Why not just put all the equipment fields in a form header and the readings in a single subform below? No matter which date you're looking at, the equipment fields stay populated until you change the equipment selection in the equipment combo in the header - then they repopulate with new data and so does the subform. If the subform isn't a data sheet, 2 date fields on its header could provide you with all the temp readings over the period, or if it's just one date textbox, for the selected date. With 1, you'd have no records for a date that has no readings, so that might be less useful. If it is a datasheet, the unbound date fields can be on the header of the main form. It's just a bit trickier to reference them from the subform (but quite doable). Either way, the subform requires date field(s) in the recordset even if they're not visible.
It looks to me that you might be repeating text data in child tables where there is a relationship. Rather than putting the id (PK) of the equipment in the readings table as a FK, you seem to be repeating the text value. Can work ok if you have cascading updates enabled in established table relationships, but it's not the best way to go.
Rather difficult to decipher if you are calibrating thermometers (the equipment) and recording the test readings, or if some type of equipment is being subjected to temperature readings and the thermometer is just a tool. It appears to be the former, but either way, I don't see why a single subform wouldn't work.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.