There’s widespread confusion about the use of “Standardized Volumetric Flow Rate” as “Mass Flow Rate” and which standard conditions to use for calculations. Official sources often don’t clarify this, and many online articles add to the confusion with incorrect or misleading explanations. This guide aims to clear things up once and for all.
"Standardized Volumetric Flow Rate" as "Mass Flow Rate"
What is “Standardized Volumetric Flow Rate”? The most straightforward way to define it is as the volumetric flow rate calculated by dividing the True Mass Flow Rate (Mf) by the fluid’s density under specific, standard conditions of temperature (Ts) and pressure (ps). So:
- Actual Volumetric Flow Rate (Q) = Mf / δ
- Standardized Volumetric Flow Rate (Qₛ) = Mf / δₛ, where δₛ = density at Ts, ps
This implies that if the temperature and pressure of the flow match the standard reference conditions, the Actual Volumetric Flow Rate would be equal to the Standardized Volumetric Flow Rate. Because Standardized Volumetric Flow Rate depends on a reference density, it varies with the specific fluid and the selected standard conditions.
The apparent paradox of using “Standardized Volumetric Flow Rate” as “Mass Flow Rate” applies mostly to gas flow rather than liquid flow, but this distinction is often overlooked, which adds to the confusion.
Liquid Flow
For liquids, the volumetric flow rate is more clearly distinct from the mass flow rate, and in most practical cases, there’s no need for a “standardized” volumetric flow rate. This is because a liquid’s density remains nearly constant over a wide range of temperatures and pressures. However, a liquid’s viscosity and flow characteristics (important for calculating energy loss) do change with temperature and pressure. Therefore, standard conditions are used mainly as a reference to understand changes in viscosity and flow characteristics rather than changes in density.
If we applied the concept of using “Standardized Volumetric Flow Rate as Mass Flow Rate” to liquids, it would generally result in the same value as the Actual Volumetric Flow Rate, making it redundant and confusing. It’s essential, however, to report the Actual Volumetric Flow Rate according to the range of operational conditions rather than standard reference conditions, as they are not the same.
So, in practice, for liquid flow, we have the following:
- Liquid Volumetric Flow Rate → Q =actual volume/time, e.g., liters per minute (L/min or LPM)
- Liquid “Mass Flow Rate” → Mf = mass/time, e.g., kilograms per hour (kg/hr)
When higher precision is needed to account for density variations, liquid flow can be reported in a “Standardized Volumetric Flow Rate,” similar to gases. In that case, it will differ from the Actual Volumetric Flow Rate.
Gas Flow
The density of gases varies greatly with changes in temperature and pressure, so using Actual Volumetric Flow Rate data without specifying operational conditions can be misleading. This is why Standardized Volumetric Flow Rate is preferred, as it references constant conditions that don’t need to be listed with each flow rate value.
Advantages of Standardized Volumetric Flow Rate for Gas Flow
- Ease of reading and comparison: Standardized Volumetric Flow Rate allows for straightforward, systematic comparisons across devices.
- Data consistency: It ensures data consistency in diverse conditions, making it ideal for applications such as medical device calibration, where standardized data is critical for accuracy.
- For example, when tabulating measurements for a medical device, a Standardized Volumetric Flow Rate is recorded instead of the Actual Volumetric Flow Rate, allowing anyone to interpret the data consistently, regardless of local conditions.
- From Equations (1) and (2):
3) Qₛ/Q = δ / δₛ - Using the Real Gas Law, we have:
δ = p / (R₉ T Z)
where R₉ is the specific gas constant and Z is the gas compressibility factor. - Substituting the Real Gas Law into Equation (3), we get:
Qₛ / Q = (p / pₛ) * (Tₛ / T) * (Zₛ / Z)
Using Equation (4), we can substitute volume units for volume rates in the left term, allowing us to calculate the Standard Volume Units in a vessel if we know its volume, internal pressure, and temperature. This also works in reverse. Since Standard Volume Units directly indicate mass, we often use them as a practical metric, which is why Standard Volumetric Flow Rate is generally preferred over both Actual Volumetric Flow Rate and True Mass Flow Rate for many applications, such as gas transfer and storage.
