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Pressure Sensor Accessories

Multiple accessories exist for pressure-sensing devices to function optimally in challenging process environments. Sometimes, we must use special accessories to protect the pressure instrument against hazards of certain process fluids. One such hazard is pressure pulsation, for example at the discharge of a piston-type (positive-displacement) high-pressure pump. Pulsating pressure can quickly damage mechanical sensors such as bourdon tubes, either by wear of the mechanism transferring pressure element motion to an indicating needle, and/or fatigue of the metal element itself.

Valve manifolds

An important accessory to the DP transmitter is the three-valve manifold. This device incorporates three manual valves to isolate and equalize pressure from the process to the transmitter, for maintenance and calibration purposes.

The following illustration shows the three valves comprising a three-valve manifold (within the dotted-line box), as well as a fourth valve called a “bleed” valve used to vent trapped fluid pressure to atmosphere:


The illustration shows the three valves comprising a three-valve manifold (within the dotted-line box), as well as a fourth valve called a

While this illustration shows the three valves as separate devices, connected together and to the transmitter by tubing, three-valve manifolds are more commonly manufactured as monolithic devices: the three valves cast together into one block of metal, attaching to the pressure transmitter by way of a flanged face with O-ring seals. Bleed valves are most commonly found as separate devices threaded into one or more of the ports on the transmitter’s diaphragm chambers.

The following photograph shows a three-valve manifold bolted to a Honeywell model ST3000 differential pressure transmitter. A bleed valve fitting may be seen inserted into the upper port on the nearest diaphragm capsule flange:


Three-valve manifold bolted to a Honeywell model ST3000 differential pressure transmitter

In normal operation, the two block valves are left open to allow process fluid pressure to reach the transmitter. The equalizing valve is left tightly shut so no fluid can pass between the “high” and “low” pressure sides. To isolate the transmitter from the process for maintenance, one must first close the block valves, then open the equalizing valve to ensure the transmitter “sees” no differential pressure. The “bleed” valve is opened at the very last step to relieve pent-up fluid pressure within the manifold and transmitter chambers:

 

Normal operation and removed from service DP transmitter manifold valves and bleed states.

 

A variation on this theme is the five-valve manifold, shown in this illustration:

 

a five-valve manifold variation attached to a differential pressure transmitter

Manifold valve positions for normal operation and maintenance are as follows:

 

Manifold valve positions for normal operation and maintenance

It is critically important that the equalizing valve(s) never be open while both block valves are open! Doing so will allow process fluid to flow through the equalizing valve(s) from the high-pressure side of the process to the low-pressure side of the process. If the impulse tubes connecting the manifold to the process are intentionally filled with a fill fluid (such as glycerin, to displace process water from entering the impulse tubes; or water in a steam system), this fill fluid will be lost. Also, if the process fluid is dangerously hot or radioactive, a combination of open equalizing and block valves will let that dangerous fluid reach the transmitter and manifold, possibly causing damage or creating a personal hazard. Speaking from personal experience, I once made this mistake on a DP transmitter connected to a steam system, causing hot steam to flow through the manifold and overheat the equalizing valve so that it seized open and could not be shut again! The only way I was able to stop the flow of hot steam through the manifold was to locate and shut a sliding-gate hand valve between the impulse tube and the process pipe. Fortunately, this cast iron valve was not damaged by the heat and was still able to shut off the flow.

Pressure transmitter valve manifolds also come in single block-and-bleed configurations, for gauge pressure applications. Here, the “low” pressure port of the transmitter is vented to atmosphere, with only the “high” pressure port connected to the impulse line:

 

Single block-and-bleed configurations for gauge pressure applications.

The following photograph shows a bank of eight pressure transmitters, seven out of the eight being equipped with a single block-and-bleed manifold. The eighth transmitter (bottom row, second-from left) sports a 5-valve manifold:

 

A Bank of Pressure Transmitters

Bleed (vent) fittings

Before removing a pressure transmitter from live service, the technician must “bleed” or “vent” stored fluid pressure to atmosphere in order to achieve a zero energy state prior to disconnecting the transmitter from the impulse lines. Some valve manifolds provide a bleed valve for doing just this, but many do not1. An inexpensive and common accessory for pressure-sensing instruments (especially transmitters) is the bleed valve fitting or vent valve fitting, installed on the instrument as a discrete device. The most common bleed fitting is equipped with 1/4 inch male NPT pipe threads, for installation into one of the 1/4 inch NPT threaded pipe holes typically provided on pressure transmitter flanges. The bleed is operated with a small wrench, loosening a ball-tipped plug off its seat to allow process fluid to escape through a small vent hole in the side of the fitting. The following photographs show close-up views of a bleed fitting both assembled (left) and with the plug fully extracted from the fitting (right). The bleed hole may be clearly seen in both photographs:

 

Bleed Fittings and Hole of a Differential pressure transmitter.

