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G1. Theory measurement principles

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The VPFlowScopes have 3 different sensors to measure flow, pressure, and temperature. All those units are used to provide you with a better understanding of your compressed air system.

1. Flow

The VPSensor uses our proprietary insertion type thermal mass flow sensor. There is no bypass flow, which results in high robustness and less sensitivity for dirt or particles. The flow sensor is directly temperature compensated. The flow reading is under normalized conditions (DIN 1343).

The sensor response signal is directly related to the mass flow rate and can be described by the following formula:

Vout = k *λ* ρ * v * (Ts-Tg)

Vout = output voltage Volt
k = sensor (geometrical) constant
λ= thermal conductivity of the gas
ρ = density of the gas kg/m3
v = actual velocity in m/sec
Ts = sensor temperature
Tg = gas temperature

The flow sensor offers (optional) bi-directional sensitivity. See the images below on how this works. In bi-directional mode, the negative flow value will show up as a minus sign.

2. Pressure

The VPsensor features a built-in gauge pressure sensor. Check specifications for the details. The sensor membrane can handle media that are compatible with glass, silicon, stainless steel, Sn/Ni, plating and An/Ag solder. The built-in gauge pressure sensor has a range between 0 .. 16 bar | 0 .. 250 psi, where the signal is sampled with 16 bits.

3. Temperature

The built-in temperature sensor measures the compressed air/gas temperature.
At low flow rates between zero flow and 10 mn/sec, the temperature sensor may heat itself due to the heated flow sensor element. This will result in a higher readout for the temperature.

Temperature compensation effects: The flow sensor is compensated dynamically for changes in the gas temperature. When exposed to quick temperature changes or large temperature changes (for example taking the unit from outdoor to indoor during wintertime, or when mounted downstream of a heat regenerated dryer) the temperature compensation may lag, which may result in significant measurement errors.

4. Totalizer 

The totalizer keeps track of the total amount of compressed air consumed in normalized cubic meters, normalized liters per minute or in (M)(M)SCF depending on which unit you choose to readout. The refresh interval is 1 second: Actual totalizer data will be available on the display and via the Modbus interface. The totalizer value is written to its internal memory with an interval of every 15 minutes. A power failure may result in a maximum of 15 minutes of totalizer data loss.

The Transmitter features 2 totalizers, plus a combined totalizer. The first totalizer counter will count up all of the positive flow, the second counter will count up all of the negative flow. The sum of these two totalizer values is shown on the display and can also be read out via Modbus. The display will show totalizer values up to 99.999.999,9 and will then become 0,0 independent of the taken unit. The totalizer will not be cleared and higher values will be available via Modbus and VPStudio. Via Modbus, all three totalizers are available.
The totalizers can only be reset to zero and will be reset altogether. It’s not possible to set them to any different value.

Updated on March 11, 2020

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