Danfoss Barometric Pressure Data sheet

Barometric Pressure Information

Understanding how Barometric Pressure affects a refrigeration thermostat

Introduction

Barometric Pressure also known as atmospheric pressure is
the force per unit area exerted against a surface by the weight
of air above that surface in the Earth’s atmosphere. In most
circumstances atmospheric pressure is closely approximated
by the hydrostatic pressure caused by the weight of air above
the measurement point.

The barometric pressure depends on other factors like earth
location (earth is not round), weather conditions (air humidity,
air temperature, air speed) and even the sea level.

The barometric pressure is measured by a barometer
(meteorological instrument that normally uses mercury
for measurement, due to which we have the pressure unit
“mmHg”).

Barometric pressure

As barometric (atmospheric) pressure is everywhere it will
also surround the thermostat (outside and also inside).

All refrigeration thermostats filled with superheated
vapour have the same basic concept which is to transform
temperature into pressure and then convert this pressure into
force in order to open and close contacts.

This means that the filling media pressure (gas) has to
overcome the barometric pressure, meaning that the final
pressure is the difference between the filling media pressure
and barometric pressure. The final temperature changes if the
barometric pressure also changes.

Filling media (gas)
pressure inside bellows
element according to
the temperature.

Final pressure is the
pressure difference

All working temperatures are always specified at one barometric
pressure (customer request). If the barometric pressure changes
or it is different from that specified, the temperature also changes.
This occurs for all vapour filled thermostats in the world.

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AA

Peter Haas
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500000092586

2016.02.02

How to calculate the temperature changes according to barometric
pressure changes.

The temperature change is linked to the type of filling media
inside the thermostat (gas), the working temperature and the
barometric pressure changes.

This means that different thermostat designs working
at the same temperature and filled with the same media
(gas) will have the same temperature change according
to the same barometric pressure change.

To calculate the new working temperature we need to know
the following 3 data:

1. Type of filling media (gas) inside the thermostat

We need to know whether the thermostat is charged
with propane R290 or R134a because each type of filling
media has its own pressure vs temperature relationship.

Table of vapour pressure for most common refrigerants used for
charging thermostats.

2. Thermostat working temperatures
We need to know the cut-out and cut-in temperatures,

and the specified barometric pressure for these
temperatures because the filling media relation of

temperature vs pressure is not linear but exponential.

You can find this information in the Danfoss dimension

sketch or by asking Danfoss.

Example of Danfoss dimension sketch and where to find the
information.

3. Barometric pressure where the thermostat is used or
is planned to be used

This is specified by the customer. Alternatively, there are

many online sources. Or you can calculate based on the
local altitude by using the barometric formula below
(estimating by air mass).

g0 x M

R x L

b

P = Pb

T

x

b

Tb+Lb x (h-hb)

Formula Symbols

P

Static pressure at sea level (760 mmHg)

b

T

Standard temperature (293 K)

b

L

Standard temperature lapse rate (-0.0065 K/m in ISA)

b

h

Height above sea level (meters)

h

Height at sea level (0 meters)

b

R

Universal gas constant for air (8.31432 N•m /(mol•K))

g

Gravitational acceleration (9.80665 m/s2)

0

M

Molar mass of Earth’s air (0.0289644 kg/mol)

2 Danfoss Appliance Controls · DKAP.ED.100.A2.02 · ®Danfoss