There was an ancient post on this BB by a guy who installed a temperature sensor in the upper rad hose plus he tapped into the rad overflow hose (the little one that goes to the top of the reservoir) with tubing that lead to a dash mounted pressure gauge.

He said that after cold startup the first thing to show activity was the pressure gauge, which started to increase well before either temp gauge reacted. Then the Volvo dash gauge and finally the hose-monitoring gauge. (This makes sense, as the Volvo gauge monitors the coolant temp in the cyl head, and the hose doesn't get hot until the thermostat opens.)

Old-timers on this BB may remember that post and the poster's description of the action of the various needles which he described as "a bit of Kabuki theater".

I tapped into that same upper line with a little 0-30psi gauge and found the same as he did...pressure rises before any temperature indication on whatever gauges you have. It's due to thermal expansion of the coolant in a confined space. This assumes the pressure cap is sealing correctly. My system pressured up to a steady 8psi at full operating temp. with the stock 75kPa (11psi) cap.

Aftermarket gauges I have used don't even register until temp. gets to 130F or so. That's typically the minimum on their scale. I think the Volvo dash gauge is like that. At least my non-compensated 1983 is.

I have tested some aftermarket gauges both mechanical and electrical, and the only ones that had decent accuracy were the more expensive ones such as VDO, Autometer, and such. Those little 3-gauge panels from the FLAPS are particularly bad.
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Bob: Son's XC70, my 83 244DL, 89 745 (Chev LT-1 V8), 98 S90 (recently sold) and 2010 XC60. Also '77 MGB and four old motorcycles