What Is the DRM Diaphragm Seal?
The KOBOLD DRM is a diaphragm seal for pressure gauges that separates the gauge from the process medium with a flush-mounted membrane and a pressure transmission liquid. It covers measuring ranges from 0-1 bar to 0-1600 bar (0-14.5 psi to 0-23,206 psi) at media temperatures up to 662°F (350°C).
Available with plastic or stainless steel wetted parts, the DRM is designed to make it easier to measure the pressure of difficult media: highly viscous, crystallizing, polymerizing, corrosive, toxic, or solids-laden media, and media at very high or very low temperatures. ANSI flange, sanitary clamp, and NPT fittings are standard, and European (metric) fitting styles are available on request.
How a Diaphragm Seal Works
A diaphragm seal transmits the process pressure through a transmission liquid that the membrane keeps separated from the medium. The transmitted pressure acts on the pressure gauge, pressure switch, or pressure sensor, so the medium never penetrates the measuring system. Diaphragm seal and gauge are joined and adjusted at the factory to form one unit.
Each seal holds only the minimum volume of transmission liquid needed to cover the gauge displacement, the compressibility of the liquid, and thermal expansion. The seal itself has little effect on the gauge characteristics; the fill and working volume, the membrane, and the transmission line are the items that do.
Direct Mounting or Capillary Tube
The DRM may be coupled directly to the gauge or remotely connected through a filled capillary tube (transmission line) up to 49.2 ft (15 m) long, depending on model, with a protection hose or armor where required. A capillary tube allows installation of the indicator at a remote location and isolates the gauge from hot processes.
Transmission lines are indispensable where high media temperatures, system pressure, or pulsation would otherwise reach the gauge, or where the measuring point is unfavorably positioned. The capillary also protects the measuring device against vibration and damps pressure changes and pressure peaks.
Materials and Wetted Parts
Stainless steel DRM housings are 316Ti (1.4571) as a rule, with 316L (1.4404), 304 (1.4301), 1.4435, or 1.4539 stainless steel used on specific models, and other materials on request. Flush-mounted diaphragms are 316Ti stainless steel. Plastic-body models (DRM-630, DRM-631, and DRM-632) use PVC, PP, or PVDF housings and a PTFE-coated CSM (Hypalon) diaphragm.
Alternative diaphragm materials and coatings are available: an ECTFE-coated diaphragm on the two-part DRM-609, tantalum or Monel diaphragms on the DRM-626/627 Vario, and PFA, PTFE, or ECTFE coating of the diaphragm or the whole seal on request. Gaskets are NBR as standard, with FKM, EPDM, or FFKM seals on the plastic models and an FPM O-ring on the DRM-626/627.
Process Connections
Threaded DRM seals have fixed male threads from ½" to 1½" NPT, G½ to G1½, M20×1.5, or M48×3 (DRM-600/601). Flanged versions include ANSI B16.5 flanges, 1" to 4", Class 150 through 1500 (DRM-634/635), DIN flanges DN 25 to DN 100, PN 6 to PN 40 (DRM-628/629), and Ø 48 mm to Ø 100 mm mounting flanges (DRM-622 through DRM-633).
Hygienic connections include union nuts to DIN 11851 (DRM-602/603), SMS (DRM-610/611), IDF (DRM-604/605), and APV-RJT (DRM-614/615), plus clamp connections to DIN 32676 (DRM-612/613) from DN 25 to DN 100 and 1" to 4". Inline diaphragm seals (DRM-500/501/502) place the diaphragm tube in the pipe itself, DN 15 to DN 50, giving cavity-free measuring points for special hygienic conditions.
Pressure, Temperature, and Fill Liquids
Measuring ranges run from 0-0.25 bar (0-3.6 psi) with pressure sensors up to 0-1600 bar (0-23,206 psi) on the DRM-616/617, depending on model, diaphragm size, and gauge size. The threaded DRM-600/601 covers 0-1 bar to 0-1000 bar (0-14.5 psi to 0-14,504 psi) with sensors and 0-6 bar to 0-1000 bar (0-87 psi to 0-14,504 psi) with gauges.
Media temperature limits vary by model: the threaded DRM-600/601 and the DIN 11851 hygienic versions are rated -40°F to 392°F (-40°C to 200°C), the DIN and ANSI flanged versions -40°F to 482°F (-40°C to 250°C), and the capsule seals DRM-606 and DRM-607/1 -40°F to 662°F (-40°C to 350°C). Plastic-body models are rated 32°F to 104°F (0°C to 40°C).
