Bronkhorst LIQUI-FLOW mini (till 01-07-2013) - Manual (Page 10)

9.17.065 page 10
2.13 Product description
A LIQUI-FLOW mini flow meter consists of a thermal mass flow sensor with a digital electronic PC-board for sensor
enhancement and communication. The sensor consists of a straight capillary with two heat resistant wires wound
around it. The measuring principle is explained briefly in section 2.14 below. The sensor signal is measured and
digitized directly at the sensor by the PC-board. The internal software (firmware) processes the signal (linearisation
and amplification) to the desired signal level, which is then translated to the desired output. Each instrument is
equipped with an analog and a digital interface, which can be used simultaneously.
Digital operation adds a lot of extra features (compared to analog operation) to the instruments, such as direct
reading at readout/control module or host computer, identification (serial number, model number, device type, user
tag) and adjustable instrument settings.
2.14 Measuring principle
The liquid flow sensor operates on a principle of heat transfer by sensing the temperature difference along a heated
section of a capillary tube. The temperature difference sensed by the upstream and downstream temperature sensors
on the capillary depends on the amount of heat absorbed by the fluid flow. The temperature sensors are part of a
bridge circuit, as seen in the figure below. The measured imbalance of the bridge, caused by the flow in the capillary is
linearised to the mass flow rate and amplified to the desired signal level.
Thermal sensor in a bridge configuration
The transfer function between mass flow and sensor signal can be described by the equation:
m p signal
c K V
Φ ⋅ ⋅ =
with:
V
signal
= sensor signal
c
p
= specific heat of the fluid
K = constant factor
Φ
m
= mass flow
From this formula it can be concluded that for each different fluid (with heat capacity c
p
) a certain sensor signal
represents a different mass flow. Using accurate fluid data and a conversion factor it is possible to predict the mass
flows for liquids other than water.
V
signal