Monitoring temperature

[Blue Boy]

Hi Everybody.

Just joined the forum and had a question.
I bought a digital temperature gauge(not a thermometer in the true sense) some time ago from the vastly underated Lidl(£2.99).It consists of a large matchbox sized unit that is supposed to monitor inside temp and hold batteries and a thin wire with a weatherproof external sender unit(pea sized).
(enough background - get to the point!)
Basically i've made a kit up and placed the unit on top of the fv and the wire(sanitised) into the inside through the bubbler hole.
Result - Room temp 15 - 21 oC for 5 days.
Wort constant temp Day1 / 2 21oC
Day 3 / 4 20oC
Day 5 19oC
Question(s) is -Is this any use, Can I effectively monitor progress without opening FV.Have i lost the plot?
Any thoughts ?
B.B