 

When installed directly on the flanges of a pressure instrument, these bleed valves may be used to bleed unwanted fluids from the pressure chambers, for example bleeding air bubbles from an instrument intended to sense water pressure, or bleeding condensed water out of an instrument intended to sense compressed air pressure.

The following photographs show bleed fittings installed two different ways on the side of a pressure transmitter flange, one way to bleed gas out of a liquid process (located on top) and the other way to bleed liquid out of a gas process (located on bottom):

 
Bleed Fittings Installed in two different ways for gas and for liquid.

 

Pressure pulsation damping

A simple way to mitigate the effects of pulsation on a pressure gauge is to fill the inside of the gauge with a viscous liquid such as glycerin or oil. The inherent friction of this fill liquid has a “shock-absorber” quality which damps the gauge mechanism’s oscillatory motion and helps protect against damage from pulsations or from external vibration. This method is ineffectual for high-amplitude pulsations, though.

An oil-filled pressure gauge may be seen in the following photograph. Note the air bubble near the top of the gauge face, which is the only visual indication of an oil filling:

 
Oil Filled Pressure Gauge

A more sophisticated method for damping pulsations seen by a pressure instrument is called a snubber, and it consists of a fluid restriction placed between with the pressure sensor and the process. The simplest example of a snubber is a simple needle valve (an adjustable valve designed for low flow rates) placed in a mid-open position, restricting fluid flow in and out of a pressure gauge:


An Example of a Snubber is a simple needle valve.

At first, the placement of a throttling valve between the process and a pressure-measuring instrument seems rather strange, because there should not be any continuous flow in or out of the gauge for such a valve to throttle! However, a pulsing pressure causes a small amount of alternating flow in and out of the pressure instrument, owing to the expansion and contraction of the mechanical pressure-sensing element (bellows, diaphragm, or bourdon tube). The needle valve provides a restriction for this flow which, when combined with the fluid capacitance of the pressure instrument, combine to form a low-pass filter of sorts. By impeding the flow of fluid in and out of the pressure instrument, that instrument is prevented from “seeing” the high and low peaks of the pulsating pressure. Instead, the instrument registers a much steadier pressure over time. An electrical analogy for a pressure snubber is an RC low-pass filter circuit “damping” voltage pulsations from reaching a voltmeter:

 
An electrical analogy for a pressure snubber is an RC low-pass filter circuit

One potential problem with the needle valve solution is that the small orifice inside the valve may plug up over time with debris or deposits from dirty process fluid. This, of course, would be bad because if that valve were to ever completely plug, the pressure instrument would stop responding to any changes in process pressure at all, or perhaps just become too slow in responding to major changes.

A solution to this problem is to fill the pressure sensor mechanism with a clean liquid (called a fill fluid), then transfer pressure from the process fluid to the fill fluid (and then to the pressure-sensing element) using a slack diaphragm or some other membrane:

 

Slack diaphragm

In order for the fill fluid and isolating diaphragm to work effectively, there cannot be any gas bubbles in the fill fluid – it must be a “solid” hydraulic system from the diaphragm to the sensing element. The presence of gas bubbles means that the fill fluid is compressible, which means the isolating diaphragm may have to move more than necessary to transfer pressure to the instrument’s sensing element. This will introduce pressure measurement errors if the isolating diaphragm begins to tense from excessive motion (and thereby oppose some process fluid pressure from fully transferring to the fill fluid), or hit a “stop” point where it cannot move any further (thereby preventing any further transfer of pressure from process fluid to fill fluid)2. For this reason, isolating diaphragm systems for pressure instruments are usually “packed” with fill fluid at the point and time of manufacture then sealed in such a way that they cannot be opened for any form of maintenance. Consequently, any fill fluid leak in such a system immediately ruins it.

