Type S, B, R, or W? A Complete Guide to Choosing the Right Disposable Thermocouple for Your Foundry

In molten metal processing, accurate temperature measurement isn’t just important—it’s essential. An incorrect reading can mean scrapped batches, off-spec chemistry, and thousands of dollars in wasted material. That’s why selecting the right consumable disposable thermocouple for your foundry or steel mill is one of the most critical decisions you’ll make.

But with Type S, B, R, and W all available, how do you know which one is right for your operation? This guide breaks down everything you need to know—from thermocouple wire composition to temperature range, accuracy, and real-world applications—so you can make an informed, cost-effective decision.

What Is a Fast Disposable Thermocouple?

A fast disposable thermocouple—also known as an expendable or consumable thermocouple—is a single-use temperature sensor designed specifically for measuring molten steel and high-temperature molten metals. It is not a reusable industrial thermocouple with a metal or ceramic sheath. Instead, it is a lightweight, consumable probe that delivers a temperature reading in 4–6 seconds and is then discarded.

Disposable Thermocouple Structure

A fast thermocouple is mainly composed of a temperature measuring probe (tip) and a large paper tube that serves as both protection and handle. Inside the tip:

• Thermocouple wires (positive and negative) are welded to compensation wires

• Compensation wires are embedded in a support bracket covered by a small paper tube

• The thermocouple wires are supported and protected by quartz

• A slag cap protects the tip from dross and contamination

• All components are bonded with refractory filler into a single, non-reusable unit

Standards

Disposable fast thermocouples are manufactured according to the Chinese ferrous metallurgy industry standard YB/T 163 (currently YB/T 163-2008 Consumable fast thermocouples). This standard specifies the terminology, product names, thermocouple types, models, technical requirements, inspection methods, packaging, and labeling for consumable fast thermocouples. The standard was first issued as YB/T 163-1999 and later revised to YB/T 163-2008.

The thermocouple wires themselves comply with international standards: GB/T 3772-98 (Type S), GB/T 1598-98 (Type R), and GB/T 2902-98 (Type B).

The Four Types at a Glance

All four types are disposable fast thermocouples designed for single-use molten metal temperature measurement. They all work the same way: you attach the probe to a thermocouple lance, dip it into the molten metal for 4–6 seconds, read the temperature, and dispose of the used probe.

What sets them apart is the thermocouple wire material inside the tip—and that’s what determines how hot they can go, how accurate they are, and how much they cost.

Type S (KS-602/604): The Industry Workhorse

Wire material: Platinum-10% Rhodium (+) / Pure Platinum (–)

The Type S thermocouple is the most widely used noble metal thermocouple in foundries worldwide. It uses a platinum-10% rhodium alloy for the positive leg and pure platinum for the negative leg.

Key Specifications

• Model: KS-602 or KS-604 (the number indicates the paper tube length)

• Temperature range: 1500–1700°C (recommended); up to 1800°C maximum

• Tolerance: ±5°C

• Response time: 4–6 seconds

• Complies with: YB/T 163

Strengths

• Exceptional thermal stability and reproducibility

• Excellent oxidation resistance—ideal for oxidizing atmospheres

• Often used as a primary calibration thermocouple

• Most economical choice among noble metal thermocouples

Best Applications in Foundries

• Carbon steel production—the most common application

• Iron processing and cast iron melting

• Electric arc furnace (EAF) temperature measurement

• Ladle furnace (LF) operations

• General foundry temperature measurement

Bottom line: If your process operates in the 1500–1700°C range, the Type S is almost always sufficient and the most cost-effective choice among noble metal thermocouples.

Type R (KR-602/604): The High-Precision Variant

Wire material: Platinum-13% Rhodium (+) / Pure Platinum (–)

Type R is similar to Type S but contains a slightly higher percentage of rhodium (13% vs 10%). This gives it marginally higher output and improved stability, but at a higher cost.

Key Specifications

• Model: KR-602 or KR-604

• Temperature range: 1500–1700°C (recommended); up to 1800°C maximum

• Tolerance: ±5°C

• Response time: 4–6 seconds

• Complies with: YB/T 163 and GB/T 1598-98

Strengths

• Improved long-term stability compared to Type S

• Slightly higher output (better signal-to-noise ratio)

• Excellent oxidation resistance

Weaknesses

• Higher cost than Type S due to increased rhodium content

Best Applications

• Applications demanding the highest precision

• High-end metallurgical analysis

• Laboratory and calibration work

• Steelmaking where marginal stability improvements justify the cost

Bottom line: Type R offers slightly better performance than Type S but comes at a premium. For most foundry applications, Type S provides comparable results at a lower cost.

