A standard 10x10x40-mm cuvette containing 2 mL of water and a stir bar was placed in a QNW t2 Sport sample holder.
The sample temperature was monitored using a thermistor probe in the sample with the tip about 3 mm above the stir bar; the probe was connected to the external probe input of the controller.
T-App was used to:
1. turn temperature control on;
2. set the target temperature to 20 ⁰C;
3. set the magnetic stirring to 500 rpm and turn it on;
4. after equilibration at 20 ⁰C the target temperature was reset to 37 ⁰C (at time 0 in the graph);
5. the cell holder (black) and of the sample (red) temperatures were monitored at 1 second intervals.
The same data with the vertical range of the graph set to show the equilibration process in detail. The range of cell holder temperatures indicated by ß| 1 |à includes the first full minute that the sample holder temperature stays within the range 37 ± 0.05 ⁰C; this is the criterion used by the temperature controller to determine that the cell holder temperature is stable. The range indicated by |ß 10 à| is an additional 10-minute period required for the sample temperature to stabilize.
Question:
Why are the final sample holder temperature and sample (probe) temperature different.
Answer:
Consider the case when the Cell holder Temperature ( CT = 37 ⁰C) is greater than the surrounding Air Temperature ( AT = 25 ⁰C ).
The Rate of heat Gain ( RG ) of the sample is proportional to the temperature difference between CT and the Sample Temperature ( ST ).
RG = KG * ( CT ‑ ST ) where KG > 0
Similarly the Rate of heat Loss ( RL ) is proportional to the difference between AT and ST.
RL = KL * ( AT ‑ ST ) where KL < 0
The final sample temperature is reached when the two are equal.
KG * ( CT – ST ) = KL * ( AT – ST ) or ST = ( KG*CT ‑ KL*AT ) / ( KG ‑ KL )
| ‑ KG / KL | ST | insulation value |
| 500 | 36.98 | impossible |
| 75 | 36.84 | excellent |
| 23.5 | 36.51 | QNW |
| 10 | 35.91 | why bother |
| 5 | 35.00 | none |
| 2 | 33.00 | worse than none |