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General Pump

Laboratory and industrial processes often require metering precise amounts of fluids in a controlled and reproducible manner. Optos pumps were developed in response to this need. These pumps can deliver measured amounts of fluids against significant back pressure.

Optos pumps attain precise metering through a positive displacement, reciprocating piston. A stepper motor drives the piston via an eccentric. Advanced software algorithms control motor speed based on piston position, achieving rapid fluid intake and smooth, constant fluid delivery while minimizing pulsation. The optional damper can further reduce pulsation.

Ball and seat inlet and outlet valves ensure precise metering. The suction created by the piston retracting to refill the piston chamber pulls the inlet ball off the inlet seat, allowing fluid to travel through the inlet check valve. Simultaneously, the outlet ball is pulled back onto its’ seat, preventing fluid from exiting the piston chamber. During the delivery portion of the piston’s duty cycle, the inlet ball is pushed back onto its’ seat as the piston moves forward, preventing fluid from traveling back to the pump reservoir. Simultaneously, the outlet ball is forced off its’ seat, allowing fluid to exit the outlet valve. For the valves to operate effectively, a pressure differential of at least 25 psi is required on the outlet side of the pump.

The pump’s flow rate is set directly in mL/min. using the keypad or remote control. Flow rates are determined by piston displacement (diameter and stroke length) and motor speed.

For the best pump efficiency, we recommend selecting a pump that typically runs in the upper half of its flow rate range. For the valves to operate effectively, a pressure differential of at least 25 psi is required on the outlet side of the pump.

  Low Flow Liquid End Standard Flow Liquid End High Flow Liquid End
Model 1:

Flow rates from .002 to 10 mL/min.

Optimal at lower rates, e.g., 1-20 mL/min.


Flow:  .002-2.5 mL/min.

@ 6000 psi


Flow:  .003-5 mL/min.

@ 6000 psi


Flow:  .01-20 mL/min

@ 3000 psi

Model 2:

Flow rates from .003 to 20 mL/min.

Optimal at medium rates, e.g., 2.5-40 mL/min.


Flow:  .003-5 mL/min

@ 6000 psi


Flow:  .01-10 mL/min

@ 6000 psi


Flow:  .02-40 mL/min

@ 1500 psi

Model 3:

Flow rates from .01 to 80 mL/min.

Optimal at higher rates, e.g., 5-80 mL/min.


Flow:  .01-10 mL/min

@ 3000 psi


Flow:  .01-20 mL/min

@ 1500 psi


Flow:  .04-80 mL/min

@ 750 psi

Eldex pumps are designed to operate under ambient temperature conditions. Do not use the pump in an environment that exceeds 35°C. It is possible to pump fluids at elevated temperatures. Although the reaction of specific fluids at elevated temperatures to the wetted parts may vary, it is usually the plastic materials in the liquid end that will be of concern. Generally, the CTFE maintains integrity at temperatures up to 200°C. The PTFE piston seal material is generally good to 260°C.

Our pumps can move fluids with viscosity measurements up to 500 centipoise (something between SAE 30 oil and glycerin), though at viscosities above 100 centipoise (something like SAE 10), the flow rate decreases; as the viscosity rises above 100 centipoise, flow decreases in a fairly linear manner until there is no flow at 500 centipoise.

Viscosity Limits are 500 Centipoise. 500cP is a theoretical maximum viscosity that has been demonstrated in lab conditions; however, pump efficiency at this viscosity is very poor. We recommend using our pumps for applications without particulates and with aqueous, water-like viscosity. When dealing with viscous liquids, we suggest adjusting the pump refill rate from “full out” to 70/30 or 50/50.

Our pump does well with clean fluids (minimal particulates) and viscosities below 500cP (practically 100cP) for a broad range of chemical compatibility.  We offer research/pilot scale precision flow rates up to 80mL/min and pressures up to 6000psi.  Some of our customers use the pump for hydrocarbon catalyst research, petrochem pilot plants, small-scale reactors, corrosion testing, hydraulic oil, and water/solvent injection.

Our Optos and HP series pumps are reciprocating piston style, which means the pump fluid chamber is completely static. Therefore, at a constant stroke, the same amount of fluid is delivered to each pump—this means reproducibility is very high by design.

