Heating, current pulsed

Pulse-like Joule heating (current pulses in small resistors)... 

In temperature modulation, the sample may be mounted on a small heater attached to a heat sink and the temperature varied cyclically by passing current pulses through the heater. If the sample is properly conducting, the current can be passed through the sample directly. Generally, for this method must be kept below 10—20 Hz, and hence there are often problems with the 1//"noise of the detector. 

The considerations so far rely on constant heating power, and the way how this power is applied to the microhotplate does not play a role. In fact, a monolithically integrated control circuitry does not apply constant power but acts as an adjustable current source. Moreover, for measuring the thermal time constant experimentally, either a rectangular voltage or rectangular current pulse is applied. Analyzing the dynamic temperature response of the system leads to a measured time constant, which... 

The deviation between the time constants for membrane heating and cooling was measured as well (Eq. (3.37)). The heater of a single microhotplate was driven with a rectangular-shape current pulse. The pulse amplitude was adjusted to produce a temperature rise of 50 °C. In this case the measured time constant for cooling was... 

The pressurized volume in this type of apparatus is relatively large, at least a few milliliters, and easily holds an electrically heated furnace. The pistons carry the heating current. Thermocouple or other sensing wires can be led out through the gaskets. Maximum steady temperatures can be as high as 3000°C, depending on the thermal insulation used. Temperatures over 4000° C can be reached in brief (millisecond) pulses. An apparatus of this type... 

Another method proposed for minimizing polarization and related effects is to use short repetitive current pulses rather than continuous voltage on the sample (J3). This method also reduces the ohmic heating effect at higher moisture contents. Some contemporary resistance meters have provisions for switching to the pulsed current mode for wood moisture contents greater than 12% and retain the DC mode at lower moisture contents. 

In the region of negative pressures water also behaves anomalously with respect to superheat. A negative pressure in water was created when a short compression wave (DSps) was reflected from a free liquid surface. The compression wave was formed by a duralumin membrane during a discharge of a low-inductance capacitor onto a flat coil pressed to the membrane. A platinum wire heated by a current pulse was immersed in the liquid. The pressure pulse and the heating pulse were reconciled in time in such a way as to make the... 

The measurement of the output characteristics is carried out during the period that the heating current is switched off. The electronic circuits for the measurement are consisted of the comparator circuit, delay pulse generator, sampling and hold circuit and triangular wave generator, as shown in Fig.3. 

The temperature attained by the wire-screen heating elements was related to the time of current flow by a calibration method. Times required to melt pure metal powders of like particle size and amounts as the coal were determined by trial and error. A number of calibration points were thus established, and the temperature at the end of the current pulse was proportional to the time of current flow in the region to 1450 °C. The heating rate was therefore a constant 8250°C/sec. 

As mentioned before, a dynamic calorimetric pulse experiment uses resistive self-heating of an electrical conductor - typically wire-shaped samples (the diameters ranging from a few hundred micrometers up to a few millimeters), rectangular shaped samples, foils, or tubes - by passing a large current pulse... 

In a second approach, the thermistor is heated by applying a short duration current pulse and subsequently measuring the cooling curve of the thermistor as the blood flows across it. The thermistor will cool more quickly as the blood flow increases. Both these methods are relatively simple to achieve electronically, but both also have severe limitations. They are essentially qualitative measures and strongly depend on how the thermistor probe is positioned in the vessel being measured and the characteristics of the local flow around it. If the probe is closer to the periphery or even in contact with the vessel wall, the measured flow will be different than if the sensor is in the center of the vessel. 

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