 

Remote and chemical seals

Isolating diaphragms have merit even in scenarios where pressure pulsations are not a problem. Consider the case of a food-processing system where we must remotely measure pressure inside a mixing vessel:

 

Without isolation, especially in food industry, there a very big chance of microbial growth due to stagnant food materials inside the impulse tubing.

The presence of the tube connecting the vessel to the pressure gauge poses a hygiene problem. Stagnant process fluid (in this case, some liquid food product) inside the tube can support microbial growth, which will eventually contaminate the vessel no matter how well or how often the vessel is cleaned. Even automated Clean-In-Place and Steam-In-Place (CIP and SIP, respectively) protocols where the vessel is chemically purged between batches cannot prevent this problem because the cleaning agents never purge the entire length of the tubing (ultimately, to the bourdon tube or other sensing element inside the gauge).

Here, we see a valid application of an isolating diaphragm and fill fluid. If we mount an isolating diaphragm to the vessel in such a way that the process fluid directly contacts the diaphragm, sealed fill fluid will be the only material inside the tubing carrying that pressure to the instrument. Furthermore, the isolating diaphragm will be directly exposed to the vessel interior, and therefore cleaned with every CIP cycle. Thus, the problem of microbial contamination is completely avoided:


the problem of microbial contamination is completely avoided.

Such systems are often referred to as remote seals, and they are available on a number of different pressure instruments including gauges, transmitters, and switches. If the purpose of an isolating diaphragm and fill fluid is to protect the sensitive instrument from corrosive or otherwise harsh chemicals, it is often referred to as a chemical seal.

The following photograph shows a pressure gauge equipped with a chemical seal diaphragm. Note that the chemical seal on this particular gauge is close-coupled to the gauge, since the only goal here is protection of the gauge from harsh process fluids, not the ability to remotely mount the gauge:

 
The chemical seal on this particular gauge is close-coupled to the gauge, since the only goal here is protection of the gauge from the harsh process fluids.

A view facing the bottom of the flange reveals the thin metal isolating diaphragm keeping process fluid from entering the gauge mechanism. Only inert fill fluid occupies the space between this diaphragm and the gauge’s bourdon tube:

 

Only inert fill fluid occupies the space between this diaphragm and the gauge’s bourdon tube

The only difference between this chemical-seal gauge and a remote-seal gauge is that a remote-seal gauge uses a length of very small-diameter tubing called capillary tubing to transfer fill fluid pressure from the sealing diaphragm to the gauge mechanism.

Direct-reading gauges are not the only type of pressure instrument that may benefit from having remote seals. Electronic pressure transmitters are also manufactured with remote seals for the same reasons: protection of the transmitter sensor from harsh process fluid, or prevention of “dead-end” tube lengths where organic process fluid would stagnate and harbor microbial growths. The following photograph shows a pressure transmitter equipped with a remote sealing diaphragm. The capillary tube is protected by a coiled metal (“armor”) sheath:


The capillary tube is protected by a coiled metal (“armor”) sheath

A close-up view of the sealing diaphragm shows its corrugated design, allowing the metal to easily flex and transfer pressure to the fill fluid within the capillary tubing3:

 

A close-up view of the sealing diaphragm shows its corrugated design, allowing the metal to easily flex and transfer pressure to the fill fluid within the capillary tubing

Just like the isolating diaphragms of the pressure-sensing capsule, these remote diaphragms need only transfer process fluid pressure to the fill fluid and (ultimately) to the taut sensing diaphragm inside the instrument. Therefore, these diaphragms perform their function best if they are designed to easily flex. This allows the taut sensing diaphragm to provide the vast majority of the opposing force to the fluid pressure, as though it were the only spring element in the fluid system.

The connection point between the capillary tube and the transmitter’s sensor capsule is labeled with a warning never to disassemble, since doing so would allow air to enter the filled system (or fill fluid to escape from the system) and thereby ruin its accuracy:


The connection point between the capillary tube and the transmitter’s sensor capsule is labeled with a warning never to disassemble, since doing so would allow air to enter the filled system (or fill fluid to escape from the system) and thereby ruin its accuracy

In order for a remote seal system to work, the hydraulic “connection” between the sealing diaphragm and the pressure-sensing element must be completely gas-free so there will be a “solid” transfer of motion from one end to the other.

