressure.)... 

The temperature of the oven must be carefully controlled. IEC 60216-4 [7] gives procedures for measuring the temperature variation and the rate of recovery. However, the tolerances given for temperature are excessive and the useable space in the oven should be determined over which the variation is no more than 2 °C. Preferably, the tolerance should be smaller, 1 °C or, ideally, 0.5 °C. 

Differential Thermal Analysis (DTA). The differential thermal analysis test serves to examine transitions and reactions which occur on the order between seconds and minutes, and involve a measurable energy differential of less than 0.04 J/g. Usually, the measuring is done dynamically (i.e., with linear temperature variations in time). However, in some cases isothermal measurements are also done. DTA is mainly used to determine the transition temperatures. The principle is shown schematically in Fig. 2.20. Here, the sample, S, and an inert substance, /, are placed in an oven that has the ability to raise its temperature linearly. 

Another problem with the furnace pyrolysers can be the difference in the temperature between the furnace and the sample. Again, due to the poor contact between the sample and the hot source, the sample may reach a lower actual temperature than the temperature of the furnace wall. It is interesting that in microfurnace systems there were reported variations in the pyrolysis products as compared to the results obtained in inductively or filament heated pyrolysers [7,18]. As an example, a study done on Kraton 1107 [7] decomposition found linearity between the oven temperature and the ratio of two decomposition monomers (styrene and dipentene) only in a narrow temperature range, namely from 450° C to 625° C. Kraton 1107 was found to decompose in filament or Curie point pyrolysers such that linearity can be noticed between temperature and styrene/dipentene ratio from 500° C to 850° C. The reproducibility of pyrolysis in a furnace was also found lower than for other pyrolysers [7]. 

PTFE powder is put into a mold to make billets. Powder is compressed uniformly (carefully) at pressures of 2,000 to 5,000 psi (14 to 34 MPa). Tbis preform is removed from the mold and sintered by heating unconfined in an oven at temperatures 680 to 715°F (360 to 380°C) for times ranging from a few hours to several days depending on the size and shape of the billet. Billet sizes go from 2 to 1,600 lb (1 to 726 kg), among the largest thermoplastic moldings made of any plastics. Time with temperature variation during cure is closely controlled. 

Fig. 3.3. Relative variation of the dimensional change (not the dimensional change itself ( )) related to the respective maximum dimensional change as a function of oven exposure temperatures. The measurements were performed after one- and two-hour oven exposme at 90 °C, 100 °C, 110 °C and 120 °C on two HDPE geomembranes (specimen 48 and 54) in machine direction (MD) and crosswise machine direction (XMD). In all cases, where a dimensional change can be observed, their absolute value clearly increases with increasing oven temperature. Differences between a one-hoitr and two-hoirr exposme cannot be recognised. A very long oven exposme over several months at 80 °C, however, will cause dimensional changes which are in the range of the changes to be measmed at 120 °C after one horn...

Equipment. The eqnipment used in rotational molding is simple many variations are available. The most common type is the so-called carousel type (Fig. 1). This machine consists of a heating station or oven, a cooling station (frequently an enclosed chamber), and a loading and imloading station. A carousel has three to six spindles or arms where the mold or molds are moimted. Most carousels have the freedom to revolve in a complete circle. The spindles are moimted on a central hub and driven by variable motor drives. New control systems allow each arm to operate independently in movement and control of oven temperature and time. Microprocessors are incorporated in the control system. Computer simulation software can be utilized in prototyping and manufacturing. 

Thermal analysis is defined by the International Confederation of Thermal Analysis and Calorimetry (ICTAC) (1,2) as a group of techniques in which a property of a sample is monitored against time or temperature while the temperature of the sample, in a specified atmosphere, is programmed. In practice, the temperature of the oven that contains the sample actually is programmed, while the temperature of the sample in some cases may differ from the programmed temperature. Exothermic or endothermic reactions or phase transitions in the sample subjected to the programmed temperature variation may cause variations in the temperature between the sample and oven up to several degrees. 

Ovenized crystal oscillator (0X0) A crystal oscillator enclosed within a temperature regulated heater (oven) to maintain a stable frequency despite external temperature variations. 

The efficacy of GC separations is highly dependent on the experimental conditions. For example, two sets of experimental data on the heptanal-cydohexanol mixture are given below to demonstrate the effects of variations in oven temperature on retention times. 

The results just described demonstrate that the resolution of GC peaks may be very sensitive to changes in retention time resulting from instability in oven temperatures. Since the number of theoretical plates is related to resolution values, significant degradation in column plate values can occur with variations in oven temperatures. When you compare the time and effort required to obtain a two-plate fractional distillation on a 2-mL mixture (see Experiment [3B] and Technique 2) with the speed and ease used to obtain a 500 plate separation on 12.5 (xL of cyclohexanol in this experiment, it is hard not to be impressed with the enormous power of this technique. 

Temperature and pressure can be precisely controlled with commercial thermal analysis equipment. This eliminates errors caused by temperature variations commonly encountered in oven testing. 

The oven of an SFC system should meet the same requirements as a normal GC oven. A constant temperature (variation 0.1 °C) must prevail in the entire oven at any time of a positive or negative temperature gradient. This is very important for reproducible capillary column SFC analysis. These columns are very sensitive to even slight variations in temperature, which can result in peak shape deformation, peak splitting, or irre-producible retention times. 

If HPLC analyses are carried out at ambient temperatures, even slight temperature variations will lead to retention time shifts making automated peak detection difficult if not impossible. The use of column ovens therefore is highly recommended. 

Figure 7 shows the effects of catalyst concentration on isothermal cure of the resin at 150°C. Catalyst concentrations spanning the range of 0.05 phr to 1.0 phr TPP were studied in a 1.18 stoichiometry ECN-PN formulation. After the microdielectric sensors were coated with the formulations, they were placed into an oven onto a halfinch thick aluminum slab which also had been preheated. Temperatures were monitored during the cure by an on-chip thermal sensor. Since the sensor indicated temperature variations of less than 2°C during the cures, there was negUbible sample heating due to the reaction exotherm in these experiments. 

---