The unit label is only the start
A purchase request for “100 m³/h air” is incomplete. Ask whether this means volume at the measuring point, volume referred to an agreed state, or a display derived from mass flow. Record the gas and the pressure, temperature and moisture basis used for the specification.
Bronkhorst's FAQ illustrates why reference states must be stated rather than inferred from a symbol. Do not assume that every supplier uses Nm³/h or Sm³/h with the same definition. Keep the agreed numerical conditions beside the unit in the RFQ and receiving-system settings.
A limited ideal-gas conversion
For the same unchanged gas, treated as ideal and with the same dry-gas basis, the equation of state gives this comparison:
Q_ref = Q_actual × (p_actual_abs / p_ref_abs) × (T_ref_K / T_actual_K)
Q is volume per unit time. Both pressures must be absolute, expressed in the same units. Both temperatures must be absolute; T_K = t_°C + 273.15. The NASA source explains the ideal-gas equation and its limits; the flow comparison here is an editorial algebraic derivation for equal gas amount per unit time.
If a pressure reading is gauge pressure, obtain the applicable local atmospheric pressure before forming absolute pressure. Do not insert the gauge value directly or silently assume that a gauge transmitter is an absolute instrument.
Worked example with declared assumptions
Assume an illustrative dry ideal gas at 100 actual m³/h, 8.000 bar absolute and 20 °C. Choose, solely for this example, a reference state of 1.01325 bar absolute and 0 °C.
- Actual temperature: 293.15 K; reference temperature: 273.15 K.
- Q_ref = 100 × (8.000 / 1.01325) × (273.15 / 293.15).
- Q_ref ≈ 735.67 m³/h at the stated reference conditions.
The gas amount per unit time has not increased. Its equivalent volume is reported at a different state. These are invented teaching inputs, not a customer's duty or a product performance claim. For moist gas, changing composition or significant non-ideal behaviour, use an appropriate manufacturer-approved method; this simple calculation does not supply those corrections.
Convert once, in a defined place
Before adding a calculation to a PLC or platform, identify the quantity already sent by the instrument. A meter may report mass flow or an already corrected volume. Applying the example again to an already corrected value would change its meaning.
For every received value, retain the tag, physical quantity, units, reference-state settings and source parameter or register. Check the full version's documentation; a product family name does not establish an interface mapping. Compare a displayed value with the received value under the same agreed settings before acceptance.
What to send with the enquiry
Include minimum/normal/maximum flow, each with its stated basis; gas condition; working pressure and temperature; pipe information; the desired output quantity; and the receiving system. Attach any existing settings record and flag unknown values.
Endress+Hauser t-mass F 500 is a separate international-brand enquiry entry on this site. Confirm its complete configuration and applicable technical information. WESIVO 728 is a pressure-transmitter enquiry entry, not proof that it calculates gas flow. The related brewery case is attributed to KROHNE and does not demonstrate either listed product's performance.
单位名称只是核对的起点
“空气 100 m³/h”不足以定义一份采购需求。先问清这是测点工况下的体积、换算到约定状态的体积，还是由质量流量导出的显示量。记录气体名称，以及规格采用的压力、温度和干湿基准。
Bronkhorst 的技术问答说明，参考状态需要明确写出，不能只看符号。不要默认每个供应商对 Nm³/h 或 Sm³/h 的定义都一样。询价文件和接收系统设置中，应把约定的数值条件与单位放在一起。
有明确限制的理想气体换算
同一种未发生变化的气体，在按理想气体处理且干气基准一致时，可由状态方程得到：
Q_ref = Q_actual × (p_actual_abs / p_ref_abs) × (T_ref_K / T_actual_K)
Q 为单位时间内的体积。两项压力都必须是绝对压力，且单位相同；两项温度都必须是绝对温度，T_K = t_°C + 273.15。文末 NASA 来源解释了理想气体状态方程及其限制；这里的流量关系是按单位时间内气体物质的量不变进行的编辑推导。
若现场读数是表压，应先取得适用的当地大气压力，再形成绝对压力。不能直接把表压代入，也不能悄悄将表压变送器视为绝压仪表。
明确假设后的算例
仅为教学，假设干燥理想气体的工况流量为 100 m³/h，绝对压力 8.000 bar，温度 20 °C。本例单独选取参考状态为绝对压力 1.01325 bar、温度 0 °C。
- 工况温度为 293.15 K，参考温度为 273.15 K。
- Q_ref = 100 × (8.000 / 1.01325) × (273.15 / 293.15)。
- 所得参考状态体积流量约为 735.67 m³/h。
单位时间内的气体物质的量并未增加，只是换到另一状态下报告等效体积。这些是假设教学数据，不是客户工况或产品性能。湿气、组分变化或明显非理想行为，应采用适合且经厂家认可的方法；本简化算式不包含这些修正。
明确换算位置，避免重复处理
在 PLC 或平台中增加计算之前，先识别仪表已经发送的物理量。仪表可能报告质量流量，也可能已经输出换算后的体积；对已换算值再次应用本例公式，会改变原来的含义。
每个接收值都应关联位号、物理量、单位、参考状态设置及来源参数或寄存器。按完整版本核对文件，不能仅凭产品系列名称确定接口映射。验收前，在相同约定设置下比对本地显示与接收值。
询价时一起提供什么
提供最小/正常/最大流量，并逐项写明口径；附气体状态、工作压力和温度、管道资料、所需输出量及接收系统。已有设置记录应一并提交，未知项应明确标出。
本站 Endress+Hauser t-mass F 500 是独立的国际品牌询价入口，应确认完整配置及适用技术文件。WESIVO 728 是压力变送器入口，不能据此推定其计算气体流量。关联啤酒厂案例署名 KROHNE，不证明上述任一产品在该现场的性能。
Discuss your application
Send your medium, operating conditions, connection, output, quantity and destination. WESIVO will review the configuration before quotation.