Wohler Flow Measurement
Flow meters measure the flow of fluids and report the results in a volume per time or mass per time such as gallons per minute or Kg/min. Since fluid properties vary so much from corrosive, high temperature, gas, liquid to steam, so do flowmeter technologies and materials. Let’s focus on a few of the technologies carried here at TEquipment.
Anemometers
An anemometer is an instrument designed to measure wind speed that is used not only for weather measurements but for HVAC applications as well. TEquipment carries a wide selection of anemometers, including models with other functions such as humidity, temperature and light measurement. We have grouped them with other Environmental meters here:
Environmental Testers / Physical Property >
Anemometer/Air Flow
This "Flow Measurement" section of our website is dedicated to other flow measurement technologies such as the following:
Pitot Tube / Accutube Products
Orifice Plates
Orifice plates are inserted between flanges into round ducts to measure gas or steam flows. They consist of a thin metal plate, usually stainless, with a hole in the center. The resulting pressure drop as gas flows past is measured by differential pressure sensors with taps on either side of the plate in the duct. They are low cost and maintenance free, but have some limitations.
Application considerations/limitations of Orifice Plates:
- Low turndown (i.e. the range from max to min flow is limited compared to other technologies)
- High pressure drop/losses to overcome
- Low accuracy
Pitot Tube
A pitot tube flow element, like an orifice plate, creates a differential pressure square root proportional to gas, liquid, or steam flow in a duct. Chief advantages over other flow elements include relatively low purchase price, low installation cost, and low permanent pressure loss (equates to low operating costs). A pitot consists of a pipe with holes across its length inserted into the cross section of the duct. At the connection point there is a fitting on duct or pipe and mating fitting on the pitot tube. Turndown is better than an orifice plate and there is less pressure drop to overcome.
Accutube
An accutube is a true double-averaging type of pitot tube manufactured exclusively by Meriam. These tubes use a round probe design that has been used for decades, unlike new Bullet or Diamond designs. The good thing about this is that round probe is low-cost and has been proven effective over many years of use. Meriam Accutubes boast up to ±0.5% accuracy when factory calibrated
and better than ±1% uncalibrated. These tubes also feature high flow turndown of 10:1. These relatively inexpensive tubes are the best option when it comes to capability, reliability, and longevity.
Other benefits of Meriam Accutubes:
- Very low permanent pressure loss
- No moving parts = more stable and accurate over time
- Multiple mounting options (wet tap, insertion, flanged, inline)
Laminar Flow Elements (LFE's) and Molbox/Molbloc Calibrators
Laminar Flow Elements (LFE's)
Laminar Flow Elements, or LFEs, follow the same principle of orifice plates and pitot tubes by creating a pressure drop proportional to flow rate for measuring gas flows, but in an inline flowmeter style. They are excellent for measuring low gas flow rates and inherently accurate and repeatable. Better turndowns of 20:1 or better are possible. Flow rate of gas mixtures can also be measured when the percentages of component gases and mixture properties are known.
LFEs are applicable in most clean and non-condensing gas flow applications. They excel in critical gas flow measurement applications and make excellent calibration standards because of their inherent accuracy, stable calibration, excellent response time and repeatability. LFEs are used to calibrate turbine meters, variable area meters, flow regulators, mass flow meters, and thermal anemometers to name a few. Gas flow measurement for demanding flow control is a common application.
Molbox/Molbloc Calibrators
![Fluke Molbox Gas Flow Calibrator]()
Click Image for Selection of Molbox/Molbloc Calibrators
Fluke Mobox/Molbloc Calibrators are designed for calibration of gas flowmeters. Molbox/Molbloc, originating with Hart Scientific, and now part of Fluke Calibration, has many proven years in the market and should be considered as a replacement to volumetric bell provers and piston provers for gas flow calibration.
Molbox/Molbloc is ideal for calibration of flowmeters such as
- Thermal Mass Flowmeters
- Variable Area FlowMeters (Rotameters)
- Turbine Meters
Molbox/Molbloc units are much smaller bench instruments that have no moving parts, no need for long straight runs, and provide results directly in mass flow units without needing corrections from volumetric to mass flow. For labs calibrating numbers of thermal mass flowmeters, work can be automated. Contact a TEquipment sales engineer and discuss your application and learn more with these application notes.
Flow Meter/Calibrator Selection Questions
Knowing as much as possible about your application will help in selecting the right technology and specific flow meter or flow calibrator model.
1. Flowing gas data
a. Flow rate in desired units
b. Base conditions
i. Pressure
ii. Temperature
c. Flowing conditions
i. Pressure
ii. Temperature
iii. Viscosity at flowing temperature
d. Differential at maximum flow
e. Specific gravity
f. Flowing gas or liquid
2. Readout instrument
3. Description of installation configuration
4. Line size
5. Line Material
6. Accuracy required