We describe our progress in developing a novel gas flow standard that utilizes 1) microwave resonances to measure the volume, and 2) acoustic resonances to measure the average gas density of a collection tank / pressure vessel. The collection tank is a 1.85 m3, nearly-spherical, steel vessel used at pressures up to 7 MPa. Previously, using the cavity's microwave resonance frequencies, we determined the cavity's pressure- and temperature-dependent volume V BBB with the expanded uncertainty of 0.022 % (coverage factor k = 2, corresponding to 95 % confidence level). This was the first step in developing a pressure, volume, speed of sound, and time (PVwt) primary standard. In the present work, when the shell was filled with argon, measurements of pressure and acoustic resonance frequencies determined the "acoustic mass" M acst that agreed with gravimetric measurements within 0.04 %, even when temperature gradients were present. Most of these differences were a linear function of pressure; therefore, they can be reduced by further research. We designed and implemented a novel positive feedback system to measure the acoustic resonance frequencies. Using the measurements of V BBB, pressure, and acoustic resonance frequencies of the enclosed gas (nitrogen or argon), we calibrated 3 critical flow venturis that NIST has used as working standards for over 10 years. The two independent flow calibrations agreed within the long-term reproducibility of each CFV, which is less than 0.053 %. Furthermore, the feasibility of a dynamic tracking technique using this feedback loop was tested by comparing ΔM acst computed under no-flow conditions and ΔM acst computed by the rate of fall or rise during a flow. This was done for flows ranging from 0.11 g/s to 3.9 g/s.

中文翻译：

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利用微波和声共振制定气体流量标准的进展


我们描述了我们在开发新型气体流量标准方面取得的进展，该标准利用 1) 微波谐振来测量体积，2) 声学谐振来测量收集罐/压力容器的平均气体密度。收集罐是一个 1.85 m3、近球形钢制容器，使用压力高达 7 MPa。此前，我们利用腔体的微波谐振频率，确定了腔体的压力和温度相关体积 V BBB，扩展不确定度为 0.022%（覆盖因子 k = 2，对应于 95% 置信水平）。这是制定压力、体积、声速和时间 (PVwt) 主要标准的第一步。在目前的工作中，当外壳充满氩气时，压力和声共振频率的测量确定了“声学质量”M acst ，即使存在温度梯度，该质量与重力测量的一致性在 0.04% 以内。这些差异大部分是压力的线性函数。因此，可以通过进一步研究来减少它们。我们设计并实现了一种新颖的正反馈系统来测量声共振频率。通过测量封闭气体（氮气或氩气）的 V BBB、压力和声共振频率，我们校准了 3 个临界流量文丘里管，NIST 已将其用作工作标准超过 10 年。两次独立的流量校准在每个 CFV 的长期再现性范围内达成一致，该再现性小于 0.053%。此外，通过比较在无流量条件下计算的ΔM acst 和通过流量期间的下降或上升速率计算的ΔM acst ，测试了使用该反馈环路的动态跟踪技术的可行性。这是针对 0.11 g/s 至 3.9 g/s 的流量进行的。