Our Optos Series pump is a single piston pump, so there is some pressure fluctuation as the pump transitions from the pump part of the piston stroke to the refill part of the piston stroke. This can be controlled with a pulsation damper/pressure transducer.

The Optos Plus models include an integrated pulse damper for smooth flow, pressure monitoring, and the ability to set high and low pressure limits.

Our pumps are designed for constant flow rates, not constant pressure.  Pressure is a byproduct of the flow, i.e., higher flow with the same outlet restriction results in higher pressure.

Pressure is fluid delivery resistance (on the pump’s output side). The pump does not generate any pressure but can move fluid despite the resistance to fluid movement generated by something else (an HPLC column, tubing, etc.).

The accuracy requirement is different than reproducibility (precision). The accuracy of our pumps (measured flow rate vs. pump set point flow rate) is affected by check valve function, piston seal function, fluid viscosity, refill rate, system pressure, and other mechanical variables that impact performance.

It is fair to say our pumps have extremely high reproducibility, even at low flow rates. However, pump accuracy is more of a problem when the flow rate design levels are below optimal (under 1mL/min). A syringe-style pump might be a mechanically superior alternative for micro and nanofluidic rates.

Our pumps do not work well with slurries.  This is due to our check valve design, where particulates can interfere with the ball and seat sealing, leading to an inaccurate flow rate. A diaphragm pump may be more suitable for this application.

Refill rate refers to the speed of the piston retraction during the pump refill stroke.  Full Out refers to a maximum speed refill designed to minimize pulsation.  With DI water in particular, this rapid refill can actually pull gas out of the fluid, leading to bubbles in the plumbing and pump cavitation.  Adjusting the refill rate, which refers to a % of the time in pumping stroke/refill stroke, the rapid refill can be slowed down – hopefully preventing this degassing.  i.e., 50/50 refers to balanced pump cycle times – 50% pump stroke, 50% refill.

Optos pumps fitted with a pulse damper can set high/low-pressure limits. When the high/low-pressure limit is violated, the pump stops running. If Optos pumps are not fitted with the optional damper or if the pressure limits are deactivated, the motor will stall when exposed to excessive pressure. When the stall stop field is set to ON, the pump will stop when a stall condition is noted.

Our ball and seat inlet and outlet check valves ensure precise metering. We now offer integrated one-piece check valves for stainless steel and ceramic options, with seals embedded into the stainless-steel check cartridge housing.  The new items are PN#6280 for standard stainless steel and PN#6282 for ceramic.  A key advantage is fewer loose pieces to roll onto the floor!

Our Optos series is designed for the interchangeability of liquid ends, with piston sizes 3/32”, 1/8”, and 1/4”. A liquid end and piston are needed.  The customer must change the piston size setting on the pump menu.

We do not offer fittings for ¼” NPT connection, but there are adapters available, such as below.

The compressibility compensation factor depends on the fluid. As there are too many fluids, this can be looked up online (natural gas has a 0.5% correction in volume per 100psi under normal pressure and temperature conditions). Various fluids have different compressibilities at different pressures. Our pumps allow you to adjust for compressibility by setting the compressibility compensation factor. The default setting is 4. The maximum setting is 60. Each increment increases motor speed by approximately 1%.

We don’t recommend doing a dry run unless it is for a very short time, as it may lead to piston seal wear. If you need to run the pump for more than a few minutes to check the system, we recommend using IPA.


Some Optos pumps may be fitted with a pulse damper. The damper is a diaphragm-style damper, where a diaphragm made of an inert material flexes against a compressible fluid (spiked with red dye), thereby absorbing pulses in fluid delivery. The system should operate against at least 500 psi to maximize the damper’s efficiency. If your system does not normally generate such pressures and you wish to increase the efficiency of the damper, you can install a backpressure device or some narrow ID tubing downstream of the pump.

Our damper design was optimized for HPLC applications, with maximum dampening capabilities at below 10mL/min and absolute minimal dwell volume. In standard configuration, at flow rates >10mL/min, the damper diaphragm is at risk of damage. We offer a damper for higher flow rates, requiring a special “T-bracket” adapter.

