TMA/SDTA840: Rapid Cooling Between Measurements

Introduction 

It takes a certain amount of time for the TMA to cool down from the end temperature of an experiment to the initial temperature of the next measurement. To a good approximation, the cooling behavior can be described by an exponential function characterized by the cooling time constant, τ. If the cooling time constant is known, the time that the module needs to cool down from a temperature of T₁ to a temperature of T₂ is given by the following equation:

t_T1→T₂ = τ·[ln(T₁–T₀) – ln(T₂–T₀)]

where T₀ is the lowest temperature that is reached asymptotically. It lies between the cryostat temperature and room temperature. The temperature of the cryostat should be set to 22.0 °C as recommended in the operating instructions. If the temperature of the cooling flange is below 16 °C or above 45 °C, the measurement is automatically terminated and an error message is displayed.

In principle, there are several ways to reduce the cooling time constant. These are described in the following sections and are discussed briefly in connection with typical measurements.

Opening the Furnace at the End of the Measurement 

If the furnace is opened at the end of a measurement, the surrounding air is free to enter the furnace and reach the sample. The cooling time constant is however only reduced by about 20 s, which therefore only has a small effect on the furnace cooling time. On the other hand, the sample itself cools much more rapidly (cooling time constant about 50 s), which may be desirable in certain circumstances. So that the furnace can be opened immediately after it reaches the end temperature, the “Furnace Open Permission” must have been previously activated in the experiment buffer.

Summary 

Table 1 summarizes the cooling time constants obtained for different cooling methods. The table shows that a high purge gas flow rate is the most effective means of cooling the TMA840. Tables 2 and 3, show, in addition, the calculated cooling times for different start and end temperatures with no flow rate or a flow rate of air of 2 l/min. The following values were used: for τ, 7.83 min (with no purge gas) and 5.33 min (with 2 l/min purge gas flow); and for T0 20 °C. Remarks on the TGA/SDTA851e The above results also apply in principle to thermogravimetry. We have however noticed that, at high purge gas flow rates, hot gas from the furnace penetrates the housing of the microbalance and causes the balance to become warm. This in turn leads to an unacceptable balance drift, which adversely affects later measurements (about 0.1 mg/h).  

^{TMA/SDTA840: Rapid cooling between measurements | Thermal Analysis Application No. UC 136 | Application published in METTLER TOLEDO Thermal Analysis UserCom 13}