Temperature Equilibration of the Conductivity Cells
Note that the box equilibration will be faster, as only the air has to change temperature
It is very important that the temperature of the two cells stays the same during an experiment. As you can see from the above graph, there is a big delay if the cell temperatures differ, before they come back to thermal equilibrium.
Thus if any component is to be added to only one cell, it must either be matched with an equivalent addition to the other one [at the same temperature] OR the solution to be added must be brought to the correct temperature before addition. This is fairly easy to achieve if a small volume is added. I often use 0.01 ml [for the enzyme addition to one cell]. It is best to try a few control experiments before commencing a series of assays.
The exactness of the temperature balance becomes more important as the sensitivity of the experiment increases. Thus measuring mM changes as initial rates requires less attention to temperature balance than does measurements of 10 micromolar changes.
Metal-walled cells - comparison with plastic-walled cells
The stainless steel tubing shown in the diagram below was used to construct low cost cells exactly as shown for the plastic ones, except that the tubing had a 2.5 mm wall compared with 1 mm for the plastic ones.
The trace of recovery to the balance condition from a temperature mismatch (ca. 10 degrees) is shown below.
It can be seen that the initial rate at which the balance condition is approached is much faster (6-fold, based on the relaxation times) compared with the plastic walled cells. The rapid phase results from fast equilibration with the metal, which has high heat conductance. Once the liquid in the cell has 'extracted' heat from the metal to lower it's temperature to that of the liquid, the equilibration process slows down. This happens because further heat has to be extracted from the air in the box via either the metal wall or plastic base of the cell. The transfer of heat from the air to the metal is slower than that from the metal to the liquid.
This means that a large temperature difference (such as that shown above, ca. 10 degrees) is not compensated by the high heat conductivity of the metal wall. The amount of heat in the metal is not enough to bring the liquid to equilibrium with the air in the box.
However if the temperature difference is smaller i.e if a small volume of liquid is added to a cell at equilibrium, the heat capactity of the metal is sufficient to rapidly bring the system back to the balance condition by rapid heat transfer from the metal.
This is shown in the fig. below where the return to balance can be seen to be achieved in ca. 30 secs. The balance is perturbed by less than 0.001V, equivalent to a change in concentration of NaCl of less than 2 micromolar [2 nmoles]. The system is remarkably robust with respect to any normally encountered perturbations that could cause temperature changes during experiments.
This represents a good reason to fabricate the cells with metal walls!
The take-home message is: You will not have a problem with temperature control if you add small volumes that have been adjusted to a temp near the experimental temp.
1) Plastic walled cells 2) Metal walled Cells
1) Plastic Walls