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A platinum rhodium tube is platinum alloyed with rhodium and drawn or fabricated into a hollow section. The addition of rhodium raises the melting point, stiffens the metal and keeps the surface stable in air at temperatures where most metals are already reacting. The tube form is chosen when the material has to hold a shape at that temperature, with the process on one side of the wall and the furnace atmosphere on the other.
Note what that description does not say. It is the same alloy that is drawn into wire, and the hollow section is the only thing that has changed. That single difference moves the material into a different set of decisions, and those decisions are what the rest of this page is about.
The products DLX lists in this family are pure platinum, platinum-iridium and platinum-rhodium, in the compositions below. The melting point and density are the values published for this product range.
| Material | Melting point | Density |
|---|---|---|
| Pure Pt (99.99 %) | 1,772 °C | 21.45 g/cm³ |
| Pt-Rh 5 % | 1,830 °C | 20.70 g/cm³ |
| Pt-Rh 10 % | 1,860 °C | 19.80 g/cm³ |
| Pt-Ir 5 % | 1,790 °C | 21.49 g/cm³ |
| Pt-Ir 10 % | 1,800 °C | 21.53 g/cm³ |
| Pt-Ir 20 % | 1,840 °C | 21.81 g/cm³ |
| Pt-Ir 25 % | 1,840 °C | 21.70 g/cm³ |
| Pt-Ir 30 % | 1,850 °C | 22.15 g/cm³ |
| Pure Rh (99.99 %) | 1,963 °C | 12.44 g/cm³ |
Two additions do different jobs, and the range is split between them for that reason. Iridium holds the density up and hardens the metal, which is what a small part that must not deform is bought for. Rhodium raises the melting point and is the addition used where the part has to work at the top of the range, which is why the 10 % rhodium grade is the one this product is built around.
The weight of the alloy is worth noting before anything else. At roughly 20 g/cm³ a tube is about twice the density of a stainless steel tube of the same dimensions, so a bill of materials written by volume will underestimate the metal required by a factor of about two.
Platinum-rhodium is bought for stability at temperature, not for conduction. Published data for the 10 % rhodium alloy gives a thermal conductivity of 38 W/m·K at 23 °C, which is a low figure for a metal. The same alloy conducts electricity at a resistivity of 18.4 to 19.2 µΩ·cm, which is high compared with copper.
Describing the material as an excellent conductor of heat or electricity gets the reason for using it backwards. What it actually offers is chemical inertness and stability of the thermoelectric output over long periods at high temperature, and those are the properties that matter in the duties it is bought for.
| Property | Published value for Pt-Rh 10 % |
|---|---|
| Thermal conductivity at 23 °C | 38 W/m·K |
| Electrical resistivity | 18.4 to 19.2 µΩ·cm |
| Density | 19.97 g/cm³ |
| Melting range | 1,830 to 1,855 °C |
| Tensile strength | 320 to 620 MPa |
| Elongation at break | under 35 % |
| Hardness, Brinell | 90 to 165 |
| Temperature coefficient | 0.0017 K-1 |
The same alloy is supplied as wire, and the two are not interchangeable. A wire is bought on a diameter and a length. A tube is bought on an outside diameter, a wall thickness, a length and a straightness, and the wall is what separates it from a wire of the same alloy.
| Dimension | What it decides |
|---|---|
| Outside diameter | Whether the tube fits the fitting, the bore or the protection head it is going into |
| Wall thickness | The temperature drop through the wall, and how long the tube lasts before the wall is consumed |
| Inside diameter | What passes through it, and whether the tube is a conduit or a sheath |
| Length | Whether the tube is a short component or a full-length protection sheath |
| Straightness | Whether the tube can be sealed or fitted into a bore without being straightened, which work-hardens it |
| Concentricity of the wall | The strength of the finished part, because an off-centre wall is a thin wall on one side |
| Form of supply | Straight lengths or coiled, which follows from the diameter and the alloy |
The tube has two surfaces and both are in service. On a protection sheath the outside faces the furnace atmosphere and the inside holds the thermocouple and its insulation. On a glass-melting component the inside carries the melt and the outside is heated. That is the difference from a wire or a rod, where only the outside is exposed, and it is the reason the wall thickness appears on the order line as a property, not simply as a dimension.
