SA flow sensor
Technology

Calorimetric principle of operation

Thermal flow sensing technology relies on the cooling effect of flowing liquids. A higher flow velocity results in a greater cooling effect and this can be reliably measured. The images below depict heat being conducted from the sensor tip by a flowing liquid.

Heat transfers away from the sensing element as media flows by

The sensor tip houses two platinum RTD elements and a heat source. When power is applied, the tip of the sensor is heated. The difference in temperature between the two RTDs is held constant (Δ 4 °C) by varying the amount of power applied to the heater. The flow rate is determined as these RTDs react to the physical effect of a flowing medium conducting heat energy away from the sensor tip.

The position of the RTD and heater at the bottom of the sensing tip.

This temperature-based operating principle can reliably sense the flow of virtually any liquid or gas. The general rule of thumb is “if the medium is pumpable, the flow is detectable.”

The flow measurement is in units of velocity (ft/sec) and it is measured at a single point in the pipe. When the pipe size is known, velocity can be calculated in units of volumetric flow rate (gpm, lpm, etc.) by multiplying the velocity by the cross-sectional area of the pipe.

Basic modes of operation

Relative mode

Use for any flowable medium including water, oil, glycol and gases.
Sensor is installed in the pipe and high and low flow rates are taught.
Display is 0…100% representing taught flow rates.
Analog signal is scaled 4…20 mA based on the taught flow rates.

Using the SA in relative mode provides the most accurate evaluation of flow velocity. The mode incorporates influences due to insertion depth and medium. If the medium remains constant, the measurement is extremely consistent and reliable.

Absolute mode

Best suited for water, low viscosity oil (<40 cSt), consistent 35% glycol/water solution, and air.
Sensor can be scaled and configured outside of the installation without flowing media.
Display can be velocity or volumetric flow rate.
Pipe size must be entered accurately to provide the most accurate measurement.

Since the measuring probe is an insertion-style, rather than inline style, the probe itself can provide some turbulence and certainly takes up part of the cross-sectional area of the pipe. For this reason, we recommend using absolute mode only in pipes greater than 2” diameter. We offer more accurate solutions for pipe sizes smaller than 2” diameter. Review other flow options.