A system comprising an anesthetic vaporizer , and a first sensor , in communication with an inlet conduit , the inlet conduit configured upstream from the anesthetic vaporizer and to deliver a flow of carrier gas to the anesthetic vaporizer , wherein the first sensor produces a first sensor signal indicative of a composition of the carrier gas in the inlet conduit ; a second sensor , in communication with an outlet conduit , the outlet conduit configured downstream from the anesthetic vaporizer and to deliver a flow of combined carrier gas and anesthetic from the anesthetic vaporizer , wherein the second sensor produces a second sensor signal indicative of a composition of the combined carrier gas and anesthetic added by the vaporizer ; a third sensor , in communication with the inlet conduit and configured downstream or upstream relative to the first sensor , wherein the third sensor produces a third sensor signal indicative of the composition of the carrier gas in the inlet conduit ; a fourth sensor , in communication with the outlet conduit and configured downstream or upstream relative to the second sensor , wherein the fourth sensor produces a fourth sensor signal indicative of the composition of the combined carrier gas and anesthetic added by the vaporizer ; and a controller that receives the first, second, third and fourth sensor signals , computes a first anesthetic output with the first sensor signal and the second sensor signal , and further computes a second anesthetic output with the third sensor signal and the fourth sensor signal , wherein the first, second, third and fourth sensors are ultrasonic time-of-flight sensors, the first and third sensor signals are indicative of acoustic time-of-flight across the inlet conduit , and the second and fourth sensor signals are indicative of acoustic time-of-flight across the outlet conduit ; wherein the first sensor is mounted to a conduit without inclination to the direction of flow to transmit an acoustic wave perpendicular to the flow and mounted across from a respective alternatively flat and sloped surface which is configured to split the acoustic wave from the first sensor so that a portion of the acoustic wave is transmitted back to the first sensor and another portion of the acoustic wave is transmitted to the third sensor ; wherein the third sensor is mounted to the conduit without inclination to the direction of flow to transmit an acoustic wave perpendicular to the flow and mounted across from a respective alternatively flat and sloped surface which is configured to split the acoustic wave from the third sensor so that a portion of the acoustic wave is transmitted back to the third sensor and another portion of the acoustic wave is transmitted to the first sensor ; wherein the second sensor is mounted to a conduit without inclination to the direction of flow to transmit an acoustic wave perpendicular to the flow and mounted across from a respective alternatively flat and sloped surface which is configured to split the acoustic wave from the second sensor so that a portion of the acoustic wave is transmitted back to the second sensor and another portion of the acoustic wave is transmitted to the fourth sensor ; and wherein the fourth sensor is mounted to the conduit without inclination to the direction of flow to transmit an acoustic wave perpendicular to the flow and mounted across from a respective alternatively flat and sloped surface which is configured to split the acoustic wave from the fourth sensor so that a portion of the acoustic wave is transmitted back to the fourth sensor and another portion of the acoustic wave is transmitted to the second sensor . <EOS>