Pm-90 Maintenance







The procedures detailed below encompass all user services that require removal of the pump heads. They include:


  • Pump head replacement
  • Plunger seal replacement
  • Plunger free-play adjustment
  • Plunger replacement
  • Plunger seal break-in

This section will detail the entire procedure, from head removal to re-insertion. The text will indicate which sections can be skipped when performing simple maintenance.

Pump Head Removal

An entire pump head may be rapidly removed by the following procedure:


  1. Observe the pressure display to make sure there is no residual pressure in the lines.


  2. Open the purge plug. Press the PURGE key and purge the pump for 5 seconds. Then press the STOP key to stop the pump. This step moves the plungers to the full forward position for removal of the pump heads.


  3. Turn the POWER off.


  4. Remove the outlet line from the head using a 1/4" wrench.


  5. Hold the inlet check-valve assembly stationary with a 1/2" wrench and remove the inlet line with a 1/4" wrench.


  6. Using a 9/64" hex head driver, unscrew two 8-32 socket head cap screws from the pump head.


  7. Rotate the head 30° counterclockwise until it stops, then gently pull the head straight out of the housing.

Pump Head Disassembly

Servicing or inspection of the plunger seal, plunger spring, and plunger require disassembly of the pump head. Proceed as follows:


  1. Gently pull the plunger assembly straight back from the Head pulling the plunger out of the Head.


  2. Locate the two 8-32 socket head cap screws on the back of the Head and remove using a 9/64" hex head driver. Pull the Backing Disk away from the head to expose the plunger seal.

Plunger Inspection and Servicing


  1. Examine the plunger carefully. Wash the sapphire rod with water and methanol. Salts may be removed by gently scrubbing with a green Scotchbrite® scouring pad moistened with water. Replace the plunger if it is chipped, scored or scratched (if it needs to be replaced, proceed to item 4).


  2. Test the plunger free-play. Hold the plunger assembly in one hand with the sapphire rod pointing up. Wiggle the sapphire rod from side to side with the other hand. It should move freely. Rotate it radially; it should move freely through 360°, much as you would roll your head on your shoulders to stretch your neck muscles. Now try to move it up and down; there should be no movement in the longitudinal direction.


  3. If the rod passes the free-play test, proceed to the next section (seal replacement). If the sapphire rod is either too loose or too tight, perform the adjustment procedure beginning in item 6.


  4. To replace the sapphire plunger, grasp the Plunger Mount firmly and unscrew the set screw from the back of the Plunger Mount. Remove the plunger by sliding it out of the back of the Plunger Mount and replace with a new one. A small ball bearing sits at the base of the plunger and is held in place with a dab of grease. Do not lose the ball during assembly.


  5. A nylon patch locks the reusable set screw in position once the position is set.


  6. To adjust the new or old sapphire plunger, tighten or loosen the set screw while testing free play.


  7. Wiggle the sapphire plunger from side to side; it should move freely. Rotate it radially;it should move freely through 360°, much as you would roll your head on your shoulders to stretch your neck muscles. Now try to move it up and down; there should be no movement in the longitudinal direction.

Plunger Seal Replacement


  1. Place the front portion of the pump head on a flat surface with the seal facing up.


  2. Remove the seal by inserting a 6-32 threaded screw into the seal material several turns, then pulling out the seal. Be careful not to scratch the metal of the pump head with the screw.


  3. Wash the pump head with water. Use a squeeze bottle to flush liquid through the check valves and all exposed ports.


  4. Irrigate the pump head and new seal (part number MR- 4598) with methanol, and push the seal fully into its seat with your thumb. Do not scratch the seal with a fingernail.


  5. Irrigate the Backing Disk if salt deposits have collected inside the irrigation ports.


  6. Proceed with pump head reassembly. Be sure to follow the break-in procedure for new plunger seals (see below) after the pump is reassembled.

