The external temperature probe (P2) has been selected as the control probe, and the setpoint set to -20C (as shown in the photo here:
As you can see, the external control is selected (as shown on the photo), and the temeprature of the sample chamber (external temeprature probe) is showing -9.05C. The bath temeprature (as measured by P1 is currently at -19.62C. If left for sufficiently long, the bath temperature would stabilize at the target temperature (-20C). It's pretty close in this photo already.
The differential temperature setting at this point was 2C, as can be seen in this photo:-
Re-reading the description of the differential temeprature on page 33 of the manual, I thought that the differential temperature setting might have been the problem. So I changed the differential temperature setting to 20C, as shown here:-
In order to achieve -20C at the sample (P2), I would expect the bath temperature to have to be between -30C and -40C (perhaps -35C). With external control selected, and the differential setting set to 20, I would therefore expect the bath temeprature to drop. However, the opposite happened, and after a short while the temeprature of the bath had risen to approx. -17C, and the sample temeprature had risen to -7.3C. You can see that clearly here:-
So, I'm not sure what the diferential temeprature setting is supposed to do, but it doesn't appear to work as I expected.
I thought I'd try to illiminate any potential issues associated with the extremely cold -20C setting, by trying the behaviour at zero (0.0C).
The setpoint was set to 0.0C, and the differential set to 5C. The control is set to the external probe.
In order to acheive 0C at the sample, and with an ambient at approx. 23C, I would expect the bath to have to be at -5C or so. That shouldn't have been too much of a problem for the AD15R-40 unit.
However, after about 4 hours, the temperature had stabilized with the bath at almost exactly zero celsius, and hte sample at 4.8C, as you can see here:-
The conclusion is that the external temperature control does not work at all.
The unit is controlling from the internal temperature probe (P1), and not from P2.