| Item | Value or requirement | Where it is confirmed |
|---|---|---|
| Material | Pure platinum, platinum-iridium or platinum-rhodium, with the rhodium or iridium content | The analysis of the heat, on the inspection document |
| Refined metal grade | ASTM B561 covers refined platinum in Grade 99.95 and Grade 99.99, and ASTM B616 covers refined rhodium | The certificate for the input metal |
| Thermocouple-grade material | ASTM E1159 covers thermocouple materials in platinum-rhodium alloys and platinum, and sets the rhodium content of the 10 % grade at 10.00 ± 0.05 % | The analysis, where the tube is bought as thermocouple material |
| Outside diameter, wall, length | Per drawing, quoted against the delivered tolerance | Inspection record for the lot |
| Temper or hardness | As supplied, since forming work-hardens the alloy | Mechanical result on the inspection document |
| Inspection document | EN 10204 3.1 | Issued with the lot |
The dimensions published for this product on the DLX site are an outside diameter of 0.2 mm to 20 mm and a wall thickness of 0.015 mm to 2 mm, from 10 mm in length upwards and to drawing. Above that there is no catalogue figure to quote: the analysis and the mechanical results are confirmed on the inspection document for the heat the material came from, which is the same position as for any other product in this family with no catalogue data.
The applications fall into two groups, and the split follows the alloy choice, not the product name.
| Application | Form the material takes | Why platinum-rhodium |
|---|---|---|
| Glass fibre bushings | A plate pierced with many small tubes, held at temperature while molten glass is drawn through | The alloy holds its shape and resists the melt at the temperature the glass has to be held at |
| High-temperature furnace windings and heating elements | Wire and strip, in the same atmosphere as the tube | Stability of the resistance at temperatures above 1,800 °C |
| Crucibles and melting equipment for precious metals and glass | Open vessels and tubes | Inertness towards the melt, with no contamination of the charge |
| Thermocouple protection and laboratory equipment | Tube used as the sheath or the component | Inertness in air at high temperature |
| Radiopaque marking bands, ring electrodes, cardiac rhythm devices and catheters | Small machined or drawn components | The precious-metal end of this range, where radio-opacity and body compatibility are the requirement |
The last row deserves a note, because it is the row on this product listing that reads as though it belongs to a different product. The listing for the Pt-Rh tube names radiopaque marking, ring electrodes, cardiac rhythm management devices and catheters as its applications, while the product title names laboratory equipment and thermocouple production. Both are real uses of the family, but the medical parts are the platinum-iridium end of the range, where the density and the hardness are the point, and the rhodium grades are the high-temperature end. A buyer who needs a marker band and a buyer who needs a furnace sheath should not be reading the same line, and the two enquiries are quoted separately.
The alloy is inert towards many environments until a specific contaminant is present, and the failure that follows is silent.
| Contaminant or condition | What happens | What to do about it |
|---|---|---|
| Silicon, phosphorus, sulfur and arsenic, including their vapours from insulation and refractories | They alloy into the platinum and embrittle it; the part can fail at the grain boundary long before the melting point is approached | Keep the tube away from those materials, and take advice before inserting it into an existing furnace, instead of assuming the temperature rating settles it |
| A reducing atmosphere, or carbon at temperature | The surface reaction changes and the thermoelectric output moves away from the original curve | Confirm the atmosphere, because the same tube that works in air may not hold its reading in a reducing gas |
| Trace impurities picked up from insulators | The EMF output drifts, which shows up as a measurement error, with nothing visibly damaged | Type B has platinum-rhodium on both legs, so it is less sensitive to pickup of trace impurities than Type R or Type S, which is one reason it is chosen in contaminated furnaces |
| Mechanical work in service, including straightening | The alloy work-hardens and the part becomes less tolerant of further forming | Order the length and the straightness so that no straightening is needed on site |
| Vapour from a neighbouring process | Contamination does not have to come from inside the furnace | Check what else is in the same chamber before the tube is committed |
Where the tube is bought as thermocouple material, the alloy choice maps onto the letter designations, and the mapping is fixed by the standards.