A potential problem with using remote diaphragms is the hydrostatic pressure generated by the fill fluid if the pressure instrument is located far away (vertically) from the process connection point. For example, a pressure gauge located far below the vessel it connects to will register a greater pressure than what is actually inside the vessel, because the vessel’s pressure adds to the hydrostatic pressure caused by the liquid in the tubing:


pressure gauge located far below the vessel

This pressure may be calculated by the formula ρgh or γh where ρ is the mass density of the fill liquid or γ is the weight density of the fill liquid. For example, a 12 foot capillary tube height filled with a fill liquid having a weight density of 58.3 lb/ft3 will generate an elevation pressure of almost 700 lb/ft2, or 4.86 PSI. If the pressure instrument is located below the process connection point, this 4.86 PSI offset must be incorporated into the instrument’s calibration range. If we desire this pressure instrument to accurately measure a process pressure range of 0 to 50 PSI, we would have to calibrate it for an actual range of 4.86 to 54.86 PSI.

The reverse problem exists where the pressure instrument is located higher than the process  connection: here the instrument will register a lower pressure than what is actually inside the vessel, offset by the amount predicted by the hydrostatic pressure formulae ρgh or γh. In all fairness, this problem is not limited to remote seal systems – even non-isolated systems where the tubing is filled with process liquid will exhibit this offset error. However, in filled-capillary systems a vertical offset is guaranteed to produce a pressure offset because fill fluids are always liquid, and liquids generate pressure in direct proportion to the vertical height of the liquid column (and to the density of that liquid).

A similar problem unique to isolated-fill pressure instruments is measurement error caused by temperature extremes. Suppose the liquid-filled capillary tube of a remote seal pressure instrument comes too near a hot steam pipe, furnace, or some other source of high temperature. The expansion of the fill fluid may cause the isolation diaphragm to extend to the point where it begins to tense and add a pressure to the fill fluid above and beyond that of the process fluid. Cold temperatures may wreak havoc with filled capillary tubes as well, if the fill fluid congeals or even freezes such that it no longer flows as it should.

Proper mounting of the instrument and proper selection of the fill fluid4 will help to avoid such problems. All in all, the potential for trouble with remote- and chemical-seal pressure instruments is greatly offset by their benefits in the right applications.

Some pressure transmitters are equipped with close-coupled seals rather than remote seals, for applications where it is best to avoid impulse tube connections to the process (e.g. liquids that tend to coagulate or in hygienic processes). A Rosemount extended-diaphragm pressure transmitter appears in the left-hand photograph, while a Yokogawa transmitter of the same basic design is shown installed in a working process in the right-hand photograph:

 

Rosemount extended-diaphragm pressure transmitter and a Yokogawa transmitter

 

Filled impulse lines

An alternate method for isolating a pressure-sensing instrument from direct contact with process fluid is to either fill or purge the impulse lines with a harmless fluid. Filling impulse tubes with a static fluid works when gravity is able to keep the fill fluid in place, such as in this example of a pressure transmitter connected to a water pipe by a glycerin-filled impulse line:

 
filled impulse line

A reason someone might do this is for freeze protection, since glycerin freezes at a lower temperature than water. If the impulse line were filled with process water, it might freeze solid in cold weather conditions (the water in the pipe cannot freeze so long as it is forced to flow). The greater density of glycerin keeps it placed in the impulse line, below the process water line. A fill valve is provided near the transmitter so a technician may re-fill the impulse line with glycerin (using a hand pump) if ever needed.

As with a remote diaphragm, a filled impulse line will generate its own pressure proportional to the height difference between the point of process connection and the pressure-sensing element. If the height difference is substantial, the pressure offset resulting from this difference in elevation will require compensation by means of an intentional “zero shift” of the pressure instrument when it is calibrated.

 

Purged impulse lines

Continuous purge of an impulse line is an option when the line is prone to plugging. Consider this example, where pressure is measured at the bottom of a sedimentation vessel:

 

purged impulse lines

A continuous flow of clean water enters through a “purge valve” and flows through the impulse line, keeping it clear of sediment while still allowing the pressure instrument to sense pressure at the bottom of the vessel. A check valve guards against reverse flow through the purge line, in case process fluid pressure ever exceeds purge supply pressure. Purged systems are very useful, but a few details are necessary to consider before deciding to implement such a strategy:

   • How reliable is the supply of purge fluid? If this stops for any reason, the impulse line may plug!

