Thermal vs. laminar differential pressure mass flow measurement technology

When choosing mass flow measurement technology, there are many important factors to consider, including:

  • Accuracy
  • Pressure and flow limits
  • Repeatability
  • Response speed
  • Turndown ratio
  • Gas or liquid compatibility
  • Cost

Two common types of mass flow measurement technology include laminar differential pressure and thermal. While thermal technology has been around longer and is more well-known, laminar differential pressure technology is advantageous in many ways. Here we discuss those advantages and include a deep dive into their benefits using lab-grown diamonds as an example application.

Higher accuracy for multiple gases, even with fluctuating process conditions

For thermal mass flow instruments, accuracy is limited by errors in specific heat capacity assumptions and calculation restrictions such as K-factor conversions when switching between gases.

In contrast, Alicat’s laminar differential pressure mass flow instruments are preloaded with gas tables full of temperature and pressure data. As a result, they provide extremely accurate mass flow measurements even when process conditions are fluctuating or when switching between over 98 different gases. Due to their high accuracy and repeatability, laminar differential pressure mass flow instruments are often trusted as calibrating standards, such as laminar molblocs.

Faster, near-instantaneous warm up times

Thermal mass flow devices may take over 30 minutes to warmup, whereas Alicat laminar differential pressure mass flow devices are ready in less than 1 second. This time saved translates to higher output and profitability.

Less restrictive, with no need for straight pipes

Due to the nature of convective heating, thermal mass flow instruments must be installed with straight piping and perfect horizontal integration. Conversely, laminar differential pressure mass flow instruments can be installed in any configuration as measurements are not affected by adjusting their flow body orientation. The less restrictive laminar differential pressure mass flow technology can be added in any orientation, vastly increasing engineering possibilities when designing systems.

Wider operating range (turndown ratio)

Turndown ratios or control ranges on laminar differential pressure mass flow devices are generally greater than on thermal mass flow devices due to their mechanical limitations. Most of Alicat’s laminar differential pressure mass flow instruments operate across a controllable range of 0.01 to 100% of full scale (10,000:1 turndown), whereas most thermal mass flow instruments offer turndown ratios of just 100:1 or 50:1.

Laminar differential pressure mass flow technology limits the amount of individual devices that need to be purchased for controlling different flow ranges. Therefore, applications requiring a wide range of flow rate control benefit tremendously from laminar differential pressure technology.

Application highlight: Diamond CVD

Laminar differential pressure mass flow instruments provide several benefits over thermal devices in diamond CVD systems:

  • Improved repeatability (±0.1% of reading + 0.02% of full scale)
  • Wider control range (0.01 to 100% of full scale)
  • Expansive gas compatibility (over 98 preloaded gas calibrations)
  • Increased diamond production rates
  • High-accuracy gas mixing
  • Expanded system build options
  • Lower device costs

Quick comparison chart

Thermal Differential pressure
Accuracy 1–2% <1%
Control Range 50:1 or 100:1 turndown ratio 10,000:1 turndown ratio
Unknown Gas Compositions No No
Startup Times >30 minutes <1 second
Cost of Devices Variable Variable
Corrosive Gas Compatibility Limited Wide
Orientation Options Restrictive Unlimited
Low Flow Range Control Excellent Excellent
High Flow Range Control Poor Poor


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