A quartz capillary positive sample is a reference part used to confirm that a leak-testing system can detect a known, intentionally created leak path. It typically contains a small quartz capillary that allows a controlled amount of gas or air to pass through under defined test conditions. I use the term “positive sample” to distinguish it from a good, leak-free sample: the positive sample should produce a detectable response when the test equipment is functioning correctly.
In practical quality control, the sample is placed in the test fixture or connected to the same test circuit as a production component. If the instrument identifies the expected leak signal, the operator has evidence that the test method, fixture, pressure supply, and detection settings are working together. It is a functional verification tool, not automatically a calibration standard or a replacement for formal instrument calibration.
The central component is a precision capillary made from quartz, commonly associated with fused silica. The capillary creates a narrow flow path with a defined geometry, allowing a small quantity of gas to move from one side of the sample to the other. During a leak test, this controlled flow simulates the behavior of a leaking product without requiring a damaged production part.
The actual response depends on the capillary geometry, gas type, pressure differential, temperature, test duration, and the measurement method. For this reason, a positive sample should be used under conditions that are as close as possible to the production test. A sample that produces a reliable signal in one pressure range may not produce the same signal in another range.
These terms are sometimes used interchangeably, but they may describe different levels of metrological control. A positive sample is generally used to verify that a leak tester detects an expected condition. A calibrated reference leak, by contrast, is normally supplied with a documented leak-rate value and traceability requirements appropriate to the buyer’s quality system.
When I prepare a purchasing specification, I ask the supplier to state whether the quartz capillary positive sample is a functional check part, a quantified reference leak, or both. I also request the applicable test gas, pressure conditions, temperature range, identification method, and any available measurement documentation. This prevents a buyer from assuming that a screening sample has the same status as a certified calibration device.
The most important function is daily or periodic verification of leak-test sensitivity. An operator can run the positive sample before production, after fixture maintenance, or after a change to the test program. A failed response may indicate an instrument problem, an incorrect setup, contamination, a damaged fixture, or deterioration of the reference sample.
In a controlled quality process, the positive sample should have a unique identification number and a defined storage location. The inspection record can include the date, operator, instrument ID, test settings, result, and any observed damage. These records support internal troubleshooting, although the exact documentation required depends on the buyer’s industry and quality procedures.
Quartz capillary positive samples are useful wherever a manufacturer needs to verify the detection of small leaks in sealed or pressure-containing components. Typical applications may include valves, fittings, packages, tubing assemblies, sensors, medical or laboratory consumables, and other components tested by pressure decay, vacuum, mass flow, or tracer-gas methods.
The sample is especially useful when the product itself is expensive, difficult to intentionally leak, or unsuitable for repeated use as a reference. It can also support multiple production lines when each test station requires an independent functional check. However, the sample should be selected according to the actual detection principle rather than simply by material name.
For pressure-decay testing, the positive sample must create a measurable change within the selected test volume and test time. For vacuum testing, the assembly must remain compatible with the vacuum level and connection method. For tracer-gas testing, the capillary and housing must be suitable for the selected gas, while the expected signal must fall within the instrument’s useful range.
I recommend defining the test method before choosing the sample. Important parameters include the target leak-rate range, test gas, upstream pressure, downstream pressure or vacuum, stabilization time, measurement time, fixture volume, and acceptable repeatability. Without these details, a supplier may only be able to offer a general positive sample rather than a properly matched reference.
Quartz is selected because fused silica offers high chemical resistance, low thermal expansion, and stable performance across many laboratory and industrial environments. A commonly cited coefficient of thermal expansion for fused silica is approximately 0.55 × 10−6/K, although the actual value depends on material grade and temperature range. Quartz is also hard and chemically resistant, but it remains brittle and can be damaged by impact, bending, improper clamping, or contamination at the connection.
For more information, please visit Zholion.
The capillary may be supplied as an exposed tube, a protected insert, or an assembly installed in a metal, glass, ceramic, or polymer housing. The housing provides mechanical protection and connection features, while the quartz element controls the flow path. The best construction depends on the pressure, gas, fixture design, cleaning method, and required handling frequency.
