inside surface and communicating with the ports 52 and 54 an o ring 45 might be used to separate surfaces b and c in this way if the voltage of points b and c were separately measured with respect to point a if there were a pressure gradient between the upper and lower ports 52 and 54 respectively points b and c would show a voltage relative to one another with reference to point a at e g ground potential upper and lower orifices 44a and 42a allow the mud to contact inner conductor a to provide a reference value for pressure this could be used to derive a signal indicative of the pressure gradient over the distance between the centers of the ports 52 and 54 this value could in turn be used in calculation of the integral of equation 11 to yield a value for q the remaining inputs to the equation 11 are readily derivable in accordance with prior art teachings it will be appreciated that what is essentially shown in fig2 is a combination of two pressure sensors disposed in very close proximity to one another and using the same piece of piezoelectric material referenced to a common ground the distinction between this structure and the use of two identical piezoelectric transducers disposed in close proximity to one another is that such a pair would require very careful zero adjustment and balancing before an accurate value for the gradient of the pressure could be derived it will be appreciated by those skilled in the art that the actual value of the pressure.