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
			Barometric pressure
for measurement, due to which we have the pressure unit
“mmHg”).
As barometric (atmospheric) pressure is everywhere it will	Filling media (gas)
	pressure inside bellows
also surround the thermostat (outside and also inside).
	element according to
	the temperature.
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.
				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.

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.

5.95	0.05
	32.3
500000092586
Peter Haas
500000033493
2016.02.02
	AA

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
				Tb				Rx Lb
		P = Pb x
				Tb+Lb x (h-hb)
Formula Symbols
Pb		Static pressure at sea level (760 mmHg)
Tb		Standard temperature (293 K)
Lb		Standard temperature lapse rate (-0.0065 K/m in ISA)
h		Height above sea level (meters)
hb		Height at sea level (0 meters)
R		Universal gas constant for air (8.31432 N•m /(mol•K))
g	0	Gravitational acceleration (9.80665 m/s2)
M		Molar mass of Earth’s air (0.0289644 kg/mol)

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