Surface roughness vs. initial pump down time of a vacuum vessel

Don Piroli
Date:7/26/03

What is the project idea?
What specific goal do you hope to achieve?

• Correlational study comparing a vacuum vessel’s internal surface roughness to the period of time it takes to pump down the vessel and reach 1E-6 torr vacuum pressure.
• Specific variables to control the surface roughness of the internal aluminum casting, volume of the internal aluminum casting, cleanliness of the casting, the vacuum integrity of the vacuum vessel’s apparatus, and calibrated gauges and multi-gauge controller.
• By controlling these variables the data should indicate a positive linear relationship exists between the surface roughness values and the pump down time.

Premise of the project:

• Three aluminum castings were used with a 32Ra, 64Ra, and a 158Ra surface roughness.
• As the surface roughness increases so will the pump down time.
• This phenomenon can be attributed to the fact that a rougher surface would contain more gas traps and have greater surface area for water molecules to adhere to and outgas.
• Outgassing negatively effects pump down time.

Why do this? Who will benefit?

• High vacuum has long been an important tool in research and development for the physical sciences particularly those concerned with the preparation and analysis of materials.
• Need for rapidly and reliably obtaining high vacuum is also a developing need in certain production industries thin film deposition, plasma processing and vacuum instrumentation.

What are the harder problems you can foresee?

• Castings being made to print.
• Cleanliness of castings after being machined.
• Vacuum integrity of testing apparatus.
• Ensuring gauges are calibrated to multi-gauge as a match set.
• Voltage readout on data logger could be compromised if hardware connections are not properly seated into connectors.
• Data file settings for data logger is properly defined for acquiring data.
• Ensuring proper vacuum technology assembling techniques are adhered to.

What are you going to do about each one?

• Make visit to machining house to ensure that features are properly machined to datum’s on print.
• A six step washing and drying process that Varian Vacuum technology will execute will ensure cleanliness of castings after being machined.
• Checking o-rings and mating parts that act as plugs for the castings will ensure vacuum integrity.
• Calibrate vacuum pressure gauges to multi-gauge controller will minimize bias and precision errors in pressure readings.
• Voltage readout of data logger could be compromised if hardware connections are not properly seated into connectors. Lock wheels on data station to ensure that the data station cannot roll forcing cable to dis-connect from the connectors. Visually check connection prior to data logging pressure readings.
• Data file settings for data logger are properly defined for acquiring data. Conduct a test run.
• Accurately measuring surface roughness once castings have been machined. Use a surface roughness indicator (comparator) with touch and feel.
• Using gloves when assembling apparatus, properly sonicating and baking parts to ensure cleanliness.
• Ensure screws are properly torque on plug flanges.

Will you need help? Is the help available?

• System engineers at my work.
• Application engineers from Varian Vacuum technology.
• Engineers from the Foundry supplying the castings.
• Engineers from machining house.
• Engineers in Quality Inspection at my work.

What’s your big worry issue with the project?

• Having to conduct more than one 24 hr trial of pump down per casting configuration. This could possibly skew pump down results and effect the correlation between surface roughness vs pump down time.
• Not getting the hardware in time for the schedule set in place.
• Power outage during acquiring data on data logger for 24 hr trial of pump down per casting.
  
  
  
  
  
  

Cost savings:

Material:
Weldment cost: $5875
Pump down time 10 hours
Month Cost savings using a 32 Ra casting vs a weldment:
$26927 - $12361 = $14565
recoup NRE of $2100 for casting in first month.

Customer:
32 Ra casting cost: $2697
2.5 hours pump down time
64 Ra casting cost: $2287
5.5 hours pump down time
158 Ra casting cost: $2139
12.5 hours pump down time
Cost includes:
Casting
Machining of casting
Secondary machining of to achieve surface finish.
Cleaning
NRE: $2100 (tooling cost)

Pass on cost saving to Customer:

Customers are looking at purchasing an instrument that costs ¾ of a million dollars.
They want performance for purchase.
Performance = Pump down time
Pump down directly effects customer service support.
Service Support Engineer call cost to customer is $100 per hour.

Summary:

• Data supports the thesis in proposal.
• Cost effective to invest in 32 Ra surface roughness casting.

Calculations for Rough Vaccum:

DETERMINE VOLUME OF CHAMBER:

(16.27” X 21.47” X 8”) = 2794.54 CUBIC INCHES

2794.54 CUBIC INCHES EQUALS 0.046 CUBIC METERS

NOMINAL PUMPING SPEED OF PUMP: 2.5 CUBIC METER/HOUR

INITIAL PRESSURE PINITIAL: 760 TORR

FINAL PRESSURE PFINAL: 5E-1 TORR

ESTIMATE TIME CALCULATION TO EVACUATE CHAMBER VOLUME FROM 760 TORR (1 ATMOSPHERE) TO 5E-1 TORR OF VACUUM PRESSURE.
FORMULA #1
T = V/S (2.3 log10 (PINITIAL/ PFINAL))
T = time to evacuate chamber
V= volume of chamber
S = the nominal pumping speed
PINITIAL = the initial pressure in torr
PFINAL = the final pressure in torr

.132 hour = (.046 m3 / 2.5 m3/hr)(2.3 log10 (760 torr / 5e-1 torr))

.132 hour equals 7.90 minutes

FORMULA #2
T = V/S (ln (PINITIAL/ PFINAL))

.132 hour = (.046 m3 / 2.5 m3/hr)(ln (760 torr / 5e-1 torr))

.134 hour equals 8.08 minutes

Measured chamber evacuation times (760 torr (atmosphere) to 5e-1 torr:

Evacuation time for 158 Ra is 10 minutes

Evacuation time for 64 Ra is 8 minutes

Evacuation time for 32 Ra is 7 minutes