ULVAC HELIOT 900 tablet controller circular gauge display — standby mode leak rate 4.16×10⁻¹¹ Pa·m³/s — ULVAC HELIOT 900 tablet controller — standby display showing circular gauge, leak rate 4.16×10⁻¹¹ Pa·m³/s, and vacuum circuit status

Helium Leak Testing — Comprehensive Technical Guide

Helium mass spectrometer leak testing is the most sensitive quantitative leak detection method available in industrial practice. This guide explains the physics, methods, terminology, and practical considerations for engineers using ULVAC HELIOT instruments.

Why Helium?

Helium is chosen as the tracer gas for leak testing for several fundamental reasons:

Small atomic size — helium (atomic radius ~31 pm) passes through leaks more readily than larger molecules, giving a conservative (worst-case) measurement
Inert and non-toxic — safe to use in enclosed spaces; does not react with test parts or surroundings
Low background concentration — atmospheric helium is approximately 5.2 ppm, giving a stable and low baseline signal
Mass spectrometer selectivity — helium has a unique mass-to-charge ratio (m/z = 4) that is clearly resolved from all other common atmospheric gases
High diffusivity — helium diffuses rapidly through leaks, reducing test cycle times

How the HELIOT Instrument Works

The ULVAC HELIOT series uses a 90° magnetic deflection mass spectrometer as its analyser. The vacuum circuit — shown live on the tablet controller — consists of:

TMP — turbomolecular pump (maintains analyser vacuum)
FP — fore pump (backing pump)
FV — foreline valve
TV1, TV2, TV3 — test valves (control gas flow path)
CLV — counter-flow limiter valve
PG — Pirani gauge (pressure measurement)
CAL — internal calibration leak
VV — vent valve

Test Methods

1. Hard Vacuum Test (Direct Connection)

The test part is connected directly to the test port of the HELIOT instrument. Helium is applied externally (by spraying or using an enclosure). Any helium entering through a leak is drawn into the vacuum system and detected.

Sensitivity: up to 1 × 10⁻¹³ Pa·m³/s
Best for: small, rigid parts with definable leak paths
Limitations: test part must withstand vacuum applied by the instrument

2. Ultra Flow Test Mode — PASS and FAIL

ULVAC HELIOT 900 PASS result on Ultra Flow test — leak rate 5.43×10⁻⁹ Pa·m³/s below setpoint — PASS result — leak rate 5.43×10⁻⁹ Pa·m³/s is below the setpoint of 6.83×10⁻⁸ Pa·m³/s

ULVAC HELIOT 900 FAIL result on Ultra Flow test — leak rate 7.45×10⁻⁷ Pa·m³/s exceeds setpoint — FAIL result — leak rate 7.45×10⁻⁷ Pa·m³/s exceeds the setpoint — part rejected

3. Ultra Fine Leak Mode

ULVAC HELIOT 900 ultra-fine leak mode display — reading below 0.01×10⁻¹² Pa·m³/s background level — Ultra Fine Leak mode — leak rate display at maximum sensitivity, below 0.01×10⁻¹² Pa·m³/s. Green colour indicates reading within the indication range.

Ultra Fine Leak mode extends the measurement window and averages the signal for measurements near the instrument’s ultimate sensitivity limit. Appropriate for hermetic package testing where limits are below 10⁻¹² Pa·m³/s.

4. Sniffer Mode (Positive Pressure)

The test part is pressurised with helium or helium/air mixture. A sniffer probe connected to the HELIOT is swept over the surface to detect escaping helium. Sensitivity: 10⁻⁵ to 10⁻⁷ Pa·m³/s. Best for large assemblies that cannot be evacuated. After sniffer mode, helium background typically takes several minutes to clear — this is normal.

Startup Sequence

Calibration

The HELIOT is calibrated using a calibration leak — a precisely characterised orifice producing a known leak rate of helium.

Internal calibration leak — built into the instrument; activated from the tablet controller; value certified at manufacture
External reference standard — a separate calibrated leak standard (CLM-08E, CLM-09E etc.) connected to the test port for periodic comparison and independent traceability

Leak Rate Units

Unit	Equivalent	Common context
Pa·m³/s	—	SI unit; used by HELIOT instruments
mbar·L/s	= 0.1 Pa·m³/s	Common in European industry
atm·cc/s	≈ 0.101 Pa·m³/s	Common in US defence standards
std cc/s	≈ 0.101 Pa·m³/s	US semiconductor industry

HELIOT 900 Sensitivity Range

ULVAC HELIOT 900 sensitivity and startup data screen — Ultra Flow, Fine Flow, Gross Flow, Sniffer values — INFORMATION → START-UP DATA screen — records instrument sensitivity for all four test modes after each startup: Ultra Flow (5.00×10⁻¹²), Fine Flow (5.00×10⁻¹¹), Gross Flow (5.00×10⁻¹⁰), Sniffer (5.00×10⁻⁹) Pa·m³/s

Test Mode Configuration

ULVAC HELIOT 900 test mode settings — Auto Zero, Cycle Test, Vent control configuration — General settings — Gas type (Helium/Hydrogen), Test Mode, Auto Zero mode, Vent control

ULVAC HELIOT 900 general settings screen — RS-232C, RS-485, EXT.I/O and compatibility configuration — Communication settings — Control mode (Tablet/EXT.I/O/Serial), RS-232C or RS-485, legacy compatibility mode (700/710, 300, ZERO)

Common Issues and Solutions

Symptom	Likely cause	Action
High baseline / elevated background	Helium contamination from recent sniffer use	Wait; run ZERO; vent briefly
Sensitivity loss	IS-BA1 wear; O-ring leak; TMP degradation	Contact us for service assessment
Slow response	Large dead volume; roughing pump performance	Minimise dead volume; check pump
Calibration drift	Internal leak age; temperature instability	Compare against external reference; contact us if >20%
TMP alarm	Bearing issue; power interruption during operation	Do not restart repeatedly; contact us

Training and Support

We offer operator and technician training for ULVAC HELIOT instruments, covering: instrument principles, test method selection, setpoint configuration, calibration procedure, routine maintenance, and fault recognition. Training is conducted in English at your facility.

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