Vacuum Technology, PVD/CVD Thin-Film Tools & Turbomolecular Pumping
Vacuum Chamber Conductance Throttling & Pumpdown Time Formula
Calculate Foreline/Valve Throttled Effective Pumping Speed S_eff (L/s), Outgassing-Limited Ultimate Base Pressure P_ult (10^-3 mbar), Pumpdown Time to Target Vacuum (s), and Throttling Loss (%).
Connecting a 500 L/s turbopump through a 200 L/s bellows pipe wastes 71% of your pump!
Semiconductor PVD sputter chambers, electron microscopes, and space thermal-vacuum tanks fail pumpdown specs when engineers bolt a high-speed turbomolecular or cryopump S_p (L/s) onto a long narrow gate valve or elbow of Vacuum Conductance C (L/s)! Vacuum pumping speed and pipe conductance combine in reciprocal series (1 / S_eff = 1 / S_p + 1 / C -> S_eff = (S_p * C) / (S_p + C)), NEVER exceeding the smaller of S_p or C! Furthermore, chamber wall water-vapor desorption Q_outgas (mbar·L/s) sets an inescapable Asymptotic Ultimate Base Pressure P_ult = Q_outgas / S_eff (mbar), so evacuating chamber volume V (L) from initial crossover pressure P_0 down to target P_target (> P_ult) requires t = (V / S_eff) * LN((P_0 - P_ult) / (P_target - P_ult)) seconds!
Verify on a 250.0 L PVD Load-Lock Chamber—Chamber_Volume V = 250.0 L (B2), Turbopump_Inlet_Speed S_p = 500.0 L/s (B3), Valve_Manifold_Conductance C = 200.0 L/s (B4), Crossover_Start_Pressure P_0 = 1.00 mbar (B5), Target_Process_Pressure P_t = 0.0001 mbar (1e-4 mbar, B6), and Wall_Outgassing_Load Q_out = 0.002 mbar·L/s (B7): Effective Chamber Speed S_eff plunges to (500 * 200) / 700 = 142.86 L/s (E2, a 71.43% throttling loss in E5!), Ultimate Base Pressure equals (0.002 / 142.857) * 1,000 = 0.014 x 10^-3 mbar (1.4e-5 mbar, E3), and High-Vacuum Pumpdown Time equals 16.38 seconds (E4)!
| High-Vacuum Pumpdown KPI (V=250 L, Sp=500 L/s, C=200 L/s, Qout=0.002 mbar·L/s) | Formula | Verified Output |
|---|---|---|
| Effective Pumping Speed Seff (L/s) | (Pump_Sp * Pipe_C) / (Pump_Sp + Pipe_C) | 142.86 L/s (333.33 @ C = 1,000 L/s) |
| Ultimate Base Pressure (10^-3 mbar) | (Outgas_Q / Seff) * 1000 | 0.014 ubar (0.006 @ C = 1,000 L/s) |
| Pumpdown Time to 1e-4 mbar (s) | (V / Seff) * LN((P0 - Pult)/(Pt - Pult)) | 16.38 s (6.95 s @ C = 1,000 L/s!) |
| Conductance Throttling Loss (%) | ((Sp - Seff) / Sp) * 100 | 71.43% (33.33% @ C = 1,000 L/s) |
Outgassing-Asymptote Throttled Vacuum Pumpdown Time Formula
Validate V > 0, S_p > 0, Conductance C > 0, P_0 > P_target > 0, Q_outgas >= 0, and verify Target_Pressure P_target > Ultimate_Pressure Q_outgas / S_eff (otherwise target vacuum is physically unreachable without bakeout)!
=LET(vl_l, B2, sp_l, B3, cd_l, B4, p0_m, B5, pt_m, B6, qo_m, B7, IF(OR(vl_l <= 0, sp_l <= 0, cd_l <= 0, pt_m <= 0, p0_m <= pt_m, qo_m < 0), "Invalid Vacuum Input", LET(se_f, (sp_l * cd_l) / (sp_l + cd_l), pu_l, qo_m / se_f, IF(pt_m <= pu_l, "TARGET BELOW BASE PRESSURE", ROUND((vl_l / se_f) * LN((p0_m - pu_l) / (pt_m - pu_l)), 2)))))Direct-Flange DN160 Gate Valve Conductance Upgrade Check (Set Conductance C = 1,000 L/s)
Replace the narrow bellows elbow with a direct wide-bore gate valve (Conductance B4 rises from 200 to 1,000 L/s): Effective Pumping Speed E2 more than doubles to 333.33 L/s, Ultimate Base Pressure E3 improves to 0.006 x 10^-3 mbar, and Load-Lock Pumpdown Time E4 drops from 16.38 s to 6.95 s!
=IF(ISTEXT(E4), E4, LET(se_f, (B3 * B4) / (B3 + B4), ROUND(((B3 - se_f) / B3) * 100, 2)))Try the example
Use the original values or change them to test your understanding.