ultrasonic blood level chamber/materials help

[tony944]

I am looking at alternate materials for a PVC blood chamber used in hemodialysis. This chamber snaps into a housing with 2 spring-loaded ultrasonic sensors contacting the housing on opposite sides. The US pulses go from one piezo, thru the chamber, and are received by the piezo sensor on the opposite side. If the blood level drops, and air (attenuates much more than blood or water) comes btw the US pulses, the signal is attenuated (amplitude drops) and an alarm is triggered. Traditional (soft/plasticized) PVC chambers have been used in this industry for years, and I would like to replace with a more rigid and thin-wall material (for assembly purposes like US bonding done at 20KHz). The only problem is, when this rigid chamber is empty (no blood), the signal is not attenuated sufficiently and the alarm is not triggered (very dangerous for the patient - air embolism). Presumably, the more rigid chamber matl is not attenuating the signal enough. We have tried adding softer copolymers to the new material mix and this has helped considerably. The new material is an SMMA and SBC are additives we have tried with some success. I have probed the op-amp at the receiver side with an Oscope and can actually measure the recieved signal pulses - with the more rigid matl indeed there is a larger residual signal when the chamber is empty compared to the soft PVC.

Questions:

1. Anyone have any experience with US transmission in blood chambers?
2. Any good references on the web regarding attenuation values for common plastics?

Thanks!