| Type | Positive leg | Negative leg | Note |
|---|---|---|---|
| Type S | Pt-Rh 10 % | Pure platinum | The 10 % rhodium grade in this listing |
| Type R | Pt-Rh 13 % | Pure platinum | A different rhodium content from the grade listed above |
| Type B | Pt-Rh 30 % | Pt-Rh 6 % | Both legs are platinum-rhodium alloys, which is why it tolerates contaminated furnaces better than the other two |
Two groups of duties. In industry it is used where a shape has to hold at high temperature in air: glass fibre bushings, furnace windings and supports, crucibles and melting equipment, and thermocouple protection sheaths. In medical devices the same family supplies small components such as radiopaque marker bands, ring electrodes and catheter parts, which sit at the platinum-iridium end of the range.
To raise the melting point and to hold the surface stable in air at high temperature. The published melting range for the 10 % rhodium alloy is 1,830 to 1,855 °C, against 1,772 °C for pure platinum. The rhodium also stabilises the thermoelectric output over long periods, which is why the alloy is the standard choice for thermocouples used as reference instruments.
No, and this is the point most often reported wrongly. The published thermal conductivity of the 10 % rhodium alloy is 38 W/m·K at 23 °C, which is a low value for a metal, and its resistivity is higher than that of copper. The alloy is chosen for inertness and stability at temperature, not for conduction.
This product is listed from 0.2 mm to 20 mm outside diameter with a wall from 0.015 mm to 2 mm, cut from 10 mm in length and supplied to drawing. Because the wall thickness sets the life of the part and the outside diameter sets the fit, both belong on the enquiry, not only one of them.
On the inspection document issued with the lot. ASTM B561 covers refined platinum in grades 99.95 and 99.99 and ASTM B616 covers refined rhodium, so the input metal is certified before the alloy is made, and ASTM E1159 sets the rhodium content of the 10 % grade at 10.00 ± 0.05 % where the tube is bought as thermocouple material. The document is issued in the EN 10204 3.1 form.
No. Type S runs at 10 % rhodium and Type R at 13 %, and the standards fix each. A tube ordered for a Type R junction and supplied against the 10 % grade will not produce the output the instrument expects, and the error appears as a calibration offset, with nothing obviously broken, which is why the type number belongs on the order along with the alloy.
Platinum rhodium tube is a high-temperature form of platinum, and the two decisions that shape it are the alloying element and the wall. Rhodium raises the melting point and stabilises the output; iridium raises the density and the hardness for small parts. Over wire, the tube adds an outside diameter, a wall, a length and a straightness, and the wall is what sets how long the part lasts. What the material offers is inertness and stability, not conductivity, and its limit is contamination, not temperature.
Since 2002 the plant at Changzhou, Jiangsu has made nickel and nickel alloy products, and this range is produced alongside them in a 12,000 m² factory working to ISO 9001 with SGS audits. Pure platinum, platinum-iridium and platinum-rhodium tube are available from 0.2 mm to 20 mm outside diameter: send the alloy, the outside diameter, the wall and the duty for a confirmed quotation.
The property values above are published values for the alloys named and are not acceptance limits. A platinum rhodium lot is accepted against the ordered alloy, dimensions and temper, and the analysis and mechanical results are confirmed on the inspection document for the heat the material came from.
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