A buyer may describe a preliminary requirement using an example such as a 1.0 mm outside diameter capillary with a 0.5 mm inside diameter, but these figures are only examples and must not be treated as a standard Zholion product specification. The final dimensions should be selected according to the required flow or leak response, not chosen from appearance alone. The supplier should confirm whether dimensions are nominal, measured, or controlled within a defined tolerance.
| Specification Area | Information to Confirm |
|---|---|
| Capillary geometry | Inside diameter, outside diameter, length, tolerance, and end condition |
| Leak response | Expected flow or leak-rate range, test gas, pressure, and temperature |
| Housing | Material, connection type, sealing method, and mechanical protection |
| Identification | Part number, serial or batch identification, and documentation |
| Use conditions | Vacuum compatibility, cleaning requirements, storage, and handling limits |
The first specification is the intended leak response. It should be expressed using the buyer’s preferred unit and test method, such as mbar·L/s, Pa·m3/s, sccm, or another technically appropriate unit. A dimension-only specification is incomplete because two capillaries with similar visible dimensions may produce different results due to length, surface condition, connection design, or manufacturing tolerance.
The second specification is the operating condition. Buyers should define pressure, vacuum level, gas type, temperature, test volume, and measurement time. For example, a requirement may specify a test pressure of 100 kPa above ambient, but that is only an illustrative value; the correct pressure must come from the production process and instrument capability.
The third area is repeatability and verification. The supplier should explain how the reference response is determined and what documentation can be provided, without claiming a certification that has not been supplied. If a formal calibration certificate is required, the buyer should state this before quotation so the supplier can confirm feasibility, scope, and applicable cost.
Start with the smallest leak that the production process must reliably detect, then define the test method around that requirement. Confirm that the positive sample produces a response large enough for the instrument to recognize but still relevant to the acceptance threshold. The sample should not be so open that it only proves the instrument can detect a very large leak unrelated to the production requirement.
Check the connector type, sealing interface, pressure rating, chemical exposure, cleaning process, and handling method. Quartz can perform well in demanding environments, but the complete assembly includes seals, connectors, and housing materials that may have different limits. I also recommend specifying a protective design when operators will connect and disconnect the sample frequently.
Ask for a drawing or controlled dimensional description, product identification, recommended test procedure, storage guidance, and replacement criteria. Clarify whether the supplier can provide one piece for evaluation, a small pilot quantity, or a repeat supply arrangement. Lead time and minimum order quantity should be confirmed in the quotation rather than assumed from standard catalog practice.
At Zholion, I approach a quartz capillary positive sample as a product-certification and application-matching project rather than a simple tube purchase. I can organize the requirement around the test method, target response, capillary geometry, housing, connection interface, identification, and documentation needed by the buyer. Where exact performance depends on the customer’s instrument, I recommend reviewing the operating conditions before finalizing the design.
For a quotation or technical review, I suggest sending the test gas, pressure or vacuum level, target leak-rate range, test duration, fixture connection, required quantity, and documentation expectations. If the final leak-rate value has not yet been established, a preliminary drawing and application description can still help define an evaluation sample. Any performance value, tolerance, or certification should be confirmed against the agreed specification rather than inferred from a general product description.
A quartz capillary positive sample is the right choice when you need a repeatable reference that confirms a leak-testing system can detect a defined flow path under specified conditions. Its value comes from the relationship between the capillary, housing, test method, and instrument—not from the quartz material alone. I recommend treating the sample as part of the complete quality-control system and matching it to the actual production acceptance limit.
The next step is to document your test gas, pressure or vacuum, target leak-rate range, fixture interface, test time, quantity, and certification or documentation needs. Zholion can then review the requirement and help define a suitable quartz capillary sample or customized assembly for evaluation and procurement. This approach reduces specification gaps and gives your team a clearer basis for routine leak-test verification.
If you want to learn more, please visit our website Quartz capillary positive sample.