High Temperature Absolute Pressure Transmitter with Remote Diaphragm Seal up to 400°C

Standard pressure transmitters fail in high-temperature service for a simple reason: their electronics cannot survive the heat at the process connection. The QWY-323 solves this with a remote diaphragm seal - the 316L or Hastelloy diaphragm touches the hot media, while the transmitter body and its sealed vacuum reference sit at the end of a capillary, safely away from the heat. The result is a true absolute pressure measurement for media up to 400°C, with no impulse line to freeze, clog or fill with process fluid. For polymerization reactors, steam-jacketed vessels, melting tanks and high-temperature distillation, this is the instrument that keeps working when ordinary transmitters are already dead.

Why Do You Need Absolute Pressure Measurement in High Temperature Processes?

High-temperature processes are where both measurement problems meet. First, the heat: media above 150°C destroys standard sensor electronics and makes impulse lines a constant maintenance headache - they freeze, polymerize, or fill with media that later solidifies. Second, the reference: many of these processes are physically defined by absolute pressure. Polymerization reactors must stay above the vapor pressure of the monomer to prevent boiling; steam-jacketed kettles need true vapor pressure control; melting and evaporation tanks depend on the absolute pressure inside the vessel. A gauge-referenced transmitter drags the weather into all of this, shifting its reading with every barometric change and requiring a vent that can clog with viscous media. The QWY-323 answers both problems at once: the remote seal removes the heat problem, and the sealed vacuum reference removes the atmosphere problem.

Working Principle: Remote Diaphragm Seal with Capillary Transmission

The QWY-323 separates the sensing point from the electronics. A welded metal diaphragm, sized and flanged to suit your vessel, contacts the hot process media. The pressure acting on the diaphragm is transmitted hydraulically through a capillary tube filled with high-temperature silicone or fluorinated oil to the transmitter body, where a diffused silicon cell measures it against a sealed vacuum reference. Because the capillary fluid is virtually incompressible, the pressure arrives at the sensor with minimal loss, and the transmitter electronics - which never see more than ambient temperature - convert it to a 4-20 mA HART signal. The remote seal also isolates the instrument from the media itself: corrosive, viscous, crystallizing or fiber-containing fluids never enter the transmitter, so there is no impulse line to purge and no dead-leg where media can stagnate.

Structural Advantages of the QWY-323 High Temperature Absolute Pressure Transmitter

• Remote diaphragm seal isolates electronics from media temperatures up to 400°C
• Capillary lengths from 1 to 10 meters for flexible transmitter placement away from heat
• 316L standard and Hastelloy C option for corrosive and aggressive media
• Flanged or threaded remote seals in DN50/DN80 or ANSI 1"/2" configurations
• Sealed vacuum reference - no atmospheric vent, no barometric drift in outdoor plants
• High-temperature silicone or fluorinated fill fluid matched to the process
• 4-20 mA HART output for remote ranging and diagnostics from the control room
• IP67 housing with stainless steel option for washdown and weather-exposed mounting

Pain Point Analysis: 4 Working Conditions Where Absolute Reference Wins

Application Case 1: Polymerization Reactor Vapor Pressure Monitoring

Application Case 2: Asphalt Storage Tank Level and Pressure Monitoring

Eight Product Advantages of the QWY-323

1. Remote diaphragm seal design protects electronics from media temperatures up to 400°C
2. True absolute measurement with sealed vacuum reference - stable in any weather or altitude
3. No impulse line, no dead-leg, no freezing or plugging with viscous or crystallizing media
4. 316L and Hastelloy C wetted options for corrosive high-temperature service
5. Capillary lengths up to 10 m for safe, accessible transmitter mounting
6. 4-20 mA HART output for remote configuration and diagnostics
7. ±0.1% FS accuracy with fill-fluid temperature compensation
8. 24-month warranty with fill-fluid selection support from our application engineers

Technical Parameters

Why Choose Our Absolute Pressure Transmitter?