Pump Head Assembly


  1. Wash all parts with water.


  2. Place the front part of the pump head face-down on a flat surface. Irrigate the plunger seal with methanol.


  3. The backing disk has an L-shaped slot in its side for locking the head assembly to the pump housing. Orient the backing disk so the L-shaped slot is opposite the check valve assembly on the head. Insert the backing disk on the back of the head and screw down with two 8-32 socket head cap screws in the counterbored holes.


  4. Irrigate the plunger-seal area and the sapphire rod with methanol.



  5. Align the guide rods on the plunger assembly with the two holes in the backing disk and insert the plunger assembly with the sapphire plunger entering the plunger seal.


  6. Insert the plunger assembly until it is about 1/4" away from the Backing Disk. Do not press the plunger assembly all the way down against the backing disk or the head cannot be reinstalled into the pump drive.

Pump Head Installation

To properly engage the plunger to the plunger drive mechanism, the plunger drive mechanism must be fully forward. This procedure assures that the drive mechanism is properly positioned for head installation or removal.


  1. Turn Power on the pump.


  2. Press the PURGE key.


  3. Press the RUN key. Purge for 5 seconds.


  4. Press the STOP key. By stopping the pump following purging, the plunger drive mechanism(s) will automatically be moved to the full forward position for installation of the pump head.



  5. With the pump head inlet check valve facing down, align the slot in the plunger assembly with the pin in the housing and slide the head assembly straight into the housing. As the head plunger mount contacts the plunger drive in the pump, the resistance to insertion will increase as the plunger slides within the seal. Continue to slide the head forward until the head backing disk rests firmly against the face of the housing.


  6. Rotate the head clockwise 30° until the check valve is vertically down and the head mounting holes align with the threaded holes in the housing. This engages the plunger mount to the plunger drive.

    Note: If the plunger assembly is closer than 1/4" to the Backing Disk, the head will be locked out from rotating into place. Remove the head and pull the plunger assembly away from the Backing Disk about 1/4" and repeat steps 5 and 6.


  7. Tighten the head to the housing with two 8-32 socket head cap screws.


  8. Hold the inlet check valve assembly stationary with a 1/2" wrench and attach the inlet line with a 1/4" wrench.


  9. Attach the fitting on the pressure manifold line to the front of the head(s).


  10. Follow the STARTUP procedure. It is very important to:


    1. ensure that the plunger seals are moist at startup
    2. purge all air from the system


  11. If new plunger seals were installed, follow the break-in procedure below.


Plunger seals must seat properly for longest life. A salt-free solvent is recommended for break-in. Proceed as follows after installing new seals:


  1. Wet the irrigation ports in the pump heads with a few drops of water.


  2. Purge the system with filtered 40:60 (v:v) acetonitrile:water, as described in the STARTUP section.


  3. Attach a column to the system. It need not be a good column, as its only purpose is to provide backpressure.


  4. Run the pump for two hours at a pressure of 3000-3500 PSI. Adjust the flow rate as necessary to achieve this pressure.


  5. After two hours you may switch to mobile phase and begin chromatography.





The PM-90 pumps use cartridge-type check valves (part number MR-4650) that do not contain removable parts. The same valve is used for both the inlet and outlet check valves on the pump. Each check-valve cartridge contains two precision-engineered balls and seats in series, for a reliable seal. You can determine the direction of flow through the cartridge by attempting to squeeze water or methanol through it in each direction. An arrow on the side of the cartridge also indicates flow direction.When installing, remember that flow is always "up" through the pump.

In most cases, faulty check-valve performance (as indicated by large pressure fluctuations) is due to microscopic debris, salt precipitation, or trapped gas pockets. Gas can be removed by purging with freshly degassed mobile phase or solvent, as described in the STARTUP section. The check valves may be cleaned by sonication (see below), or (if necessary) replaced. When cleaning or replacing these cartridges, remember that any lint from towels, paper wipes, etc. may reintroduce the problem. The best approach is to flush the check valves with methanol and to reassemble them wet.

