Which thermocouple protection tube sizes do you keep in stock?

18 sizes, from 6×4 to 26×20 mm (OD × ID), are usually in stock in standard lengths of 300, 450, 600 and 1000 mm. We cut other lengths to order, and we have made protection tubes from 300 to 2500 mm long.

What alumina grade are the protection tubes?

Our standard grade is C799 alumina, 99.5% Al₂O₃. Where a drawing names C 610, we supply 95% white alumina instead.

What temperature can the tubes take?

In air, our corundum tubes are used up to 1,750 °C hanging vertically and 1,650 °C lying horizontally. Vacuum, load, fast heating and alkali vapour bring that limit down.

What tolerances do you quote?

On OD and ID: ±0.5 mm for OD 6–12 mm, ±0.6 mm for OD 15–17 mm, ±0.7 mm for OD 20 mm and ±1.0 mm for OD 24 mm. Length is held to ±1 mm and straightness to 0.1% of the length. If the bore matters most, we can make the ID one-sided, for example 10 +1/−0.

Will our thermocouple insert reach the bottom of the tube?

We test-fit tubes before dispatch. In a 15×10 tube, an 8.5 mm insert reaches the bottom and a 9.5 mm rod doesn't, so put your insert diameter on the order.

How fast do stock tubes ship?

Stock sizes usually ship in about 3 days for a small order and about 7 days for a bulk order. Sizes made to your drawing usually take 25–30 days, from 100 pieces.

Are your protection tubes made under ISO 9001?

Yes. Our ISO 9001:2015 certificate 06525Q02034R1S, issued by CFL Certification Center and valid to 5 August 2028, covers "Manufacturing of ceramic protective tubes for thermocouples".

Can we test samples first?

Yes. We usually send 5–10 free samples, short pieces of the size you want, and you pay the courier. Sample lengths of 500 mm or 1000 mm also work.

A wide stack of ivory tubes with rounded closed ends, grouped in bundles wrapped with clear stretch film, on a dark charcoal-grey background.

Alumina thermocouple protection tubes

Thermocouple protection tubes in C799 alumina, one end closed or open, OD 6–40 mm and cut to length. 18 sizes from 6×4 to 26×20 mm (OD × ID) are usually in stock, including 10×6, 15×10 and 24×18, and small orders ship in about 3 days.

Request a quote Standard sizes

high-purity alumina: C799; high-purity alumina
outer diameter: 6–40 mm; outer diameter
usually in stock: 18 sizes; usually in stock
to ship: ~3 days; to ship

Protection tubes in stock

About fifty long ivory tubes with rounded closed ends, standing in two groups against a white grooved wall.

Film-wrapped bundles of ivory closed-end tubes; each domed tip is cut with a single straight slot.

Long ivory tubes with rounded closed ends standing side by side, each with a single line of small holes along its length.

See all 8 photos

Graded row of alumina protection tubes with domed closed ends on a dark charcoal-grey background.

Graded row of cream alumina protection tubes on a dark charcoal-grey background, seen from the open ends, the largest in front showing its domed closed end.

Long open-ended alumina tubes laid side by side from large to small diameter, on a dark charcoal-grey background.

Diagonal row of open-ended alumina tubes in graded diameters on a dark charcoal-grey background, showing the wall thickness of each end.

Configurations

One-end closed
Open-ended
Round
Elliptical

Choose open or closed ends

A finished thermocouple held in a hand: a grey connection head on a metal sheath that ends in a long white ceramic protection tube. — Fitted to a thermocouple.

Applications

Thermocouple assemblies
Temperature-measurement instruments
Industrial furnaces
Heat-treatment furnaces

Ordering

Minimum order, non-standard parts: 100 pcs; Usually, per item made to your drawing or sample.
Samples for approval: 5–10 pcs; Usually sent for you to approve before the order runs.
Production lead time: 25–30 days; Usual production cycle for non-standard parts.
Standard sizes from stock: ~3⁠–⁠7 days; Stock covers most standard sizes. Small orders ship in about 3 days, bulk orders in about 7 days.

Typical terms. Quantities and dates for your order are confirmed in its quotation.

Standard sizes

Sizes made so far

OD: 6–40 mm
Length: 300–2500 mm

Other sizes made to drawing; tolerances confirmed per quotation.

Dimensions in mm

Standard sizes of Alumina thermocouple protection tubes, in millimetres
OD	ID
6 × 4 mm	4
7 × 5 mm	5
8 × 5 mm	5
9 × 6 mm	6
10 × 6 mm	6
10 × 7 mm	7
12 × 8 mm	8
13 × 8 mm	8
14 × 10 mm	10
15 × 10 mm	10
15 × 11 mm	11
16 × 12 mm	12
17 × 12 mm	12
18 × 13 mm	13
20 × 15 mm	15
24 × 18 mm	18
25 × 18 mm	18
26 × 20 mm	20

Standard sizes usually ship from stock in 3⁠–⁠7 days. Standard lengths 300 / 450 / 600 / 1000 mm; other lengths cut to order.

From our factory's standard-size list, September 2026.

Choose open or closed tube ends

GY Alumina makes both configurations. Start with your assembly drawing, because OD × ID alone does not tell us which end you need.

One end closed

Choose this when your assembly needs an intact ceramic tip around the measuring end. The bore stops inside the tip, so tell us the usable internal depth your insert needs as well as the overall tube length.

Both ends open

Choose this when your assembly needs a bore that passes all the way through. Check your drawing for any separate end closure. For a work tube running through a furnace, see our furnace-tube sizes.

