Variable temperature control

Even conventional hotplates can be fitted with a continuously variable temperature control thanks to platinum thin-film sensors. The encapsulated platinum sensor is positioned so that it is in contact with the cast iron plate from below and thus able to register the temperature of the hotplate. Normally the power supplied to the plate is regulated by a stage switch. The increased control sensitivity is made possible by incorporating electronic circuitry capable of interpreting and acting upon the sensor signals. 

Option 4 A two or three way variable temperature control valve with bypass line is not required... 

E-3 spectrometer. A servo-hydraulic system was built which allowed loading of the samples in a wide variety of modes, and permitted tensile tests to be conducted at constant stress rate, at constant stress (creep), in cyclic fatigue, at constant strain rate, at constant strain or step strain. Provision was made for the simultaneous recording of stress-strain data and ESR spectra and a variable temperature control unit was employed. 

Several safety features have been included in the control system. The exit gas thermocouple is connected to a variable temperature controller to maintain gasifier temperatures below safe operating limits. A pressure switch is connected to the outlet point of the gasifier which will shut the combustion fan down if the pressure exceeds 120 cm water column. Various pieces of equipment are also interlocked to protect the system. 

NMR measurement. VT- Si CP/MAS NMR spectra were obtained at temperatures from -120°C (153 K) to 120°C (393 K) using a JEOL GX270 spectrometer equipped with a variable-temperature CP/MAS accessory operating at 53.54 MHz.The sample was contained in a cylindrical-type rotor and spun at 4-5 kHz. Contact time was 5 ms and repetition time 15 s. Spectral width and data points were 10 kHz and 8 k, respectively. Spectra were usually accumulated ca. 200-300 times to achieve a reasonable signal-to-noise ratio. Si chemical shifts were calibrated indirectly through the Si peak of polydi-methylsilane and were converted to the value from tetramethylsilane (TMS). The variable temperature controller was used for all of the probe temperatures at which measurements were taken. 

Initial thermal equilibrium, often based on the lock level stability, can be reached quickly taking as little as a few minutes (e.g. a room temperature sample inserted into a regulated magnet at 25 °C). Increasing the airflow to the variable temperature controller can speed this equilibrium process but unfortunately can easily introduce microphonics in the resulting spectra due to minute vibrations (i.e. wiggling of the sample in the probe). This is of course deleterious, and a compromise between temperature stability and positional stability must be reached for each spectrometer. For cryogenically cooled probes this becomes... 

NMR Apparatus. NMR spectra were obtained on a Varian A-60 equipped with a Varian V-6040 variable temperature controller and probe. Scans were made at 7-second intervals. Care was taken to avoid radiofrequency saturation of the proton resonance. The temperatures at the beginning and end of the runs were determined in the standard manner from the chemical shift of the two ethylene glycol peaks. Calibration of the heating characteristics of the apparatus as a function of time, heater current, and temperature was carried out prior to the triglyceride spectra determinations with the ethylene glycol peaks. Knowing heater current and time elapsed from the onset of heating permitted temperatures to be determined to ztl°C. in 10 trial runs. 

Probes usually have variable temperature control to run experiments at temperatures selected by the analyst. Cryogenically cooled probes can improve the resolution of a system, so that a 600 MHz spectrometer equipped with such a probe can provide resolution equivalent to a 700-800 MHz instrument. New probe designs with flow-through sample holders are commercially available, for use in coupled HPLC-NMR instruments and HPLC-NMR-MS instruments. These hyphenated instruments are discussed under applications later in the chapter. 

Techniques. N.M.R. spectra of atactic, isotactic and syndiotactic samples were recorded using a Jeol PS-100 instrument and a Varian 220 spectrometer, equipped with variable temperature controller and frequency meter (10). Spectra were obtained using 5-10% (W/V) solutions in CCli or 0-chlorobromo-benzene with HMDS as internal standard. 

The cell designs wc have successfully utilised with uv/vis/nir and epr spectrometers are detailed briefly. The variable temperature control of our cells is an important feature since many redox products are stable only at low temperatures. The range of systems which can be studied speciroclcctrochemically are extensive. This paper considers only redox processes which are... 

Carpenter TA, Klinowski J, Tennakoon DTB, Smith CJ, Edwards DC. Sealed capsules for convenient acquasition of variable temperature controlled-atmosphere magic-angle-spinning NMR spectra of solids. 

Sample Inlet System—The sample inlet sy em must have variable temperature control capable of operating continuously at a temperature up to the maximum column temperature employed. The sample inlet system must allow a constant volume of liquid sample to be injected by means of a syringe or liquid sampling valve. 

A gas sampling valve placed in an unheated zone of the gas chromatograph in conjunction with a splitter injector heated with a variable temperature control. 

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