Question:
When attempting to measure ice nucleation points in leaves, is it better to attach a thermocouple probe to the outside of the leaf or to insert it directly into the leaf?
Background: One of the aspects of my research is measuring ice nucleation point in leaves ranging from nano phyllous ( 20 cm long). Some of these also have succulent leaves. I would like to do this using thin (0.2 mm) wire thermocouples, in a commercial freezer.
The problem: I have heard that inserting the thermocouple into the leaf (literally poking a hole in it) is inadvisable, as this causes cellular damage making it impossible to distinguish between the two different exotherms associated with ice nucleation (intra- and extracellular exotherms). I have tried this method and was only able to pick up one exotherm.
I also have heard that it is advisable to attach the thermocouples to the outside of the leaves, as they will then measure the external air temperature leading to loss of accuracy and a lack of proper exotherm measurement (essentially, background 'noise' deafening the sensor).
If it is better to attach the thermocouple to the outside of the leaf, how should I attach it in a way that 1) reduces thermocouple contact with the air inside the freezer, and 2) minimizes damage to the leaves. I have read of using 'thermally insulated clips' but am worried that these will damage the leaf cells, as many of the species I wish to analyse are quite delicate.
Suggestions would be most welcome!
Thanks,
Rob
I think you a have a difficult task on this type of measurement ... and if you put the thermocouple attached to the leave with a very bit sellotape.
We have used this method and the thermocuples can detect the exotherm even if attached to the leaf surface (externally)...in this way you preserve the integrity of the leaf tissues
Thermocouples can be glued to leaf by surgical tape small bits and this would do the job.provided the samples are kept in a closed polystirene foam box inside the freezer in order to smooth temperature decrease. The tape brand should ensure '... artificial silk tape with hypoallergenic acrylic adhesive. Very well tolerated by the skin. Its artificial silk fabric support enables correct transpiration...' . Water content, water potential, and calcium pectates as well a number of surface metabolites (...hardening) of inner cell walls would outplay size effect. We wrapped leaves in aluminium foil to avoid water sublimation. For plant freezing theory you may check Olien chapter in Li and Sakai book of 1978 if available
Hi There, thank you for answering my questions.
I will try using tape, but I am not sure about the glue? Will this not accumulate on the thermocouples?
Surgical tapes are self adhesive and you do not need extra glue. The fabric (silk or other type) allows transpiration and this reduces artifacts; Every medical supply store has a choice of them. It is relevant to decrease temperature smoothly. A couple of thermocouples attached to an inert like a sheet of polythene that is cut in leaf shape can be used as a reference. One is used as differential calorimeter .by taking the difference with an actual leaf and the other can test box temperature, At end the limits are on data handling during experiments avoiding too many variables..
Externally is the way to go. Exuded sap will also give an altered nucleation site. Bonding the lower surface of the TC to the leaf is best. While the TC still has exposure to air at a different temperature than the leaf (which cools radiatively), in energy transfer the conduction contact with the leaf dominates over the convective contact with air. This makes the measurement very close to leaf T. Pick your locations well, or sample multiple places on a large leaf; T varies from variation in the boundary layer. See MC Ball's work on this. Good luck.
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