Remote seal instruments are unforgiving of guesswork - the wrong fill fluid, capillary length or diaphragm size quietly destroys accuracy. Qinwei Instruments engineers select the fill fluid and seal configuration from your actual process data: media temperature, ambient temperature, mounting height and pressure range. Every QWY-323 is assembled, filled and calibrated in our factory, with the complete system tested at elevated temperature before shipment. With more than a decade of instrument manufacturing experience and customers across the chemical, polymer and energy industries, we provide the application engineering that makes remote seal systems work - and we stay on the line until yours does.

SEO FAQ

Standard transmitters have their electronics directly behind the process connection. When media exceeds roughly 120-150°C, heat conducts through the housing and raises the electronics temperature beyond their rated limit, causing drift, premature failure and eventually permanent damage. Impulse lines are the traditional workaround, but they introduce their own problems: hot media can flash, freeze, polymerize or solidify in the line, and the line must be purged and traced. A remote diaphragm seal eliminates the root cause - the electronics never see the process temperature because the measurement is transmitted hydraulically through a capillary. That is why high-temperature service is the classic application for remote seal transmitters, and the QWY-323 extends this to absolute pressure measurement.

Every remote seal system adds small errors from fill-fluid thermal expansion, capillary length and mounting height. The QWY-323 handles these in three ways: the fill fluid is selected to match your temperature window, the system is calibrated as a complete assembly (sensor plus seal plus capillary) rather than as a bare transmitter, and the electronics apply fill-fluid temperature compensation. The result is a stated system accuracy of ±0.1% FS across the calibrated range. For the best accuracy, give us the actual media temperature, ambient temperature and the vertical distance between the seal and the transmitter - we will size the capillary and fill fluid so the installed error stays inside your tolerance.

For media above 250°C, standard silicone oil degrades, so we supply high-temperature silicone oil for service up to about 300°C and fluorinated fill fluid for the highest temperatures and for oxygen or chlorine service where silicone is not permitted. The choice also depends on the ambient temperature at the transmitter end, because the fill fluid must remain fluid across the whole temperature span. Our application engineers confirm the fill fluid, capillary length and diaphragm size from your process data before we build the unit. If you are unsure of the exact media temperature, we will specify a margin and document the limits on the datasheet.

Yes. Mounted at the bottom of an open or vented tank, the QWY-323 measures the hydrostatic head of the liquid column, which is directly proportional to level for a known liquid density. The flush-mounted diaphragm version is particularly suited to tanks with viscous, slurry or solidifying media, because there is no nozzle dead-leg where material can accumulate and distort the reading. For closed tanks under pressure or vacuum, the absolute measurement gives you the true head plus vapor pressure picture, and we can help you derive level from the two measurements if you install top and bottom instruments. Tell us your tank geometry and media density and we will confirm the calibration.

The capillary is armored stainless steel and the diaphragm seal is welded, so damage requires physical abuse - impact, crushing or sharp bending. If a leak ever occurs, the symptom is a slow zero shift followed by a reading that no longer responds to pressure, which HART diagnostics can flag as a sensor fault. The capillary and seal assembly is replaceable as a module, and because the transmitter body is standard, we can ship a replacement seal assembly quickly to minimize downtime. For installations where capillary damage is a real risk, we offer armored capillary and protective conduit options, and we will recommend the right protection level for your plant layout.

You need four inputs: the nozzle size and flange standard (for example DN50 PN16 or ANSI 2" 150 lb), the media and its maximum temperature, the pressure range you need to measure, and the mounting height difference between the seal and the transmitter. From these, we select the diaphragm diameter, fill fluid, capillary length and calibration. For hygiene-sensitive or highly corrosive media we also confirm the wetted material - 316L standard, Hastelloy C for chlorides and strong acids. If you are replacing an existing failed instrument, send us a photo of the old seal and nozzle and we will match the configuration exactly.