## Usage Information

This Wireless Atmospheric Pressure Sensor is designed to report atmospheric pressure.

There are additional facilities with this sensor, to allow the calculation of altitude and consequently sea level pressure from an observation position. The device can evaluate altitude, calculated from a defined baseline and utilising an internal calibration table. Temperature is also available for recording, from this single integrated sensor unit.

The sensor is a compact rugged device for logging key changes in atmospheric pressure patterns. A comprehensive "meteorological station" may be produced with additional wind flow and humidity data logging. It is a very versatile device for helping to understand both complex meteorological behaviour and some of the fundamental laws that govern gas behaviour.

## Pressure Measurements

When discussing pressure, we need to make a distinction between absolute pressure and atmospheric pressure.

Empty space has zero pressure and is therefore a reference point for absolute pressure: this is the force per unit area exerted in a defined region with respect to a vacuum. Absolute pressure is uncorrected for any local conditions. The standard way to refer to this quantity is Pabs. If the total force F is distributed over a defined area A, then:

Pabs = F/A

Atmospheric pressure is the force per unit area, acting on a defined surface as the result of the column of air above it and may be referred to as Patm. If the average density of air, ρ, the height of the air column present, h, acceleration due to gravity g, are known then:

Patm = ρgh

NOTE: Pressure drops, by observation, 1.2 kPa for every 100 m above sea level (approximately 1.1%) when values of h are less than 2000 m.

The atmospheric density is influenced by several factors. For example, temperature and higher humidity tends to lower the density. Since atmospheric pressure is caused by the gravitation pull of the Earth, g, the pressure at altitude, Ph , does not remain constant. It is influenced by the gas composition, the exact radial position of the measurement, temperature, wind and rotational effects due to the Earth's spin.

In a gravitational field, the essential differential pressure expression for a static fluid, with respect to elevation (h) is given by:

dPh/dh = - ρg

The form of pressure distribution that will result, with a molar mass of dry air, m, absolute temperature, T, and Universal Gas Constant R, can be calculated. Assigning Patm to be the atmospheric pressure at "sea level", we can arrive at the pressure at altitude Ph:

Ph = Patm exp[-mgh/(RT)]

This approximates to the following expression, when values of h constrained so that the exponential term < 0.1 :

Ph = Patm [1-mgh/(RT)]

or

Ph = Patm {1-mgh/[(Cp-Cv)T]}

The constants Cp and Cv are the specific heat capacity values at constant pressure and volume respectively.

This predicts a 1% drop in pressure for every 100 m displacement from sea level below 2000 m.

## Pressure Temperature and Altitude

To compare pressure conditions around the world, meteorologists adjust the measured pressure to sea-level conditions - called relative pressure. Air pressure decreases with altitude; the relative pressure at a location is higher than the measured pressure, as it has been corrected to the sea level equivalence. Relative pressure is larger than measured atmospheric pressure - unless one measures below sea-level (see below).

All three of the measurement types available in this sensor can be linked together in a single relationship that is represented by the following equation. Setting P0 to be the atmospheric pressure at sea level, T the temperature in Celsius and h to be the displacement between the measurement and P0 and Ph, then:

P0 = Ph[1 - 0.0065×h/(T+0.0065×h+273.15)]-5.257

The above relationship forecasts a pressure drop of approximately 1% for every 100 m elevation, at 15 Celsius, below 2000 m.

All of the Atmospheric Pressure Sensor's collection data can be gathered using the EasySense App. Temperature can be measured to 0.1 0C precision, altitude to within 0.14m and atmospheric pressure to within 0.01 kPa.

Altitude is calculated with respect to a reference pressure (at sea level), notionally 101.325 kPa. If differential altitude values are required not so referenced, then see below (EasySense) to produce a more convenient output.

## Measurement in EasySense

The Atmospheric Pressure Sensor is designed to be used with EasySense software, available from the Data Harvest website. The sensor may be used in the following way.

1. Turn on the sensor.

2. Connect the sensor to the EasySense app, using the Devices Icon.

3. Select the measurement types that you need and close the Devices panel.

4. Select, from Setup, either Continuous or Snapshot mode - depending on the type of experiment to be conducted. Time-based measurements utilise the Continuous mode, while singular evaluation makes use of the Snapshot mode.

5. Use the "Set Tare" facility, available from the "Live Data" panel, if relative altitude reporting is required.

6. Data may be exported to other packages for further analysis as required.

## Advice for Correct Usage

- Please observe the operational limits for the sensor.
- This Atmospheric Sensor measures absolute atmospheric pressure. The sensor end cap needs to be open to the atmosphere.
- It is not intended for use as a universal gas pressure monitor.
- Any protective container used (for meteorology) must be suitable for the task and able to sustain the free air flow conditions.
- Protect from the weather – keep the sensor dry and free of condensation.
- Do not place the sensor in an environment in which there are very high long - term humidity levels. This may result in damage or malfunction.
- It may be cleaned using a soft moist cloth.
- Do not immerse in water or detergent.
- Do not expose to chemical vapours such as acetone, organic solvents, or chlorine.

## Units of Measurement and Definitions

Pressure is defined as force per unit area and the standard SI unit of pressure is the pascal (Pa) - 1 Pascal = 1 Newton per square meter (1 N/m2).

The average pressure exerted at sea level by the atmosphere is 101.325 kPa (1 atmosphere).

One atmosphere is also 760 mm or 29.9 inches of Hg. The conversion from kPa to mbar is simply to multiply by 10 (1 kPa = 10 mbar = 0.14504 psi).