   • Is the purge fluid supply pressure guaranteed to exceed the process pressure at all times, for proper direction of purge flow?

   • What options exist for purge fluids that will not adversely react with the process?

   • What options exist for purge fluids that will not contaminate the process?

   • How expensive will it be to maintain this constant flow of purge fluid into the process?

Also, it is important to limit the flow of purge fluid to a rate that will not create a falsely high pressure measurement due to restrictive pressure drop across the length of the impulse line, yet flow freely enough to achieve the goal of plug prevention. In many installations, a visual flow indicator is installed in the purge line to facilitate optimum purge flow adjustment. Such flow indicators are also helpful for troubleshooting, as they will indicate if anything happens to stop the purge flow. In the previous example, the purge fluid was clean water. Many options exist for purge fluids other than water, though. Gases such as air, nitrogen, or carbon dioxide are often used in purged systems, for both gas and liquid process applications.

Purged impulse lines, just like filled lines and diaphragm-isolated lines, will generate hydrostatic pressure with vertical height. If the purge fluid is a liquid, this elevation-dependent pressure may be an offset to include in the instrument’s calibration. If the purge fluid is a gas (such as air), however, any height difference may be ignored because the density of the gas is negligible.

 

Heat-traced impulse lines

If impulse lines are filled with liquid, there may exist a possibility for that liquid to freeze in cold weather conditions. This possibility depends, of course, on the type of liquid filling the impulse lines and how cold the weather gets in that geographic location.

One safeguard against impulse line freezing is to trace the impulse lines with some form of active heating medium, steam and electrical being the most common. “Steam tracing” consists of a copper tube carrying low-pressure steam, bundled alongside one or more impulse tubes, enclosed in a thermally insulating jacket.


heat-traced impulse lines

Steam flows through the shut-off valve, through the tube in the insulated bundle, transferring heat to the impulse tube as it flows past. Cooled steam condenses into water and collects in the steam trap device located at the lowest elevation on the steam trace line. When the water level builds up to a certain level inside the trap, a float-operated valve opens to vent the water. This allows more steam to flow into the tracing tube, keeping the impulse line continually heated. The steam trap naturally acts as a sort of thermostat as well, even though it only senses condensed water level and not temperature. The rate at which steam condenses into water depends on how cold the impulse tube is. The colder the impulse tube (caused by colder ambient conditions), the more heat energy drawn from the steam, and consequently the faster condensation rate of steam into water. This means water will accumulate faster in the steam trap, which means it will “blow down” more often. More frequent blow-down events means a greater flow rate of steam into the tracing tube, which adds more heat to the tubing bundle and raises its temperature. Thus, the system is naturally regulating, with its own negative feedback loop to maintain bundle temperature at a relatively stable point5.

The following photograph shows a picture of a steam trap:

 
steam trap

Steam traps are not infallible, being susceptible to freezing (in very cold weather) and sticking open (wasting steam by venting it directly to atmosphere). However, they are generally reliable devices, capable of adding tremendous amounts of heat to impulse tubing for protection against freezing.

Electrically traced impulse lines are an alternative solution for cold-weather problems. The “tracing” used is a twin-wire cable (sometimes called heat tape) that acts as a resistive heater. When power is applied, the cable heats up, thus imparting thermal energy to the impulse tubing it is bundled with. This next photograph shows the end of a section of electrical heat tape, rated at 33 watts per meter (10 watts per foot) at 10 degrees Celsius (50 degrees Fahrenheit):

 
this electrical heat tape is rated at 33 watts per meter at 10 degrees Celsius

This particular heat tape also has a maximum current rating of 20 amps (at 120 volts). Since heat tape is really just a continuous parallel circuit, longer lengths of it draw greater current. This maximum total current rating therefore places a limit on the usable length of the tape. Heat tape may be self-regulating, or controlled with an external thermostat. Self-regulating heat tape exhibits an electrical resistance that varies with temperature, automatically self-regulating its own temperature without the need for external controls.

