The safest way to select an RTD probe is to match one documented construction to the process, not to apply a family maximum to every probe. In the Focusens FWZ Series source, SUS304 and SUS316 are listed housing materials. The family technical table lists -50 to +200°C, -50 to +350°C and -50 to +600°C ranges, plus a 10 s response entry labelled 0.63τ. Individual drawings are more specific: they show -50 to +200°C through -50 to +500°C, diameters from 3 to 16 mm, customer-defined lengths, and 20 or 30 s response entries in still air.

Those values are useful only with their original conditions. The FWZ pages do not provide a common t50/t90 comparison, a material-specific corrosion envelope, or an IP65/IP67 rating. Inconel 600, Hastelloy, a PTFE probe sheath and a mineral-insulated FWZ construction are also not listed as standard options in the reviewed FWZ pages. Treat those as engineered requests, not published Focusens specifications.

What is included in an RTD probe?

An RTD sensor is the resistance-based sensing element. An RTD probe is an assembly that can include the element, a sheath or housing, filler or insulation, extension leads and a connector or termination. TE Connectivity's RTD overview makes the same element-versus-assembly distinction.

This distinction affects every purchasing decision. A Teflon lead-wire option does not prove that the wetted probe has a PTFE sheath. A housing material does not define the cable limit. A terminal head rating does not automatically cover the gland, connector and installed assembly.

The Focusens FWZ ordering code lists Pt100, Pt1000, Cu500 and Ni element choices, DIN Class A, B, C or a special class, simplex or duplex construction, eight mounting categories, five wire-material categories and a customer-defined wire length. The technical table separately lists PT100, PT500 and PT1000. Confirm the final element and accuracy class on the quotation and drawing when these two source sections are used together.

Focusens FWZ specifications available from the supplied catalogue

The public Focusens straight-pipe Pt100 product page also lists SUS304/SUS316, the 10 s (0.63τ) response entry and the same cable-material group. Because the live page contains several temperature statements for different contexts, the quotation should still identify the exact product code and drawing rather than using one page-wide maximum.

RTD sheath and housing material selection table

Use SUS304/SUS316 when transcribing the Focusens source. SS304/SS316 can remain in search copy as familiar aliases, but the quotation and drawing should carry the exact material designation used by engineering.

When is Inconel 600 required?

There is no universal temperature at which every RTD probe must switch from stainless steel to Inconel 600. The decision depends on the complete assembly, atmosphere, oxidation cycle, contaminants, pressure, vibration, sheath wall, joining method and required service life. Inconel 600 becomes a serious candidate when the application combines high temperature with oxidation or furnace-atmosphere exposure and the selected stainless steel construction is not suitable.

For an FWZ enquiry, provide the normal temperature, peak temperature, time at peak, atmosphere, process chemistry, pressure, velocity, vibration and cleaning cycle. Ask Focusens to return the exact alloy designation, drawing revision and supported operating limit. Until that response is issued, Inconel 600 remains a custom request.

Structure, diameter, temperature and response-time table

The following values are transcribed from FWZ drawings on PDF pages 21-22 of the supplied Focusens catalogue. A, L and other letters are drawing dimensions; they should not be renamed as insertion length, overall length or sensitive length without the controlled drawing.

The threaded drawings show examples including M8x1, M12x1, M14x1, M16x1.5, G1/4 and G1/2. The head-style drawing shows M20x1.5 and NPT1/2 markings, but the reviewed page does not state the head material, terminal arrangement, transmitter compatibility or IP rating.

Ø4.5 mm is not shown as a standard FWZ diameter on these pages. It may fall within a displayed diameter range, but that is not the same as confirming an available standard size. Request the exact outside diameter, tolerance, sheath wall, sensitive length and response result on the quoted drawing.

How should RTD response time be compared?

Do not subtract the catalogue's 10, 20 and 30 s entries as if they were one controlled test series. The general row says 10 s (0.63τ). Selected drawings say 20 s in still air or 30 s in still air. The source does not give one test medium, flow velocity, temperature step, immersion, diameter and t50/t90 definition for all three values.

A valid comparison request should contain:

• the response metric: t50, t63 or t90;
• the temperature step and starting condition;
• the test medium and flow velocity;
• probe diameter, sheath wall and tip construction;
• insertion depth and mounting arrangement;
• the exact product code and drawing revision.

If two proposals use different response definitions or media, mark them not directly comparable. A smaller diameter often changes the thermal path, but this article does not assign a guaranteed speed advantage to Ø3, Ø4.5 or Ø6 mm without a common test.

How should insertion and immersion depth be determined?

Start with the required sensing position, not a catalogue maximum. The sensing region must reach representative process temperature while the assembly still has enough clearance for installation, flow, thermal expansion and removal. In a pipe, provide the internal diameter, wall thickness, flow direction, velocity, nearby fittings and thermowell design. In a tank, provide the fill range, agitation, stratification, heater location and cleaning access.

Keep these dimensions separate on the RFQ:

• insertion length from the stated mounting reference;
• overall probe length;
• sensitive length and sensitive-zone position;
• thermowell length and bottom thickness, when used;
• cable length and cable exit direction;
• process connection and thread engagement;
• installation and removal clearance.