Once we have Qₛ, we could multiply it by δₛ to get Mₓ (Mass Flow Rate). However, in practice, we typically work only with volumetric flow rate units. If we were to convert to Mₓ and then back, we would end up dividing Mₓ by δₛ to get Qₛ again. This extra step is unnecessary since Qₛ already effectively represents Mₓ (because δₛ is a known constant).
A key advantage of using volumetric flowmeters instead of mass flowmeters is that, at low pressures commonly found in operational settings, we can approximate Zₛ / Z ≈ 1. This allows Qₛ to be calculated without knowing the specific gas, based only on Q, T, and p. In contrast, calculating Mₓ typically requires knowledge of the gas type. This makes Qₛ data more versatile and universally applicable, as it enables comparisons and calculations independently of the gas in use. In this way, Qₛ can be thought of as a simplified form of Mₓ.
Why is it Called "Mass Flow Rate"?
All the reasons above make Standardized Volumetric Flow Rate a preferred metric over True Mass Flow Rate, but why is it called “Mass Flow Rate” if it’s actually volumetric? This terminology became common because, in practice, Qₛ and Q are used almost exclusively. Using Qₛ prevents confusion with Q (which refers to the Actual Volumetric Flow Rate) while still representing True Mass Flow Rate.
- Gas Volumetric Flow Rate → Q = actual volume/time (e.g., l/min or LPM)
- Gas “Mass Flow Rate” (“Std Vol Flow Rate”) → Qₛ = standardized volume/time (e.g., SLPM)
"Mass Flow Rate" and "True Mass Flow Rate"
Since “Standardized Volumetric Flow Rate” is often called “Mass Flow Rate,” the term “True Mass Flow Rate” is used to indicate the actual mass per time, for both gases and liquids. This term is commonly used with flowmeters like Coriolis flowmeters, which measure True Mass Flow Rate directly.
So, to summarize:
- Liquid Volumetric Flow Rate → Q = actual volume/time , e.g. l/min (aka LPM)
- Gas Volumetric Flow Rate → Q = actual volume/time , e.g. l/min (aka LPM)
- (Note that, unless otherwise specified, “Volumetric Flow Rate” means “Actual Volumetric
- Flow Rate”)
- Liquid “Mass Flow Rate” → Mf = mass/time , e.g. kg/hr
- Gas “Mass Flow Rate” (“Std Vol Flow Rate”) → Qₛ = standardized volume/time, e.g. SLPM
- Gas/Liquid “True Mass Flow Rate” → Mf = mass/time , e.g. kg/hr
What are "Normal Conditions" and "Standard Conditions"?
There’s confusion about the values of NTP (Normal Temperature and Pressure) and STP (Standard Temperature and Pressure). Both terms generally refer to a chosen reference temperature and pressure for a specific application, rather than universally fixed values.
Definitions by ISO
ISO defines these terms in ISO 14532 as follows:
- Normal Conditions:
- Pressure, temperature, and humidity of 101,325 kPa and 273.15 K for dry gases.
- Standard Conditions:
- Pressure, temperature, and humidity of 101,325 kPa and 288.15 K for real gases in a dry state.
- Key Points on Terminology and Use
- The terms NTP and STP can vary by context and aren’t universally fixed.
- “Standardized” values add an “S” to units (e.g., SLPM), while “normalized” values add an “N” (e.g., NLPM).
- If someone claims “this is the standard,” ask for the specific context or application.
- For instance, ISO 13443 defines “Standard reference conditions” for international gas trade, while some other standards, like those from NIST or IUPAC, define slightly different values depending on their applications.