Both steam and electrical heat tracing are used to protect instruments themselves from cold weather freezing, not just the impulse lines. In these applications it is important to remember that only the liquid-filled portions of the instrument need freeze protection, not the electronics portions!

 

Water traps and pigtail siphons

Many industrial processes utilize high-pressure steam for direct heating, performing mechanical work, combustion control, and as a chemical reactant. Measuring the pressure of steam is important both for its end-point use and its generation (in a boiler). One problem with doing this is the relatively high temperature of steam at the pressures common in industry, which can cause damage to the sensing element of a pressure instrument if directly connected.

A simple yet effective solution to this problem is to intentionally create a “low” spot in the impulse line where condensed steam (water) will accumulate and act as a liquid barrier to prevent hot steam from reaching the pressure instrument. The principle is much the same as a plumber’strap used underneath sinks, creating a liquid seal to prevent noxious gases from entering a home from the sewer system. A loop of tube or pipe called a pigtail siphon achieves the same purpose:

 

water traps and pigtail siphons

The following photograph shows a pigtail siphon connected to a pressure gauge sensing pressure on a steam line:

 

the photograph shows a pigtail siphon connected to a pressure gauge sensing pressure on a steam line

 

Mounting brackets

An accessory specifically designed for a variety of field-mounted instruments including DP transmitters is the 2 inch pipe mounting bracket. Such a bracket is manufactured from heavy gauge sheet metal and equipped with a U-bolt designed to clamp around any 2 inch black iron pipe. Holes stamped in the bracket match mounting bolts on the capsule flanges of most common DP transmitters, providing a mechanically stable means of attaching a DP transmitter to a framework in a process area.

The following photographs show several different instruments mounted to pipe sections using these brackets:

 
these photographs show several different instruments mounted to pipe sections using these brackets

 

Heated enclosures

In installations where the ambient temperature may become very cold, a protective measure against fluid freezing inside a pressure transmitter is to house the transmitter in an insulated, heated enclosure. The next photograph shows just such an enclosure with the cover removed:

 
an enclosure with the cover removed

 

Not surprisingly, this installation works well to protect all kinds of temperature-sensitive instruments from extreme cold. Here, we see an explosive gas sensor mounted inside a slightly different style of insulated enclosure, with the lid opened up for inspection:

 
an explosive gas sensor mounted inside a slightly different style of insulated enclosure, with the lid opened up for inspection
 
 
 

1The standard 3-valve manifold, for instance, does not provide a bleed valve – only block and equalizing valves.

2This concept will be immediately familiar to anyone who has ever had to “bleed” air bubbles out of an automobile brake system. With air bubbles in the system, the brake pedal has a “spongy” feel when depressed, and much pedal motion is required to achieve adequate braking force. After bleeding all air out of the brake fluid tubes, the pedal motion feels much more “solid” than before, with minimal motion required to achieve adequate braking force. Imagine the brake pedal being the isolating diaphragm, and the brake pads being the pressure sensing element inside the instrument. If enough gas bubbles exist in the tubes, the brake pedal might stop against the floor when fully pressed, preventing full force from ever reaching the brake pads! Likewise, if the isolating diaphragm hits a hard motion limit due to gas bubbles in the fill fluid, the sensing element will not experience full process pressure!

3Like all instrument diaphragms, this one is sensitive to damage from contact with sharp objects. If the diaphragm ever becomes nicked, dented, or creased, it will tend to exhibit hysteresis in its motion, causing calibration errors for the instrument. For this reason, isolating diaphragms are often protected from contact by a plastic plug when the instrument is shipped from the manufacturer. This plug must be removed from the instrument before placing it into service.

4Most pressure instrument manufacturers offer a range of fill fluids for different applications. Not only is temperature a consideration in the selection of the right fill fluid, but also potential contamination of or reaction with the process if the isolating diaphragm ever suffers a leak!

5In fact, after you become accustomed to the regular “popping” and “hissing” sounds of steam traps blowing down, you can interpret the blow-down frequency as a crude ambient temperature thermometer! Steam traps seldom blow down during warm weather, but their “popping” is much more regular (one every minute or less) when ambient temperatures drop well below the freezing point of water.

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Comments (1)Add Comment
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question
written by tp, September 20, 2013
can we use diaphragm seals and valve manifolds together? what will be the lines? tubing or capillary?

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