Pulsation dampers stabilize the flow and pressure in circuits with volumetric or dosing pumps. They are used in a wide range of applications.

A pulsation damper is a device used in fluid systems to reduce pressure fluctuations or pulsations caused by reciprocating pumps, compressors, or other pulsating flow sources. It is designed to absorb or dampen the rapid pressure fluctuations, providing a more stable and uniform flow.

Our Optos Series pump is a single-piston pump, so there is some pressure fluctuation as the pump transitions from the pump part of the piston stroke to the refill part of the piston stroke. This can be controlled with a pulsation damper/pressure transducer. The system should operate against at least 500 psi to maximize the damper’s efficiency.

The basic design of a pulsation damper typically consists of a pressure vessel with an elastomeric or gas-filled bladder inside. When the pulsating flow enters the damper, the bladder compresses and expands, absorbing the excess pressure during high-pressure peaks and releasing it during low-pressure troughs. This action helps to dampen the pressure fluctuations and maintain a more constant flow downstream.

Metering pumps, also known as dosing pumps, are designed to pump a precise fluid volume in a controlled fashion. They typically operate in a reciprocating manner, which can produce a pulsating flow. Pulsation dampers can significantly improve the efficiency and reliability of the pumps.

  • Smooth Flow: Metering pumps are precision devices designed to deliver a consistent amount of fluid. Without a pulsation damper, the pump’s natural reciprocating action can create pulsations in the flow, leading to inconsistencies in the delivered volume. By dampening these pulsations, the damper ensures that the metering pump delivers a more uniform flow.
  • Enhance Measurement Accuracy: In systems where the fluid being pumped is also being measured or monitored (for attributes like flow rate, concentration, etc.), pulsations can make readings erratic and less accurate. The smoother flow produced by introducing a pulsation damper can lead to more consistent and accurate measurements.
  • Improved Discharge Pressure Control: Pulsation dampers can help stabilize discharge pressure, particularly in systems with variations in downstream resistance. A more stable discharge pressure can enhance the pump’s dosing accuracy.

Our Optos S and M models with pulse dampers come standard with a 10-32 outlet valve. We offer the #6092 T-bracket adapter for Swagelok-style compression fittings for 1/8” tubing connections. Optos H Models come standard with 1/8 Swagelok-style compression fittings.

Pump Controls

Our HP Series pumps do not have electronic controls; they use a manually adjusted micrometer to set the piston stroke length, which adjusts the flow rate. Our Optos series is generally a superior product unless it is operating in a harsh environment where electronics experience failure.

Our Optos pump can be controlled with an analog signal (0-5V, 4-20mA) or via an RS232 interface. High- and low-pressure limits can only be enabled when the Optos Series pump is fitted with the optional pulse damper.

Our Optos Pumps require DSR/DTR controls to work properly and specific wiring on the RJ12. We sell the cables: Eldex P/N: 6143 CABLE, RS232, USB to RJ12

Below is the information on the rear panel’s RS232 modular jack (connector is RJ-12).  It is configured for 9600 baud, 8 data bits, 1 stop bit, and no parity.  Pinout is:

RJ 12 Pin Function DB 9 pin
1,6 (white, purple) Ground 5
2 (black) DSR (Input) 4
3 (red) RXD (Input to pump) 3
4 (green) TXD (Output from pump) 2
5 (yellow) DTR (Output) 6

The pump uses hardware handshaking. It does not transmit on the TXD output if the DSR input is at a low logic level, and it does not receive on the RXD input when the DTR output is at a low logic level. A low logic level is –3.0 to –15 volts and a high logic level is 3.0 to 15 volts.

Our pumps can be controlled with an analog signal or via an RS232 interface. To connect the pump to your system, you can use a USB to RS232 RJ12 (which we sell) or a USB to RS232 DB9 Serial 9-pin cable (which we don’t sell). The pump itself does not have a DB9 connector.


We do not offer on-site service. Our H/D instrument has been around for 30+ years, so a wide amount of literature is available. Please see our user guide, which details the customer setup procedure and required equipment (including vacuum pump and nitrogen source). There is little/no calibration or maintenance needed on-site over time, so you should get years of trouble-free operation.