ASHRAE Journal - July 2019 - 72

Overcoming the Glycol Measurement Challenge
By Philip Holoch, Senior Development Engineer for Sensors at BELIMO Automation AG, Hinwil, Switzerland

Glycol is commonly used to prevent heat transfer fluids from freezing in HVAC systems. The advantage
of using glycol-water mixtures is for systems to be able to operate below 32°F. There are a number of
issues associated with these mixtures; more expensive than water, have a lower heat capacity, and require
up to 30% higher flow to achieve the same energy transport. At low temperatures, glycol-water mixtures
become quite viscous, requiring between 50 and 100% additional pump energy to match the heat transfer
of water alone. Another complication is that ethylene glycols are toxic and not environmentally friendly.
Heat transfer fluids are complex mixtures of around 60 to 75% deionized water, 25 to 40% monoethylene
or propylene glycol with typical additives. Providers offer different formulas for speciﬁed applications.
The composition also changes for cost optimization, new legislation, or over time due to:
* degradation due to thermal overexposure;
* water addition for pressure losses in the system;
* variations in concentration due to on-site mixing.

Figure 1

transducer

Figure 2
laminar

Composition changes impact the density, heat capacity, and viscosity. Change effects the measured
thermal energy (equation below) and the volumetric flow. The change in viscosity will have an impact
on the flow proﬁle or velocity distribution. If a thermal energy meter for use with water is employed with
a glycol water mixture, changes in fluid properties can result in a cumulative heat measurement error up
to 40%. If the correct fluid parameters are known, it is possible to compensate.

turbulent flow

Thermal energy (Q) deﬁned by the formula:
0

V = volumetric flow
ρ = density
cp = heat capacity
∆T = differential temperature (supply - return)
Figure 3

Trusted flow measurement
This issue has been recognized for years. The option was to accept the measurement error or avoid
glycol. However, Belimo has developed an inline flow sensor that offers automatic, continuous glycol
measurement and compensation, providing trusted flow measurements without drift. With ultrasonic
transit-time technology, it provides an accurate method for thermal energy measurement that is
insensitive to magnetite problems and suitable with a wide range of heat transfer fluids.

This data, together with the fluid temperature, can be used to calculate the kinematic velocity and
determine the volumetric flow. As volumetric flow requires an average of the flow velocities across the
diameter of the pipe, not just a single pathway, this is affected by differing flow proﬁles (Figure 2). In
laminar flow (grey), the velocity is greatest in the center and slower at the pipe wall. In contrast, turbulent
flow (blue), the flow velocity is virtually identical across the pipe diameter. This effect is compensated
during production by using correction curves to calibrate the sensor. As the flow proﬁle is influenced by
fluid viscosity, the presence of glycol can lead to signiﬁcant measurement error.

Figure 4
40%
30%

rel. error [%]

Belimo flow sensor uses two offset transducers (Figure 1) to determine the flow velocity by measuring
the speed of sound in the heat transfer fluid. The downstream transit time is measured by passing an
ultrasonic pulse from the lower transducer to the upper transducer; the process is then reversed to
obtain the upstream time. Transit times differ based on the flow of fluid through the sensor tube; the
downstream pulse is accelerated, the upstream pulse is impeded and slowed. The ultrasound pulses
can be used to determine the measured velocity (
).

20%
10%
0%
-10%

-20%

This was used to develop a patented automatic glycol compensation algorithm that selects the correct
fluid properties for the flow and energy calculation. The algorithm can be applied to a range of heat
transfer fluids, ensuring accurate and repeatable measurements.
Without compensation (Figure 4), volumetric flow measurements can show errors up to 30%. The
application of automatic glycol concentration (Figure 5) minimizes drift and signiﬁcantly reduces the
degree of measurement error.
Conclusion
The Belimo flow sensor with ultrasonic transit-time technology that automatically measures and
compensates glycol concentration is an advancement in thermal energy. A 'ﬁt and forget' sensor to
compensate variable and changing viscosities. The sensor has no moving parts, rugged design, requires
no calibration, and provides accurate, repeatable measurements improving the control and efﬁciency of
HVAC systems.

-30%
Volumetric Flow
Error

Band

77°F

194°F

Error

Band

77°F

194°F

Figure 5
20%
15%

rel. error [%]

Another approach, the Reynolds number (Re=v*D/mu) which is interpreted as the ratio between inertial
forces to viscous forces within a fluid. On one side of the ratio, the different fluid velocities in a flow
create turbulences. On the other side, the viscosity is counteracting the creation of turbulences. The
flow velocity distribution is similar for a given Reynolds number. The correction factor used to relate the
volumetric flow to the average velocity across the cross-sectional area of the pipe plotted as a function
of Reynolds number is independent of the fluid viscosity. The water and glycol-water calibration curves
are superimposed (Figure 3). The sensors can be calibrated with water and measure accurately if the
viscosity of the fluid is known.

10%
5%
0%
-5%

1100

15
15

20
2

-10%
-15%
-20%

Volumetric Flow
Error 6%

eth.
eth.
eth.
prop.
prop.
prop.
glycol 51% glycol 51% glycol 51% glycol 47% glycol 47% glycol 47%
77°F
23°F
77°F
23°F
194°F
194°F

ASHRAE Journal - July 2019

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