False Statements to Avoid
a) Specific values assigned to NTP/STP without context:
Assigning specific values to NTP (Normal Temperature and Pressure) and STP (Standard Temperature and Pressure) as if they are universally applicable is misleading. No single set of values can represent all engineering conditions.
- If someone claims “these are the standard values,” it’s essential to ask, “For which context?” If the answer doesn’t reference a specific standard or ignores the type of flow, field, or application, the values provided are unreliable.
- For instance, the “Scope” section of ISO 13443 defines “Standard reference conditions” as follows:
“The temperature, pressure, and humidity used for measurements and calculations of natural gases, substitutes, and similar fluids. The main purpose is for international trade of natural gases, where standardization helps simplify assessing both quality and quantity for global trade.” - Some articles incorrectly present certain values as “the standard” without mentioning that those values were set for specific applications. For example, some sources state that “the NIST standard is 20°C,” but this applies only to specific measurement tools, like calipers, not flowmeters, which often use 0°C instead.
- The misconception that NTP/STP have universal values leads authors to use these terms without clarifying the actual values for each context. Some add further confusion by referencing institutions (e.g., “STP from NIST”), which only provide limited context.
b) Assigning specific institutions to STP values
Statements like “the NIST standard is 20°C” or “the IUPAC standard is 0°C” ignore the fact that institutions set specific references only for particular uses. Both examples (“NIST uses 20°C” and “IUPAC uses 0°C”) serve different purposes based on their respective tasks.
c) Associating specific fields with NTP and STP
Claims such as “STP is used for chemistry, and NTP for fan evaluation” incorrectly imply that these terms are exclusive to certain fields. There isn’t a fixed rule for using these terms in particular fields; rather, the associated values vary depending on the specific standards and applications.
Conclusion
- “Standardized Volumetric Flow Rate” is often used to measure gas flow because it simplifies measurements, calculations, and data comparisons. However, it’s important to specify the gas type and the exact conditions used for its calculation, as these can vary based on the application.
- The term “Mass Flow Rate” became associated with “Standardized Volumetric Flow Rate” because, in practice, only Qₛ (Standardized Volumetric Flow Rate) and Q (Actual Volumetric Flow Rate) are used. Referring to Qₛ as “Mass Flow Rate” helps avoid confusion with Q, which typically represents the “Actual Volumetric Flow Rate.”
- “NTP” and “STP” (Normal and Standard Temperature and Pressure) do not have universally fixed values. Instead, they usually refer to a single temperature and pressure condition selected for a particular context or application. Therefore, assumptions should not be made based on these terms alone; it’s essential to verify the exact values in each specific situation.
- When selecting a standard reference condition, it’s likely that a suitable standard already exists for the case at hand. Therefore, it’s important to carefully check and ensure compliance with any relevant established standard before choosing conditions independently.
References
- ISO 14532:2014 Natural gas — Vocabulary.
- ISO 13443:1996 Natural gas — Standard Reference Conditions.
- ISO 6976:2016 Natural gas — Calculation of calorific values, density, (…).
- ISO 15970:2008 Natural gas — Measurement of volumetric properties: density (…).
- ISO 7504:2015 Gas analysis — Vocabulary
- ISO 5011:2020 Inlet air cleaning equipment for internal combustion engines (…).
- ISO 24062:2023 Measurement of fluid flow in closed conduits — Ultrasonic meters (…).
- 20 Degrees Celsius , Theodore D. Doiron, NIST, 2007.
- Unit Conversions , NIST, 2024.
- Calibration of Laminar Flow Meters for Process Gases , NIST, 2012.
- Gas flow standards and their uncertainty , NIST, 2022.
- Gas Flowmeter Calibrations with the Working Gas Flow Standard , NIST, 2009.
- Glossary of Atmospheric Chemistry Terms , Jack G. Calvert, IUPAC, 1990.
- Quantities, Units and Symbols in Physical Chemistry , IUPAC, 1993.