There is no universal insertion-depth formula that fits every RTD construction. An Endress+Hauser RTD technical document gives different minimum immersion values for different thermowell constructions and notes that process temperature, pressure, velocity, immersion length, material and medium all affect thermowell limits. Use that as general engineering context, not as a Focusens dimension.

What does a spring-loaded RTD design do?

The FWZ ordering code includes compression spring installation type, so spring loading is a documented configuration category. The reviewed FWZ page does not state spring travel, preload, contact force, temperature limit or the exact mechanical purpose for every design.

In a thermowell assembly, spring loading is commonly used to keep the measuring insert in positive contact with the bottom of the thermowell bore. WIKA states this function for its spring-loaded TR10-2 RTD assembly. That generic function explains why the option exists, but it is not a Focusens performance guarantee. For FWZ, request the spring travel, preload, stack-up tolerance, mounting interface and approved drawing.

How should IP65 or IP67 be selected for an RTD connection head?

An IP rating belongs to the tested enclosure or complete assembly named by the source. It cannot be transferred from a transmitter box, connector or another sensor family to an FWZ probe.

IEC 60529 is the governing enclosure-protection standard referenced by the FRT source. In the IP code, the first digit 6 denotes dust-tight protection; a second digit 5 concerns water jets, while 7 concerns immersion conditions. These are different tests. Schneider Electric notes that an IP67 result should not automatically be treated as proof of a different water-jet rating; see its IP66/IP67 test explanation.

For a connection head, state the actual exposure: indoor dust, outdoor rain, washdown jets, condensation, temporary immersion, cleaning chemicals, ambient temperature and installation orientation. Ask the supplier to identify the head, cover seal, cable gland, connector, thread, probe entry and cable that are included in the rating. If any one part changes, the assembly rating may need a new review.

RTD probe RFQ checklist

Frequently asked questions

Which RTD probe housing materials are listed by Focusens?

The supplied FWZ source lists SUS304 and SUS316 housing. It lists Teflon as a cable/wire material, not as a PTFE probe sheath. Inconel 600, Hastelloy and a mineral-insulated FWZ RTD construction are not listed on the reviewed FWZ pages.

What are the upper temperature limits of SUS304 and SUS316 in the FWZ range?

The source does not assign a separate maximum to each housing material. The family technical table lists ranges ending at +200°C, +350°C and +600°C, while the reviewed drawings end at +200°C, +250°C, +300°C, +350°C or +500°C depending on structure. Use the limit attached to the selected drawing; do not assign +600°C to every SUS304 or SUS316 probe.

How many seconds faster is one FWZ structure than another?

The available entries are not a controlled t50/t90 comparison. The general table lists 10 s (0.63τ), while drawings list 20 or 30 s in still air. Request one response definition and one test condition before calculating a difference.

When must an RTD use Inconel 600?

No universal threshold applies to every RTD. It is an engineering candidate for some high-temperature oxidation and furnace-atmosphere applications when stainless steel is unsuitable. Focusens availability and the permitted range require a written custom response.

How is RTD probe immersion depth selected?

Define the representative sensing position, process geometry, thermowell, flow, clearance and service access. Then request a drawing that separates insertion length, overall length and sensitive length. Do not copy a depth from another probe.

Should a connection head be IP65 or IP67?

Choose the required tests from the real exposure. IP65 addresses dust-tight construction and water jets; IP67 addresses dust-tight construction and immersion conditions. Require the rating for the complete installed head, gland, connector, probe entry and cable. The supplied FWZ pages state neither rating.

What is the purpose of a spring-loaded RTD probe?

The FWZ code confirms a compression-spring installation category. In thermowell systems, spring loading commonly keeps the insert against the thermowell bottom, but the FWZ travel, preload and performance must be defined by its own drawing.

Related Focusens RTD resources

• Browse the Focusens RTD sensor collection.
• Review the straight-pipe Pt100/Pt1000 RTD probe.
• Compare the stainless-steel Pt100 RTD probe.
• See the Pt100/Pt1000 coffee-machine RTD assembly.
• See the high-temperature Pt1000 boiler probe.

To request a selection review, send the completed RFQ fields through Focusens Contact Us. Ask for the current product code, controlled drawing, material callout, response definition and IP evidence before approving production.

Source and review note

Focusens product values in this guide were transcribed from the supplied Focusens Catalogue.pdf, FWZ Series RTD Sensor pages 21-23 in PDF numbering, and checked against the supplied FRT product sheet where IP65 is discussed. Public engineering links provide general material, immersion, spring-loading and IP-code context; they do not expand Focusens product availability.

• Source revision shown in the catalogue: V2.0; modification rights reserved.
• Local source review date: 2026-08-24.
• Author: to be assigned before publication.
• Technical reviewer: to be assigned before publication.
• Drafting method: prepared with AI assistance from the supplied Focusens documents and cited public technical sources; human technical and editorial review is required.
• Publication decision: hold until the current catalogue/drawing revision, target URL and named technical reviewer are approved.