GB865073A - Apparatus for simultaneously carrying out thermogravimetric, differential-thermal and derivative-thermogravimetric analysis - Google Patents

Abstract

<PICT:0865073/III/1> <PICT:0865073/III/2> <PICT:0865073/III/3> Apparatus for analysing mixtures by the thermogravimetric, differential thermogravimetric and differential thermal methods comprises two crucibles in a furnace, one containing the test material and attached by an electrically-insulating carrier to one arm of a balance and the other fixed and containing an inert reference material, two thermoelements to register the instantaneous temperatures of the contents of the two crucibles respectively, and means for measuring the rate of movement of the balance arm. In Fig. 1 the sample is placed in a crucible 5 arranged in a furnace 1 together with the reference material 16. Each crucible contains a thermocouple to measure the temperature of the contents either directly or through the walls of the crucible. The instrument 8 reads the difference between the thermocouple voltages and the instrument 6 can be joined by links to a or d to measure the individual temperature of either crucible. By plotting the difference between the temperatures against the reference temperature a "differential thermmal analysis" (DTA) curve can be obtained. The crucible 5 is attached to the left-hand side of the beam of the balance by a steel hoop 10 and variations of weight can thus be determined. The thermocouple leads 4 are preferably brought out not as shown but via torsion-free coils, one pair in the plane of the stirrup knife-edge of the balance and one pair in the plane of the balance beam (Figs. 5 and 6, not shown), to avoid drag on the beam. If a torsion balance is used the torsion filaments themselves can be used as current leads. Attached to the right-hand side of the beam is a permanent magnet or electromagnet 13 which moves in a coil and induces an E.M.F. in it proportional to the rate of movement and hence representative of the rate of loss of weight of the sample. By plotting the readings of the meter 14 against temperature a "differential thermogravimetric analysis" (DTG) curve can be obtained. The positions of magnet and coil can be interchanged (Fig. 7-10, not shown). In an alternative arrangement (Figs. 11-13, not shown) a shutter having a wedge-shaped pointer is attached to the balance pointer 41 and varies the light passing from a lamp to a photo-cell. The output of the photo-cell is differentiated by a transformer or a CR network to give the required output. The curves can be obtained automatically as follows (Figs. 16 and 19):-the TG curve is produced on continuously-moving photographic paper 49 by focusing the image of a slit in a plate 48 attached to the balance beam or pointer. The DTA and DTG curves are obtained similarly from mirrors on the galvanometer 8 and 14 respectively. A "temperature" (T) curve is obtained from the galvanometer 6. The vertical calibration lines h are produced from lamps 55, 56 shining through slits in screens 53, 54 in front of the casing of the paper. Four further lamps 57 light up periodically by means of a clockwork switch to give the time markers i and from these are drawn the horizontal calibration lines j which facilitate the reading of the chart in terms of temperature instead of time. A direct recording in terms of temperature can be made if the drum is driven by a motor energized from the amplified output of the thermoelement.