Then mark any tip slots, side holes or drilled holes on your drawing. We have made these features; hole dimensions, position and tolerances need confirmation for your part. A hole through the tip leaves an opening, so make sure the drawing shows the end condition you actually need.

Pressure and leak ratings belong to the complete assembly. WIKA notes that gas diffusion through ceramic protection tubes can still occur at high temperature (TE 65.80, p. 10). Send your atmosphere, operating temperature and any pressure or vacuum requirement with the drawing for review.

For example, you can write: “OD 10 mm × ID 6 mm × overall length 600 mm, 99.5% Al₂O₃, one end closed. Please review the attached insert drawing.” That gives us the tube geometry and the end condition together.

Specify tube ends in your quote request

Secondary operations

Done in the factory after forming. Hole sizes and positions are confirmed from your drawing.

Cut to lengthTubes cut to the ordered length.
End slotsA slot across a closed tip, or slots cut into open tube and insulator ends.
Side holes and perforated tubesOne or two side holes, or a line of holes along the tube.
Closed ends with drilled holesOne-end-closed tubes drilled along their length.

Grade and material data

Corundum and mullite. Our standard grade is C799 alumina, 99.5% Al₂O₃, and we supply 95% white alumina where a drawing names C 610. Purity and maximum temperature are confirmed per SKU.

Full material data

Typical values by material class, Alumina thermocouple protection tubes
Material class	Al₂O₃ %	Working temperature °C	Hardness, Mohs
High-alumina	>85	1300–1450	8
Corundum	99–99.5^*	1600–1800^*	8-9
Mullite	Data supplied with quotation

* Stated per SKU: the grade and maximum temperature of each part are confirmed in its quotation.

Certified and tested

Quality management

ISO 9001:2015

GB/T19001-2016 · certified by CFL Certification Center

Scope: Manufacturing of ceramic protective tubes for thermocouples
Registration: 06525Q02034R1S
Valid: 6 August 2025 – 5 August 2028
Accreditation: CNAS C065-M · IAF

The issuer listed this certificate as valid when checked on 6 October 2026. Check the issuer’s record (Chinese).

A copy of the certificate is available on request.

Independent lab test

99.61% Al₂O₃

Alumina tube sample, tested by Chinalco Testing Technology (Zhengzhou) Co., Ltd., 13 May 2026. Report 26-04928.

Chemical composition of the tested alumina tube sample
Item	%	Method
Al₂O₃	99.61	GB/T 24487-2022
SiO₂	0.15	GB/T 6609.3-2004
Fe₂O₃	0.014	GB/T 6609.4-2004
Na₂O	0.054	GB/T 6609.1-2018
CaO	0.13	GB/T 6609.1-2018
Loss on ignition	0.033	GB/T 6609.2-2022

Tested by the same lab every year

202499.55% Al₂O₃, report 24-04372
202599.57% Al₂O₃, report 25-04979
202699.61% Al₂O₃, report 26-04928

Lab reports are available on request.

Guides

Sizes, grades and tolerances in more detail.

Questions buyers ask

Which thermocouple protection tube sizes do you keep in stock?: 18 sizes, from 6×4 to 26×20 mm (OD × ID), are usually in stock in standard lengths of 300, 450, 600 and 1000 mm. We cut other lengths to order, and we have made protection tubes from 300 to 2500 mm long.
What alumina grade are the protection tubes?: Our standard grade is C799 alumina, 99.5% Al₂O₃. Where a drawing names C 610, we supply 95% white alumina instead.
What temperature can the tubes take?: In air, our corundum tubes are used up to 1,750 °C hanging vertically and 1,650 °C lying horizontally. Vacuum, load, fast heating and alkali vapour bring that limit down.
What tolerances do you quote?: On OD and ID: ±0.5 mm for OD 6–12 mm, ±0.6 mm for OD 15–17 mm, ±0.7 mm for OD 20 mm and ±1.0 mm for OD 24 mm. Length is held to ±1 mm and straightness to 0.1% of the length. If the bore matters most, we can make the ID one-sided, for example 10 +1/−0.
Will our thermocouple insert reach the bottom of the tube?: We test-fit tubes before dispatch. In a 15×10 tube, an 8.5 mm insert reaches the bottom and a 9.5 mm rod doesn't, so put your insert diameter on the order.
How fast do stock tubes ship?: Stock sizes usually ship in about 3 days for a small order and about 7 days for a bulk order. Sizes made to your drawing usually take 25–30 days, from 100 pieces.
Are your protection tubes made under ISO 9001?: Yes. Our ISO 9001:2015 certificate 06525Q02034R1S, issued by CFL Certification Center and valid to 5 August 2028, covers "Manufacturing of ceramic protective tubes for thermocouples".
Can we test samples first?: Yes. We usually send 5–10 free samples, short pieces of the size you want, and you pay the courier. Sample lengths of 500 mm or 1000 mm also work.

What to send for a quote

01Outside diameter (OD), inside diameter (ID) and length
02One end closed or both ends open
03Round or elliptical section
04Al₂O₃ grade, or your operating temperature and atmosphere
05Thermocouple type, if you want us to check the fit
06Quantity per order and expected reorder rhythm

Next family:Alumina thermocouple insulators and multi-bore tubes

Request a quote

Get a quotation for protection tubes.

Tell us the dimensions, grade and quantity. We confirm the grade, dimensions and tolerances in each quotation. Our yearly lab reports are available on request.

Request a quote sales@gyalumina.com