A method, comprising: placing a material in a flow tube while exciting a vibration mode of the flow tube , wherein exciting the vibration mode of the flow tube comprises the steps of: periodically driving a first driver with a first signal; periodically driving a second driver with a second signal, wherein the second driver is driven in phase with the first driver , wherein the first driver's drive amplitude modulated signal reaches a maximum amplitude when the second driver's drive amplitude modulated signal reaches a minimal amplitude, and the first driver's drive amplitude modulated signal reaches a minimum amplitude when the second driver's drive amplitude modulated signal reaches a maximum amplitude; measuring the relative phase between a first pickoff and a second pickoff ; and determining a relative phase of a right eigenvector for the flow tube , characterized by : measuring a frequency shift that occurs between when the first driver's drive amplitude modulated signal reaches a maximum amplitude and when the second driver's drive amplitude modulated signal reaches a maximum amplitude; offsetting the first signal with a phase shift; adjusting the first signal offset until the frequency shift is substantially undetectable; and determining the relative phase of the left eigenvector coefficients from the first signal offset necessary for rendering the frequency shift substantially undetectable. placing a material in a flow tube while exciting a vibration mode of the flow tube , wherein exciting the vibration mode of the flow tube comprises the steps of: periodically driving a first driver with a first signal; periodically driving a second driver with a second signal, wherein the second driver is driven in phase with the first driver , wherein the first driver's drive amplitude modulated signal reaches a maximum amplitude when the second driver's drive amplitude modulated signal reaches a minimal amplitude, and the first driver's drive amplitude modulated signal reaches a minimum amplitude when the second driver's drive amplitude modulated signal reaches a maximum amplitude; periodically driving a first driver with a first signal; periodically driving a second driver with a second signal, wherein the second driver is driven in phase with the first driver , wherein the first driver's drive amplitude modulated signal reaches a maximum amplitude when the second driver's drive amplitude modulated signal reaches a minimal amplitude, and the first driver's drive amplitude modulated signal reaches a minimum amplitude when the second driver's drive amplitude modulated signal reaches a maximum amplitude; measuring the relative phase between a first pickoff and a second pickoff ; and determining a relative phase of a right eigenvector for the flow tube , characterized by : measuring a frequency shift that occurs between when the first driver's drive amplitude modulated signal reaches a maximum amplitude and when the second driver's drive amplitude modulated signal reaches a maximum amplitude; offsetting the first signal with a phase shift; adjusting the first signal offset until the frequency shift is substantially undetectable; and determining the relative phase of the left eigenvector coefficients from the first signal offset necessary for rendering the frequency shift substantially undetectable. measuring a frequency shift that occurs between when the first driver's drive amplitude modulated signal reaches a maximum amplitude and when the second driver's drive amplitude modulated signal reaches a maximum amplitude; offsetting the first signal with a phase shift; adjusting the first signal offset until the frequency shift is substantially undetectable; and determining the relative phase of the left eigenvector coefficients from the first signal offset necessary for rendering the frequency shift substantially undetectable. <EOS>