How is heat exactly measured in DSC

[ussername]

In order to obtain DSC curves the instrument has to measure a HEAT when changing the temperature.
During the measuring cycle with increasing temperature, I bet the heat is measured via Joule heating.
But how is the sample cooled down together with measuring required heat during a cooling cycle?

 

[Borek]

Have you tried googling for the construction of DSC units?

 

[ussername]

Yes I found out the principle for one of the heat flux sensors. That's simply a thin plate which has termocouples on its both sides. The thermal conductivity of the plate is known. The plate is situated perpendicular to the heat flow and we measure temp. difference on both sides in thermal steady state. From measured values and from thermal conductivity one can obtain a heat flux.

But there is a problem of controlling a heat flow during cooling cycle of DSC. The instrument needs to control heat flow very precisely. How can it achieve? Is it possible to do it simply via controlling the flow of cooling media?

 

[Borek]

Does it really have to be very precise? DSC is a differential method, in which you compare your sample against a known medium. The most important part is that both the sample and the reference are cooled/heated simultaneously, and the constancy of the heating/cooling speed is not that important. Sure, it is better to have it under control, but I don't think it is the most important aspect of the whole method (and device design).

I can be wrong though.

 

[ussername]

Whether it is precise or not, there must exist some mechanism of heat flow regulation during the cooling cycle.

 

[DrStupid]

Whether it is precise or not, there must exist some mechanism of heat flow regulation during the cooling cycle.

The heat flow is not regulated but measured. This measurement should be as precise as possible. The environmental temperature is regulated. The required precision of this regulation depends on the principle of the measurement.

 

[ussername]

The heat flow is not regulated but measured. This measurement should be as precise as possible. The environmental temperature is regulated. The required precision of this regulation depends on the principle of the measurement.

As there is programmed some temperature profile to undergo and the heat capacity of sample is generally changing with temperature, the heat flow must be regulated during measurement (in order to implement the temperature profile). If you keep constant heat flow during measurement you will heat up the reference crucible faster than the sample of course..

 

[DrStupid]

the heat flow must be regulated during measurement (in order to implement the temperature profile).

No, the temperature is regulated and the corresponding heat flow is measured.

 

[ussername]

No, the temperature is regulated and the corresponding heat flow is measured.

Ok if you regulate the temperature, you simultaneously regulate the heat flow. So the question is how to regulate the temperature during a cooling cycle.

 

[DrStupid]

Ok if you regulate the temperature, you simultaneously regulate the heat flow.

Not necessarily. In case of a heat flux DSC there is a single environmental temperature (which is regulated) but two different heat flows (sample and reference).

So the question is how to regulate the temperature during a cooling cycle.

With a suitable control circuit. The details depend on the specific technical solution for heating and cooling.

 

[ussername]

With a suitable control circuit. The details depend on the specific technical solution for heating and cooling.

Do you know some common example of that technical solution? I'm curious what physical quantity is changing in order to regulate heat flow (in power compensated DSC). I proposed changing the flow of cooling media as it changes the heat transfer.

 

[DrStupid]

Do you know some common example of that technical solution?

No, but the easiest way would be a combination of non-regulated cooling and regulated heating.

I proposed changing the flow of cooling media as it changes the heat transfer.

That would be a more sophisticated solution.

 

[Kevin McHugh]

In order to obtain DSC curves the instrument has to measure a HEAT when changing the temperature.
During the measuring cycle with increasing temperature, I bet the heat is measured via Joule heating.
But how is the sample cooled down together with measuring required heat during a cooling cycle?

There are two types of cooling in DSC, ambient and cryogenic. In either case, cooling is affected by liquid nitrogen, or ambient water.