TEMPERATURE DETECTION CIRCUITRY

Ambient temperature variations will affect the accuracy and reliability of temperature detection instrumentation. Variations in ambient temperature can directly affect the resistance of components in a bridge circuit and the resistance of the reference junction for a thermocouple. In addition, ambient temperature variations can affect the calibration of electric/electronic equipment. The effects of temperature variations are reduced by the design of the circuitry and by maintaining the temperature detection instrumentation in the proper environment. 

Density compensation may also be accomplished through electronic circuitry. Some systems compensate for density changes automatically through the design of the level detection circuitry. Other applications compensate for density by manually adjusting inputs to the circuit as the pressurizer cools down and depressurizes, or during heatup and pressurization. Calibration charts are also available to correct indications for changes in reference leg temperature. 

Fig. 9. Schematic diagrams of the major components of cw and pulsed EPR-ENDOR instruments. The sample is in a resonant microwave cavity, situated between poles of a magnet and surrounded by a temperature-control system (not shown). The structure of the circulator directs microwaves from the source to the cavity, and from the cavity to the detection system. A radio frequency synthesizer provides rf to coils situated around the cavity. Note that this diagram shows an arbitrary orientation of the rf coils. For convenience the magnetic field modulation coils are not shown for the cw spectrometer. For the pulsed EPR spectrometer (B), fast switches (ovals) are used to control pulse timing for the rf and microwave pulses, as well as to protect the detector. For simplicity, several features including the timing circuitry are not shown. The signal from the detector is sent to a boxcar integrator. Both spectrometers are computer-interfaced for data collection and storage. Further details may be found elsewhere.

A major advance in detection of NMR signals has been the development of probes in which the RF coil and the preamplifier are cooled close to the temperature of liquid helium, but with the sample remaining at ambient temperature. These so-called cryoprobes have a S/N ratio improvement of 500% over conventional probes of the same sample diameter. This is because the thermal noise level in the circuitry scales approximately as the square root of the ratio of the absolute temperatures. There are some limitations to this improvement for highly conducting... 

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