Temperature Instrumentation for Research and Industry


Thermowells - General Information

Thermowells provide maximum protection for thermal sensors from corrosion, pressure and flow induced stresses. When selecting thermowells these parameters determine the type and material that should be used. In general, thermowells are machined from solid bar stock for "A" dimensions to 24" but for londer lengths a built-up design is used.

Application Considerations

  • Select sensor location for representative temperature measurement.
  • Provide sufficient depth of immersion so that heat transfer along the instrument does not influence temperature measurement.
  • Select materials that are compatible with corrosive media elements.
  • Select thermowell with sufficient stiffness to resist destruction from flow induced stresses.

For Mechanical Application Considerations -Click Here!

Thermowell Materials

Strength at operating temperature and resistance to corrosion are the primary considerations in material selection.

Carbon Steels
Can be used to 1300°F (700°C) usually in oxidizing atmospheres.
Austenitic Stainless Steels (300 Series)
Can be used to 1600°F (870°C) mostly in oxidizing atmospheres although type 316 can be used in some reducing environments.
Ferritic Stainless Steels (400 Series)
Can be used to the 1800°F (982°C) - 2100°F (1149°C) range in both oxidizing and reducing atmospheres.
High Nickel Alloys
Can be used to 2100°F (1149°C) in oxidizing atmospheres.


Melting Point (°F)


Max. Operating Temperature in
Atmosphere (°F)

304 304 SS 2560 ORNV 1650
310 310 SS 2560 ORNV 2100
316 316 SS 2500 ORNV 1700
321 321 SS 2550 ORNV 1600
347 347 SS 2600 ORNV 1600
446 446 SS 2700 ORNV 2000
CS Carbon Steel 2500 ON 1300
INC InconelTM 2550 ONV 2100
INX Inconel XTM 2620 ONV 1500
INY IncoloyTM 2500 ON 1600
HTX Hastelloy XTM 2300 O 2200
HTC Hastelloy CTM 2310 O 1800
HTB Hastelloy BTM 2375 OR 1400
MON MonelTM 2460 OR 1000
BR Brass 1850 O 650
AL Aluminum 1220 O 700
NCK Nickel 2647 O 1400
TRN Tantalum 5425 V 5000
TIT Titanium 3035 VN 2000
O=Oxidizing, R=Reducing, N=Neutral, V=Vacuum

Velocity Rating

Once the selection of materials is made attention should be given to the parameter of flow induced stresses. Fluids flowing by a well form a turbulent wake called the Von Karman Trail. This wake has a definite frequency based on the diameter of the well and the fluid velocity. It is important to provide a well with sufficient stiffness so that the wake frequency will never exceed the natural frequency of the well itself. Should the natural frequency of the well coincide with the wake frequency, the well would vibrate to destruction. Tapered shank wells provide greater stiffness for the same sensitivity than a straight shank well. The higher strength to weight ratio gives these wells a higher natural frequency and therefore are able to operate at higher fluid velocities. Recommended maximum velocity rating can be found for every standard well length and material catalogued. Ratings are based on operating temperatures as shown in the table below.

MATERIAL Velocity Rating
Operating Temperature
Carbon Steel (C1018)
304 SS
316 SS
Monel 900°F
Brass 350°F

Single values that appear in the velocity tables may be considered safe for water, steam, air or gas. Double values distinguish between water (paranthesized) and steam, air and gases. These values are intended as general guides to selection. If you have operating conditions requiring special well designs our engineering staff is available to assist you.

Pressure Rating

The limit pressure versus temperature ratings are tabulated for various materials for each thermowell series. Here is a typical table.

70° 200° 400° 600° 800° 1000° 1200°
Brass BR 5000 4200 1000 - - - -
Carbon Steel CS 5200 5000 4800 4600 3500 1500 -
A.I.S.I. 304 304 7000 6200 5600 5400 5200 4500 1650
A.I.S.I. 316 316 7000 7000 6400 6200 6100 5100 2500
Monel MON 6500 6000 5400 5300 5200 1500 -

NOTE: Selection of material and/or equipment is at the sole risk of the user of this site. The data presented does not and should not preclude professional engineering design and consulting for your particular application. Marlin Manufacturing Corporation, its distributors, representatives, and the contributors to this site specifically deny any warranty expressed or implied.

For more information regarding Thermowells
Consult your Marlin Catalog.