An understanding of flow through the pump and check valves is helpful in diagnosing check-valve problems. Flow during a full cycle of two pump strokes for the PM-92 pump is as follows:

When the left pump head is compressing, the inlet check valve below it closes, preventing any backflow. The outlet check valve above it opens, allowing mobile phase to proceed towards the system pressure transducer in the center of the pressure manifold, then out of the pump.

While this is occurring, the right pump head is aspirating. It's inlet check valve is open, allowing mobile phase to enter from the solvent reservoir. The outlet check valve is closed, which prevents any backflow into the right pump head from the left head and manifold. The LCD displays an "L" during this part of the pump cycle.

After the left pump head finishes its stroke, the roles of the two pump heads reverse. The right pump head begins compressing. The inlet check valve closes, and the outlet check valve opens. The left pump head begins aspirating; its inlet check valve opens and its outlet check valve closes. The LCD displays an "R" during this part of the pump cycle.

Pump-related malfunctions that produce pressure fluctuations can be localized by observing the pump stroke and pressure readout. When the display shows "L" while pumping, the left head is compressing. It's inlet check valve must shut, and it's outlet check valve must open. In addition, because the pressure manifold chamber is open to both pump heads, the outlet check valve of the right pump head must close. If it didn't, fluid would flow from the left pump head into the manifold, then down into the right pump head. The opposite argument applies to the right pump head.

Because of this relationship, there are three likely places to check when pressure drops as one pump head compresses: the pump head itself (plunger seal, plunger), the inlet check valve for that pump head, and the outlet check valve for the opposite head.

Check Valve Removal


  1. Stop the pump and allow pressure to dissipate. Then turn off the POWER.


  2. Hold the check-valve assembly stationary with a wrench and remove the inlet or outlet line with another wrench.


  3. Remove the check-valve assembly, which contains the check-valve cartridge.

Check Valve Cleaning


  1. Place the entire check-valve assembly in a solution of laboratory detergent warmed to 50 °C. We recommend a 50% solution of RBS-35® (Pierce Chemical Company). Soak for 1-2 hours. (Do not warm the solution above 60 °C, because plastic parts in the check valves may become distorted.)


  2. Sonicate the check-valve assembly for 15 minutes in the detergent solution.


  3. Flush with deionized water, then methanol.


  4. Reinstall the check-valve assembly following the instructions below. If check-valve problems continue, install a new cartridge.

Check-valve Cartridge Replacement


  1. Remove check-valve assembly from the pump head.


  2. If the cartridge does not shake out of the assembly, push it out with a paper clip. This may require some force. Alternatively, you may be able to insert a small screw into the cartridge and pull it out of the assembly. Discard the old cartridge, as it is now damaged.


  3. Clean the new cartridge, as above.


  4. Insert the new cartridge. Be certain to install the cartridge so it allows flow in the proper direction. Flow is always up through the pump; inlet check valves permit flow from the inlet line to the pump head, outlet check valves permit flow from the pump head to the outlet line. The cartridges may have an arrow to indicate flow direction. If not, the end with the small hole is the inlet, and the end with either several holes or a large hole is the outlet.

Check Valve Installation


  1. Squirt some methanol through the check valve. This will both wet it and allow you to observe whether the direction of flow is correct.


  2. Install the check-valve assembly onto the pump head with a wrench.


  3. Hold the check-valve assembly stationary with the wrench and connect the inlet or outlet tubing with a second wrench.


  4. Follow the STARTUP procedure to prime the pump and remove all air from the check valves and lines.





There are three pressure transducers located in the pressure manifold of the PM-92 pump. Two pressure transducers are located in the manifold of the PM-91 pump. They should rarely need attention. If a leak ever develops, there is a PTFE o-ring on the pressure transducer that needs to be replaced (Part Number MR-1254). Follow these steps:


  1. Stop the pump and allow system pressure to fall to 30 PSI or less. Turn the POWER off.


  2. Using a 1/2" wrench to hold the outlet check valve, remove the fitting attached to the outlet check valve using a 1/4" wrench. Remove the fittings from the other outlet check valve.


