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.
5.95 0.05
32.3
AA
Peter Haas
500000033493
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
Table results by formula
Altitude from sea level
(m)
0 760
500 717
1000 675
1500 636
2000 599
2500 563
3000 529
The temperature change follows the barometric change.
This means that the exact value changed by the barometric
pressure will be followed by the filling media (gas inside the
thermostat).
Example
1 Thermostat filled with R290
2 Thermostat working temperature cut-out -22.0 °C and
cut-in -10.0 °C at 760 mmHg (sea level)
3 Now working at 500 meters from sea level equals to
717 mmHg
Barometric pressure
(mmHg)
Temperature
R290
[°C] [bar] [°C] [bar]
-23.3 1.156 -11.0 2.333
-23.2 1.164 -10.9 2.344
-23.1 1.172 -10.8 2.355
-23.0 1.180 -10.7 2.367
-22.9 1.188 -10.6 2.378
-22.8 1.196 -10.5 2.389
-22.7 1.204 -10.4 2.401
-22.6 1.212 -10.3 2.412
-22.5 1.221 -10.2 2.423
-22.4 1.229 -10.1 2.435
-22.3 1.237 -10.0 2.446
-22.2 1.246 -9.9 2.458
-22.1 1.254 -9.8 2.469
-22.0 1.262 -9.7 2.481
-21.9 1.271 -9.6 2.492
-21.8 1.279 -9.5 2.504
The optional formula below can be used instead of the
vapour table.
Over
pressure
Temperature
R290
Over
pressure
When moving to a higher altitude (lower barometric
pressure) we still need to have the same final pressure
so: The barometric pressure changed from 760 mmHg to
717 mmHg (reduced external pressure) meaning the
thermostat gas needs a lower pressure to get the same final
pressure as before. This difference of -43 mmHg (specified
pressure – working pressure) is equal to -0.057 bar.
Original cut-out specification is -22.0 °C with R290 at
760 mmHg. Using the vapour saturated gas properties table
you find that pressure related to this temperature is equal to
1.262 bar.
To achieve the same final pressure the gas will need now
1.205 bar (1.262–0.057), using again the vapour saturated gas
properties table again you find the temperature that is equal
to -22.7 °C.
The same calculation is valid for the cut-in temperature. See
the calculation in the table for R290 gas.
Barometric pressure change : 760 to 717 = -43 mmHg =>
-0.057 bar difference in pressure
Cut-out -22.0 °C => 1.262 bar; -0.057 bar = 1.204 bar =>
-22.7 °C (0.7 degree colder)
Cut-in -10.0 °C => 2.446 bar; -0.057 bar = 2.389 bar =>
-10.5 °C (0.5 degree colder)
B
T =
A - Log - P0 + P
Formula Symbols
T
Final temperature at final barometric pressure (°C)
T
Temperature at initial barometric pressure (°C)
o
P
Final barometric pressure (mmHg)
P
Initial barometric pressure (mmHg)
o
A
Filling media constant A
B
Filling media constant B
C
Filling media constant C
Constant table for filling media
Gas A B C
R 1270 6.89297398 816.3970935 251.1933122
R 134a 7.133349824 915.0119095 241.3949807
R 152a 7.130027652 941.3515628 245.5657134
R 290 6.878127193 833.1521839 250.5091034
R 600a 6.782784136 887.1594508 239.1543191
10
A -
B
T0 + C
- C
For more information please contact Danfoss Appliance
Controls.
The 077B….EBD, 077B….L EBD can be applied on systems with R290 and R600a as the working fluid.
For countries where safety standards are not an indispensable part of the safety system Danfoss recommend the installer to get a
third party approval of the system containing flammable refrigerant.
DKAP.ED.100.A2.02 © Danfoss | DCS (rja) | 2019.02
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