All-welded 316L stainless steel pressure transmitters with no internal static O-rings are now a defined product class, with 200 bar static pressure, 0.04-0.25% accuracy, and IP68 sealing cited as standard performance by multiple OEMs as of mid-2026 [S1][S2][S3].
The architecture eliminates elastomeric seals on the wetted side, bonding the 316L diaphragm directly to a 316L body via fusion welding, so the fill oil, silicon die, and process media are separated by a single metallic barrier; this is the same principle described in patent CN203365046U for an all-welded differential pressure sensor with strong static-pressure bearing and good sealing [S5].
What "no static O-ring" means in the welded architecture
Static O-rings in a conventional pressure transmitter sit between the diaphragm and the process port; they are the dominant leak path when the elastomer ages, swells in hydrocarbons, or cold-flows under sustained static pressure [S2][S5].
Welding the diaphragm to a 316L port removes that interface, which is why Ould's PT-516 lists "no internal O-rings" alongside IP68 ingress protection and reverse-polarity protection, and why the MDM291 datasheet explicitly says "No O-rings, all welding construction, possible for various fluid media" with 316L diaphragm and housing as the only wetted metals [S2][S3]. The PC90D monocrystalline-silicon differential sensor repeats the pattern: 316L all-welded integrated structure, positive/negative pressure symmetrical, no O-ring inside, supplied with a small flange kit [S8]. For a primer on how elastomer selection typically fails in this role, see the O-ring reference page.
Performance numbers from current OEM datasheets
The MDM291 welded differential pressure sensor specifies 0-0.35 bar to 0-20 bar range, 200 bar max static pressure, 2x overpressure, ±0.15%FS BFSL linearity (max ±0.25%FS), ±0.05%FS repeatability and hysteresis, 100 MΩ insulation at 100 VDC, 0-70 °C compensated with -40 to +125 °C working range, and ±0.3-0.5%FS/year long-term stability, all in a 316L diaphragm and 316L housing with FKM as the only non-wetted elastomer on the lead-wire side [S3].
Comparable gauge/absolute devices on the fully-welded industrial list (Jumo MIDAS S22 Ex, MIDAS S05, LH-10, GE UNIK5000H, Baumer CombiPress PFMH, and the 4262A) cover 0.4 bar to 700 bar, with 0.04-0.3% precision, -40 to +125 °C process temperature, and 0.1% long-term stability; the UNIK5000H is specifically framed for hydrogen service where the absence of an elastomer is the main reason hydrogen embrittlement and permeation of an O-ring are avoided [S1]. For a wider view of gauge/absolute ranges, the pressure transmitter reference lists the same architecture family.
Gauge, absolute, and differential compared against decision criteria
All-welded 316L is offered in three topologies: gauge/absolute (single port, 1-side isolation), differential (two ports, both sides isolated by welded diaphragms, as in MDM291 and PC90D), and submersible level (PT-516, IP68, single cable outlet) [S2][S3][S8].
For high static-pressure service the MDM291's 200 bar static rating with 2x overpressure is a useful threshold: differential devices that rely on an internal O-ring typically derate above 100-160 bar, while the all-welded 316L differential construction holds 200 bar with a published zero drift of ≤0.05 mV/bar against static pressure [S3]. For media compatibility the welded 316L construction inherits the same corrosion profile as standard stainless steel 316L, which is suitable for water, steam, oils, and many chemicals but not for concentrated chlorides or strong reducing acids; for those, diaphragm-seal assemblies in Hastelloy-C, tantalum, or Monel are the standard upgrade, as KOBOLD lists on the PAD datasheet [S4]. For absolute-pressure applications where the reference side must be hard vacuum, dedicated absolute pressure transmitter designs extend the same welded approach.
Application fit and failure modes
All-welded no-O-ring transmitters are specified for low-pressure gas, hydrogen, sanitary/CIP-SIP, submersible level, and differential flow in chemical and oilfield service, where the user wants to take elastomer ageing, swelling, and cold-flow off the failure-mode list [S1][S2][S3][S5].
They are NOT a free upgrade for every job. A conventional O-ring transmitter with a Kalrez or FFKM seal typically outperforms a welded 316L diaphragm on highly corrosive media where 316L is the wrong alloy; in that case the same welded-architecture devices are sold with Hastelloy-C or tantalum diaphragms through diaphragm-seal adapters rather than as a direct swap [S4]. Welded construction also complicates field repair: a damaged diaphragm or blocked pressure port usually means replacing the whole sensor rather than re-seating an O-ring, which raises lifecycle cost for low-criticality installations; users should weigh this against the longer mean time between failures the O-ring-free design delivers in high-cycle or hard-to-access service [S2][S3]. For differential flow on orifice plates, the differential pressure transmitter reference notes the same trade-off between welded and O-ringed isolations.
Process connection and welding practice
Welding the transmitter to the piping is the other half of the "no static O-ring" design: the bond between sensor and piping becomes the long-term seal, so procedure matters as much as the diaphragm weld inside the device [S7].
Industry guidance for pressure-sensor process connections recommends a compatible stainless steel filler rod, full-penetration welds, and post-weld passivation when the wetted side is 316L; the same guidance notes that welded connections "form a strong, permanent bond between the pressure sensor and the equipment or piping" and that they are the preferred method for high-pressure, high-temperature, or hazardous-media service where a threaded connection with sealant could leak [S7]. For the related static-pressure molding machine note that the word "static" in a different context refers to molding pressure and is unrelated to the static O-ring being removed here. The all-welded concept as a whole is also covered in pressure transmitter product-class definitions.
Standards, sourcing, and what to verify on a datasheet
The KOBOLD PAD differential pressure transmitter datasheet lists ATEX approval and HART protocol digital communication, with available wetted materials including stainless steel, Hastelloy-C, tantalum, and Monel [S4].
When verifying a candidate datasheet, check four numbers explicitly: (1) "no static O-ring" stated on the wetted side, with the only elastomer (if any) named and located on the non-wetted side; (2) 316L or higher alloy for diaphragm and port, with a compatible filler rod specified for the process weld; (3) ingress rating, where IP68 is typical for submersible service and IP65/IP67 for gauge/absolute; and (4) long-term stability as %FS/year, where ≤0.1%FS/year is common for the higher-end welded devices and ≤0.5%FS/year for OEM-grade elements like the MDM291 [S2][S3]. Buyers should also confirm the overpressure rating (2x is the typical baseline) and the static pressure rating separately, since the two numbers are independent on a differential device [S3][S4].
Trackable signals: (a) more IP68-rated submersible variants moving from 316L into higher-nickel alloys for seawater and chlor-alkali service; (b) the all-welded no-O-ring topology extending into sanitary 3-A / EHEDG bodies that currently still use clamped gaskets, per the Baumer CombiPress PFMH example [S1]; (c) wider availability of HART 7 / HART-IP on welded differential devices as ATEX/IECEx dual-certified Ex d / Ex ia becomes the default on European chemical-plant builds [S1][S4].
Background reading: Stop Category 0 vs Stop Category 1 per IEC 60204-1: Selection Rules, Wiring, and Drive.