  3. Using a 1/4" wrench, remove all the fittings from the pressure manifold.


  4. Remove the two 8-32 socket head cap screws which mount the pressure manifold to the front panel.


  5. Pull the pressure manifold straight back. The pressure transducers are mounted to the back of the pressure manifold.


  6. Use a 3/8" wrench on the flat of the leaking pressure transducer to unscrew the transducer from the pressure manifold. Pull the transducer straight back from the manifold.


  7. Remove the o-ring from the transducer and replace with a new o-ring (Part Number MR-1254).


  8. Before screwing the pressure transducer back into the pressure manifold, twist the pressure transducer four turns counter-clockwise so that when the transducer is screwed into the manifold, the transducer cable is unwound and is left untwisted when the transducer is mounted tightly to the pressure manifold.


  9. Screw the pressure transducer into the manifold finger tight, so the o-ring is squeezed against the back of the pressure manifold. Using a 3/8" wrench, tighten the transducer another 3/4 turn.


  10. Mount the pressure manifold to the front panel with two 8-32 socket head cap screws.


  11. Reconnect all fittings to the same ports they were connected to previously.


  12. Purge the pump as described in the STARTUP section.





Check all fittings for leaks every day. Large leaks will be obvious. Small leaks will be apparent only by the accumulation of salt deposits around the fitting. It is not normal for fittings to have even small leaks, and these should be tended to.

The first approach to treating a small leak is to clean up the salts with water and tighten the nut slightly. About 1/8 turn should do it, unless the nut is loose. If this doesn't stop the leak, it's best to replace the fitting and line entirely. Why?

It's certainly tempting to use a lot of force to stop a small leak. But several bad things can happen when you overtighten a fitting:


  • The nut may break off with its threads still in the hole. A machinist will have to get it out for you, and if you're very lucky the part won't be damaged.


  • The nut may fuse in place, and break off the next time you try to open it.


  • The tubing end may become crimped, restricting flow and causing performance problems.

It makes more sense to replace a fitting or line before any of these events occur. This figure gives part numbers and ferrule types for all tubing used in the PM-90 pump:




The in-line filter should be suspected whenever system pressure rises above normal. The filter becomes clogged by doing its job: preventing particles from the mobile phase or pump seals from lodging in the injector or on the column. To replace the filter frit do the following:


  1. Stop the pump and allow pressure to drop to zero.


  2. Remove the inlet and outlet tubing to the filter assembly. Be sure to note the flow direction.


  3. Loosen the two halves of the filter assembly with two wrenches, but do not separate them yet.


  4. Hold the assembly vertically, with the smaller section on the bottom. Now unscrew the larger section and remove.


  5. The two frit supports and the frit can now be removed from the smaller section. Keep the supports separate so they can be reassembled in the same order: they seal best when installed in the section from which they came.


  6. Flush water through each support to ensure that its passageway is not clogged.


  7. Place the appropriate support on the smaller section, then put a new filter frit (MR-4136) on top of it. Put on the remaining support, then screw on the larger section. Tighten with two wrenches.


  8. Attach the inlet line from the pump to the assembly, observing proper flow direction. Pump some mobile phase through the unit to remove air, then attach the outlet tubing.





The fan filter should be removed for cleaning at least once per year (more often if it is visibly dirty.) Proceed as follows:


  1. Gently pry off the retaining grid with a screwdriver. Do not unscrew the mounting screws!


  2. Carefully pull out the filter. You may either vacuum it clean, or wash it gently in warm sudsy water. Be careful not to tear the filter.


  3. If you've washed the filter, blot it well between sheets of paper towels, then allow it to dry.


  4. Reinstall the filter by holding it in place over the fan opening, then snapping the retaining grid in place. Position the grid with its ridges facing in towards the filter.





Regular maintenance will keep pump performance up to specifications. We recommend:


Every Day Inspect all fittings in the flow path for leaks.
Every Three Months Replace plunger seals and inspect interior of pump head.
Every Year Clean the fan filter.