Well, it's negative, relative to atmospheric. The pump induces a pressure differential, regardless of the atmospheric pressure.
What you measured was the pump outlet versus atmospheric.
I think it was pHaestus that took some readings like that, or maybe it was someone from OC.... but I remember the negative reading at the pump intake.
What you (also) need is pump inlet versus atmospheric: that way you'll know the pressure increase from the pump in the closed loop.
Alternatively, you can do what BillA (aka unregistered) suggested: run a couple of Ts at the inlet and outlet, hook up some vertical tubing, and calculate the height difference in the columns of water, then translate it in psi. The tubes need to be of some length though...
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