measuring vacuum

[nadeem majeed]

when we vacuum the a/c unit the vacuum gauge goes to -1bar. can we get more vacuum on the same atmospheric pressure

[dogma]

when we vacuum the a/c unit the vacuum gauge goes to -1bar. can we get more vacuum on the same atmospheric pressure

Anyone who uses their compound guage to read and measure a vacuum is a cowboy and shouldn't have their licence.

A vaccustat is the only way to know what vacuum you have in a system and your codes of practice will tell you the minimum requirements for all systems.

[Sturt]

Hi nadeem

It sounds like you are quoting -1 bar from your manifold gauge set?
This is not suffiecient for accurate measurement of vacuum.

The scale is vast and mbar is expressed differently in low vacuum industry/applications.
1000mbar or atmospheric will appear as 1 x 10 3 (this should be a small 3 next to the 10).

With a rotary vane vacuum pump the lowest point really necassary for dehydration in the refrigeration industry is anything <3 mbar.
This type of pump will go to 1 x 10-3 or 0.001 mbar.

Lower vacuum is possible and the best recorded in test conditions is 1 x 10-14 or 0.0000000000000mbar!

Hope this helps....

Sturt.

[dogma]

I use microns as the unit for measuring a vucuum.

My Vac pump pulls a vacuum of 27 microns.

On a new install I like to pull a vac of 50 microns. If I shut the system off and leave for the lenght it takes me to have a ciggarette and the vac reads 1 micron above what I've pulled. I start to worry about a leak.

On existing "wet" plant I pull at least 1000 microns, however I usually tripple vac and pull to 500 or less.

-DOGMA_

[Peter_1]

Dogma, 500 microns is far above vapor pressure of water at room temperature. :eek:

[dogma]

Extract from Australia/NZ Codes of practice.


PART 1
6 Evacuation
This section refers to evacuation in the field only – not evacuation during the manufacturing
process.
As the systems covered by this code are supplied pre-charged with refrigerant there is
no need to evacuate the system upon installation. If evacuation is required at a later stage,
however, the following procedure must be followed:
6.1 Instructions must be followed if the system manufacturer has supplied instructions
for evacuation, except where the instructions specify a practice that will lead to
emission of refrigerant.
6.2 The system must be evacuated to less than 117 Pa absolute (900 microns of mercury)
if the system manufacturer has not supplied instructions with the system for
evacuation.
6.3 After the system has been evacuated the vacuum pump should be isolated from
the system. As a guide, with constant ambient conditions, the vacuum should not
rise more than 13 Pa (100 microns of mercury) in one hour. A greater rate of rise may
indicate a leak or the presence of moisture (see also 7.1.9).
6.4 Absolute vacuums must be measured using accurate measuring equipment selected for the specific application.


PART 2
6 Evacuation
This section refers to evacuation in the field only – not evacuation during the manufacturing process.
6.1 The manufacturer’s instructions for evacuation must be followed if the system is
factory-matched (ie: the manufacturer has supplied a matched evaporator and
condenser) and the manufacturer has supplied instructions with the system, except
where the instructions specify a practice that will lead to emission of refrigerant.
Provided the instructions do not specify a practice that will lead to emission of
refrigerant, if the manufacturer’s instructions are followed then the installation is
exempt from items 6.1.2 to 6.1.5.
The relevant parts of this section must be complied with if there are any parts of the
evacuation procedure not covered by the manufacturer’s instructions.
Installation of all other systems, or systems where manufacturer’s instructions are not
supplied, must comply with section 6 of this code in its entirety.
6.2 Evacuation should be carried out with dedicated evacuation hoses (large diameter
/ as short as practical) and gauges and not service manifolds / gauges.
6.3 The system must be evacuated to remove moisture and non-condensables after
determining that there are no refrigerant leaks when the system is pressurised,
6.4 Evacuation must be either the deep evacuation method, or triple evacuation using dry
nitrogen only as the moisture absorber, following the procedures described below.
Deep vacuum method: Pull a deep vacuum to a pressure of less than 65 Pa absolute
(500 microns of mercury). After isolating the vacuum pump, allow the system to stand for
60 minutes to ensure the vacuum is maintained at or below 78 Pa absolute (600 microns of mercury); OR
Triple evacuation method: Use a vacuum pump to pull a vacuum to a pressure
of at least 65 Pa absolute (500 microns of mercury). Break the vacuum with dry
nitrogen and allow the system to stand. Re-evacuate the system and repeat the
procedure twice more, breaking the vacuum each time with dry nitrogen.


That is the guide I use Peter_1 but I always prefer to go beyond the recomendation.

Like I said though, in a new/dry system I go to 50u and 500 for existing/wet plant.


I do usually try to reach 50u on all systems if time persists.

I was working with my boss on industrial plant a couple months back and he used a vac stat for the first time. He reached 3000 u and said that was enough. I kept my mouth shut.

:(

[nike123]

Dogma, 500 microns is far above vapor pressure of water at room temperature. :eek:


Nope! Vapor pressure of water at 20°C is 0,0234 Bar which is 17,55144338 Torrs or 17.551,44338 microns.
That mean that 500 microns is way below vapor pressure of water at room temperature.

[dogma]

Hi nike123 and Peter 1.

What are the recomendations and codes in your countries for evacuation etc?.... and do you have a ph diagram for water/steam you can post?



-Dogma_

[nike123]

Hi nike123 and Peter 1.

What are the recomendations and codes in your countries for evacuation etc?.... and do you have a ph diagram for water/steam you can post?



-Dogma_

This is from CPS:
2000 microns lowest average industrial requirements for A/C systems
1000 microns medium average industrial requirements for A/C systems
600 microns highest average industrial requirements for A/C systems
400 microns lowest average industrial requirements for refrigeration systems
200 microns medium average industrial requirements for refrigeration systems
100 microns highest average industrial requirements for refrigeration systems
25 microns deep vacuum for special requirements and for testing of vacuum pump efficiency


This (http://www.refrigeration-engineer.com/forums/showthread.php?t=14826&highlight=vacuum) could be helpful!

This (http://en.wikipedia.org/wiki/Vapor_Pressure_of_Water_at_Various_Temperatures) is water vapor pressure temperature table!

[Peter_1]

Sorry Dogma, misinterpreted IP units. You're 100% right.
I also use the triple vacuum method but use now OFN instead of refrigerant like in the old days.
What we have to use legally now is the EN378 which states lower than 290 Pa.
I personally say in school lower than 100 Pa, preferable in the region of 50 Pa to 70 Pa.

[dogma]

"...but use now OFN instead of refrigerant like in the old days."


OFN? please explain

[nike123]

OFN? please explain
Oxygen Free Nitrogen (O2 < 0,5 ppm)

[dogma]

Ahh... I see..


nadeem,


buy 1 of these mate.


http://www.toolstop.co.uk/cps-vg200-...um-gauge-p1327