The (P)R-series probes were the original series with a 45 degree angle (+/-10% spec; lower offset). These were very long along the vessel to maintain the correct transducer to reflector geometry
which was inappropriate for some vessel applications (ascending aorta, pulmonary artery, other tight anatomical locations) and also caused the probes to be bulky and heavy as they got larger in size.
The R-series were termed "high resolution".
(P)S-Series probes are smaller bodied because the angle of the transducers to the reflector is steeper (55-60 degrees) allowing a shorter separation of the transducers along the vessel. Thus the housing
of the probe is more compact but the resolution of the S-series probes was slightly lower at ±15% for the T101D/T201D (the unit available at the time of release)
When the the 400-series flowmeters were introduced, the electronics were improved as was the accuracy and offset specs. At this stage, the larger PR-series probes began to be phased out. The smaller
PR series are retained for specific applications.