Design and development of a computerized high temperature pulse calorimeter

A computerized pulse heating calorimeter was designed to measure the specific heat of electrical conductors. A computer assisted data acquisition and control system was built and computer programs for acquisition and specific heat calculation were written.The calorimeter was tested from room temperature to 480°C with nickel as the specimen.

The next few paragraphs contain a brief description of the vacuum pumping system, instrument rack, power supply, data acquisition and control programs, overall manager program and preliminary data analysis.

The vacuum pumping system consisted of a mechanical pump and a diffusion pump whose outlet was connected to the vacuum chamber where the specimen was held in a vertical position by the specimen holder. The electrical connections (current leads, voltage taps and thermocouple wires) coming from the specimen holder were connected to terminals on the top platform. These terminals are connected to the instruments situated in the rack and then to a personal computer.

The instrument rack mainly consisted of (1) three separate Preston Amplifiers for amplification of the acquired signals, (2) programmable power supply, (3) a standard resistor and (4) a digital voltmeter. The amplifiers were set to accept thermoelectric voltage from the thermocouple welded to the specimen, voltage across the effective specimen length and the current signal from across a standard resistor. The amplified analog signals were fed to the analog-to-digital converter, and converted to digital signals which were then fed to the computer.

The input of the power supply was set to accept a digital to analog input signal from the computer, process it, then send the specified current to the specimen for the specified length of time.

Two main computer programs were written to conduct the specific heat experiment from the keyboard of the computer. The first program is a control and acquisition program and directs the power supply to send a one step current pulse to the specimen. The operator can specify the current for the pulse and its duration by typing their values using the keyboard or by selecting some standard runs. The acquisition part of the program loads the current, voltage and temperature signals from the specimen and arranges them in three data arrays. Calculations are performed on these data to compute the instantaneous power and the electrical resistivity of the specimen which are displayed on the screen for each data point. The data array for time, power and temperature can be stored on the hard disk to be used in another program.

The second program is the overall manager program and read the raw data created by the acquisition program from the hard disk and carries out conversions, differentiations and other calculations to get equations for the rate of change of temperature during pulsing and cooling and the equation for the power input to the specimen with time (t) and temperature (T). The calculated power and slope values are then substituted into the specific heat (C_p) equation and aCp-T curve in the desired range is obtained.

A section on preliminary data analysis has been included which shows the effect of current reversal, changing interval limits for data fitting and degree of fit on the specific heat curve. However, the main goal of the project was the design of the calorimeter rather than data analysis.

The main problems in the design of the calorimeter were due to vacuum leaks in the pumping system and disturbance in the thermocouple emf due to external electromagnetic fields.