Type B (KB-602/604): The High-Temperature Specialist

Wire material: Platinum-30% Rhodium (+) / Platinum-6% Rhodium (–)

The Type B thermocouple uses a platinum-30% rhodium alloy for the positive leg and platinum-6% rhodium for the negative leg. With higher platinum-rhodium content, it delivers excellent stability at extreme temperatures.

Key Specifications

• Model: KB-602 or KB-604

• Temperature range: 1500–1700°C (recommended); up to 1800°C maximum

• Tolerance: ±5°C (standard); precision grades available at ±3°C

• Response time: 4–6 seconds

• Complies with: YB/T 163 and GB/T 2902-98

Strengths

• Best-in-class stability at extreme temperatures

• Excellent oxidation resistance

• Superior performance at high temperatures where Type S and R may start to struggle

• Often the preferred choice for high-temperature steelmaking

Weaknesses

• Higher cost than Type S (more platinum means a higher price tag)

• Minimum recommended temperature of ~600°C—below this, the signal is weak

Best Applications

• Stainless steel production

• Specialty alloys and superalloys

• High-temperature steelmaking

• Electric arc furnace (EAF) operations at elevated temperatures

• Any application where the melt approaches 1800°C

Bottom line: If accuracy at high temperature is non-negotiable and your process runs at the upper end of the range, the Type B is worth the investment.

Type W (KW-602 / Tungsten-Rhenium): The Cost-Effective Alternative

Wire material: Tungsten-3% Rhenium (+) / Tungsten-25% Rhenium (–)

Tungsten-rhenium thermocouples represent a fundamentally different approach—built on refractory metal performance at a fraction of the cost of precious metals. The alloy is designated WRe3/25 (3% rhenium in the positive leg, 25% rhenium in the negative leg).

Key Specifications

• Model: KW-602

• Temperature range: 1500–1700°C (recommended); up to 1800°C maximum

• Tolerance: ±5°C to ±7°C

• Response time: 4–6 seconds

• Complies with: YB/T 163 and JB/T 9496-99 (compensation wires), JB/T 9497-99 (thermocouple wires and scale tables)

Strengths

• Significantly cheaper than platinum-based materials

• Excellent value for high-volume, cost-sensitive operations

• Can measure up to higher temperatures than platinum types in appropriate conditions

Weaknesses

• Lower accuracy compared to platinum types (up to ±7°C vs ±5°C for KS/KB)

• Will oxidize and fail quickly if exposed to air or oxidizing atmospheres at high temperatures

• Requires compensation wires (WC3/25 type)

Critical Atmosphere Constraint

Tungsten-rhenium thermocouples are not suitable for use in air or oxidizing environments at high temperatures. They are designed for:

• Vacuum environments

• Inert gas (argon, nitrogen)

• Reducing atmospheres (hydrogen, CO)

Bottom line: If cost is a critical factor and you’re working in a protected atmosphere, tungsten-rhenium (Type W) is the practical, economical choice. For most conventional foundry applications in air, platinum-rhodium types (S or B) remain the standard.

Quick Decision Guide

Note: All four types are manufactured according to YB/T 163, with measurement accuracy of ±0.2%t, loop resistance ≤3Ω, insulation resistance ≥5MΩ, and response time of 3–5 seconds.

Summary Comparison

Final Takeaway

Choose based on your temperature needs, atmosphere, and budget:

• Below 1700°C in air → Type S is your answer (most cost-effective noble metal choice)

• High-precision work with budget to spare → Type R offers marginal improvements over S

• High-temperature applications approaching 1800°C → Type B is your best platinum option

• Cost-sensitive operations in protected atmospheres → Type W (tungsten-rhenium) is the way to go

For most foundries, Type S and Type B cover the vast majority of applications. Type W is the go-to for high-volume, cost-sensitive operations where the atmosphere can be controlled.

All our fast thermocouples are manufactured in strict compliance with YB/T 163 and are available in various paper tube lengths to suit your specific application—typically 300mm, 500mm, 600mm, or 1000mm.

Need help selecting the right thermocouple for your specific melt conditions? Contact our technical team for personalized recommendations.

Email: info@sf-foundry.com
WhatsApp: +8618636913699