Sensor Science Division, Physical Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Dr., Gaithersburg, Maryland 20899, USA.
Rev Sci Instrum. 2023 Mar 1;94(3):034904. doi: 10.1063/5.0143819.
The National Institute of Standards and Technology measured gas flows exiting large, unthermostated, gas-filled, pressure vessels by tracking the time-dependent pressure P(t) and resonance frequency f(t) of an acoustic mode N of the gas remaining in each vessel. This is a proof-of-principle demonstration of a gas flow standard that uses P(t), f(t), and known values of the gas's speed of sound w(p,T) to determine a mode-weighted average temperature ⟨T⟩ of the gas remaining in a pressure vessel while the vessel acts as a calibrated source of gas flow. To track f(t) while flow work rapidly changed the gas's temperature, we sustained the gas's oscillations using positive feedback. Feedback oscillations tracked ⟨T⟩ with a response time of order 1/f. In contrast, driving the gas's oscillations with an external frequency generator yielded much slower response times of order Q/f. (For our pressure vessels, Q ∼ 10-10, where Q is the ratio of the energy stored to the energy lost in one cycle of oscillation.) We tracked f(t) of radial modes in a spherical vessel (1.85 m) and of longitudinal modes of a cylindrical vessel (0.3 m) during gas flows ranging from 0.24 to 12.4 g/s to determine the mass flows with an uncertainty of 0.51 % (95 % confidence level). We discuss the challenges in tracking f(t) and ways to reduce the uncertainties.
美国国家标准与技术研究院（NIST）通过跟踪剩余气体的声模态 N 的压力 P(t) 和共振频率 f(t)，测量了大型无恒温、充满气体、压力容器中的气体流量。这是一种使用 P(t)、f(t)和已知气体声速 w(p,T)的气体流量标准的原理验证演示，用于确定压力容器中剩余气体的模式加权平均温度 ⟨T⟩，同时压力容器充当经过校准的气流源。为了在流动功快速改变气体温度时跟踪 f(t)，我们使用正反馈来维持气体的振荡。反馈振荡以约 1/f 的响应时间跟踪 ⟨T⟩。相比之下，使用外部频率发生器驱动气体的振荡会产生响应时间慢得多的约 Q/f。（对于我们的压力容器，Q ∼ 10-10，其中 Q 是存储的能量与一个振荡周期中损失的能量之比。）我们在气体流量从 0.24 到 12.4 g/s 范围内跟踪了球形容器（1.85 m）中的径向模式和圆柱形容器（0.3 m）中的纵向模式的 f(t)，以确定质量流量，不确定度为 0.51 %（95 %置信水平）。我们讨论了跟踪 f(t)的挑战和降低不确定性的方法。
