# 3 way check valve

I am working on a micro dispensing system, using syringe pump. The design involves a syringe on top to be moved by stepper motor. There would be one liquid reservoir form which the syringe would pull liquid from, and push it to eject liquid from other end.

When we pull the syringe, the liquid is sucked into the syringe, while the other opening is shut. When the syringe is pushed, the liquid is ejected from the other end.

The quantity of liquid to be dispensed would be very small (400mg) so i am using small syringe of 1 or 2 ml .. as per my measurement, after every 100 dispensing operations, 1 ml syringe would be empty and we would need to pull liquid from the reservoir into the syringe, and do the dispensing again.

My question is, I am unsure about the check valve here. Is there a 'Single' check valve available which would allow this kind of flow to happen ?

One, you don't need a double/dual check valve; one on the inlet line and one on the outlet would work. See the image below. You haven't explained why two distinct check valves are less desirable than an all-in-one solution, so I'm suggesting it in the event it was overlooked.

Two, I'm not sure how your measurements add up. You state,

"The quantity of liquid to be dispensed would be very small (400mg)"

and then later you state

after every 100 dispensing operations, 1 ml syringe would be empty.

Assuming the dispensing liquid is anywhere close to water, it will have a density of approximately 1g/cm3, or 1g/mL. So, a 400mg dispensing quantity means your 1mL syringe is empty after 2.5 doses. If you're going to get 100 doses of 400mg in a 1mL syringe, then that means that you've got 100*400 = 40,000mg, or 40g, in the 1mL syringe. That's 40g/mL, or about four times the density of lead.

• 1. the liquid to be dispensed is mercury ... 2. I haven't checked but as the outlet of liquid would be small, I am guessing would there be any issues of error in filling by multiple check valves? Furthermore, which kind of check valves be implemented .. size wise ? Everytime we'd have to dispense around 29uL .. Feb 26 '16 at 5:31
• @Muzammil - They would have to be small enough that the pressure required to lift the valve internals wouldn't over-discharge. I think you may find precise dosing of quantities that small through check valves to be difficult. 29uL is very small (but still - 100 doses of 0.029mL is still almost 3mL). Have you considered maybe using a peristaltic pump instead of what you're trying to design, which is effectively a very small reciprocating positive displacement pump? Feb 26 '16 at 18:05
• Also, I believe peristaltic pumps are currently what is in use in the medical field for precision dosing of IV drugs, and apparently also for aquarium chemistry maintenance. Feb 26 '16 at 18:15
• I actually considered those pumps .. but was worried about two factors .. 1 would peristaltic pump be that precise .. secondly .. IV drugs aren't usually that dense .. would peristaltic pump be able to drive a dense liquid like mercury without a huge error .. (small error margin are ofcourse acceptable) Feb 26 '16 at 19:10
• Regarding density, peristaltic pumps are positive displacement, not positive suction, so it would have trouble sucking mercury from a reservoir, but you could resolve that by putting the reservoir above the pump (like an IV bag). But all of the information you've provided should by included in your question for other people to respond to, and I can then update my answer to include all I've said. I feel like we're going off on tangents here. Feb 27 '16 at 14:09

The dual check valve here should work.