Identifying and selecting copper capillary tubes of different precision cannot be done by visual inspection alone; a combination of "look, test, and ask" is necessary. Since the precision of the capillary tube directly determines the flow stability of the refrigeration system, different application scenarios (such as precision refrigeration and general flow guiding) have drastically different tolerance requirements.
The following is a selection and identification guide compiled based on industry standards and practical experience:
1. Look at the Appearance and Markings (Initial Screening)
High-quality copper capillary tubes will have obvious visual characteristics; this is the first line of defense.
● Look at the Color:
● High Purity (TP2/T2): High-quality copper tubes have a reddish hue, a metallic luster, and a bright, clean surface.
● Inferior/High Impurity: The color is dark or black, or the surface has oxidation spots or pinholes.
● Check Packaging and Labeling:
● Products from reputable manufacturers (such as Hongtai and Jintian) usually come in protective film or cardboard boxes. The label will clearly indicate the material (TP2/T2), specifications (outer diameter × wall thickness), and the applicable standard (e.g., GB/T 1531).
● Note: If the packaging is flimsy and lacks any labeling, it is usually a standard or inferior tube, and its accuracy cannot be guaranteed.
2. Measure Dimensions and Physical Properties (Core Identification)
This is the most direct method to determine the accuracy grade. You will need a micrometer (screw gauge), as the accuracy of a vernier caliper (0.02mm) may not be sufficient for high-precision capillary tubes.
● Measure Outer Diameter Tolerance (Determine Accuracy Grade):
● High Accuracy (Refrigeration Grade): The outer diameter error is usually controlled within ±0.02mm. For example, a tube nominally 2.0mm should have an actual measurement between 1.98-2.02mm.
● Standard Precision (Industrial Grade): Outer diameter error may be ±0.05mm or even larger.
● Operation: Measure several times at different locations on the tube (beginning, middle, and end). Large fluctuations in the values indicate poor concentricity, making it prone to breakage during bending.
● Measuring Wall Thickness and Inner Diameter:
● Since the inner diameter is difficult to measure directly, it is usually estimated by measuring the outer diameter and wall thickness.
● Formula: Inner Diameter = Outer Diameter - (2 × Wall Thickness).
● Important: Refrigeration capillary tubes require extremely high wall thickness uniformity; uneven wall thickness will lead to unstable flow.
● Bending Test (Assessing Material and Toughness):
● High-Quality Copper: Softer and more ductile. It bends easily with minimal springback when gently bent by hand, and there are no cracks or whitening at the bend.
● Inferior/Brass: Harder, harder to bend, springs back easily, or shows fine cracks at the bend (indicating excessive impurities).
3. Inquire about parameters and applications (precise matching)
When purchasing, directly confirm the following three key indicators with the supplier to avoid buying the wrong material:
● Inquire about the material grade:
● Preferred: TP2 (phosphorus deoxidized copper): Best weldability, less prone to hydrogen embrittlement, the first choice for refrigeration repair.
● Second choice: T2 (pure copper): Good thermal and electrical conductivity, but weldability requirements are slightly higher.
● Avoid: H62/H65 (brass), although cheap and hard, it is prone to cracking during welding, has poor thermal conductivity, and should never be used for refrigeration throttling.
● Inquire about the applicable standards:
● Inquire whether it conforms to GB/T 1531 "Copper and Copper Alloy Capillary Tubes" standard. If it is for refrigerator repair, it is best to require "high-grade" or "precision-grade" tolerances.
● Inquire about flow characteristics (professional grade):
● For bulk purchases or precision repairs, you can request the manufacturer to provide a flow test report (the number of milliliters flowing per minute under a specific pressure). This is the most "hardcore" indicator for verifying the accuracy of the inner diameter.