The pressure transmission liquid is selected for the operating conditions. Silicone-free fills are glycerine, rated to 176°F (80°C), and liquid paraffin, rated 14°F to 248°F (-10°C to 120°C). Silicone oils are available for -40°F to 392°F (-40°C to 200°C), -4°F to 662°F (-20°C to 350°C), and -4°F to 752°F (-20°C to 400°C). For special applications the seal can be filled with MF7 (mercury), -4°F to 662°F (-20°C to 350°C).
Typical Applications
KOBOLD DRM diaphragm seals are used when the pressure gauge alone cannot stand up to the requirements of the measuring point. Areas of application include media enriched with solids; crystallizing, polymerizing, and corrosive media; toxic and environmentally hazardous media; high-viscosity media; very low or very high medium temperatures; hygienic requirements in the pharmaceutical sector; and batch changing without product residues in the measuring system.
Common Questions about DRM - Diaphragm Seal
The DRM shields the measuring device from aggressive, highly viscous, solidifying, and crystallizing media, and from media carrying suspended solids. It is also used where metals are incompatible with the medium, where the medium is toxic or environmentally hazardous, and where the medium temperature is too high or too low for the gauge alone.
The highest rating is 662°F (350°C) on the capsule seals DRM-606, DRM-607/1, DRM-620, and DRM-620/1 with MF7 fill. For the threaded DRM-600/601, the maximum medium temperature is 212°F (100°C) with direct mounting and 392°F (200°C) when a capillary tube separates the gauge from the seal.
Capillary length depends on the seal model: 49.2 ft (15 m) maximum on the threaded DRM-600/601, the capsule seals, the DRM-622/623 and DRM-624/625, and the DRM-633; 39.4 ft (12 m) on the DRM-621; and 32.8 ft (10 m) on the hygienic, DIN flange, ANSI flange, Vario, and plastic models. A protection hose or armor is fitted where required.
ANSI flange seals DRM-634/635 cover 0-15 psi to 0-1500 psi with a gauge and 0-4 psi to 0-1500 psi with a sensor. Sanitary clamp seals DRM-612/613 cover 0-0.4 bar to 0-16 bar (0-5.8 psi to 0-232 psi). Plastic seals reach 10 bar (145 psi) in PVC or PP and 16 bar (232 psi) in PVDF.
The fill is chosen for the operating conditions: glycerin or liquid paraffin where a silicone-free liquid is required, silicone oil for wider temperature spans, and MF7 for special applications. Other requirements, such as an absolutely silicone-free fill, also enter into the choice, so KOBOLD technicians select the transmission liquid for the stated requirements.
The DRM-634 (direct mounting) and DRM-635 (capillary tube) use a sealed-in membrane and a flange to ANSI B16.5 in eight sizes from 1" through 4" and in Class 150, 300, 600, or 1500. Flange and diaphragm are 316Ti stainless steel, PFA coating is available on request, and the device connection is a G½ female thread.
Because the reading depends on the temperature of the fill liquid, the assembly is configured for the ambient and medium temperatures of the application; otherwise gauges are calibrated at 68°F (20°C) ±3.6°F (±2°C). Gauges with a transmission line are set with no height difference between gauge and seal, so any other height difference must be stated on the order.
The DRM-630 (PVC), DRM-631 (PP), and DRM-632 (PVDF) have plastic housings with stainless steel housing screws, a PTFE-coated CSM (Hypalon) diaphragm, and NBR, FKM, EPDM, or FFKM seals. Process connections are G or NPT female or male threads from ¼" to 1", with a G¼, G⅜, G½, or NPT female device connection or a capillary tube.
A diaphragm seal barely influences the characteristics of a pressure gauge. The reading is affected by the temperature of the transmission liquid, so the measuring system is tuned to the operating conditions, and the gauge and seal are set up together at the works as a single unit.
Suitable devices include KOBOLD MAN series pressure gauges, SEN series pressure sensors, and PAD series pressure transmitters, among others. Each DRM model carries a separate measuring range for sensors and for gauges, since the range depends on the size of the diaphragm and of the gauge; the sensor range typically starts lower than the gauge range for the same seal.