liquid dispensing control - anyone got any experience? - Page 1
Forum section: electronics / projects

author: Corporate666
author_rank: Unknown
timestamp: March 29, 2013, 04:38:30 am
content: I am working on a little side project that would require somewhat accurate dispensing and mixing of various chemicals. When I saw somewhat accurate, let's say a mixture has 10% of A and 90% of B.. the ratio should be within a couple of percent or better. Also, the viscosity of the liquids varies. The application is mixing different low viscosity epoxies that range from almost water-like to motor oil. The reservoir will be above the dispensing apparatus, so I am looking into electronically controllable valves (I guess ball valves?) to control.The problem is measuring the amount dispensed. Basing it on time doesn't account for viscosity differences. But measuring seems to be a lot of work.Anyone got any novel ideas?And anyone got any suggestions for electrically operable valves, or even better, any experience in the field?This is just a little project to speed up a process at work, so I have to keep an eye on costs - but I want to make something that is very reliable and as accurate as I can reasonably make it.

author: smashedProton
author_rank: Frequent Contributor
timestamp: March 29, 2013, 04:46:33 am
content: Ben Krasnow has something like this on his YouTube channel. It will be difficult to measure without using a scale like he didwww.GarrettBaldwin.com

author: smashedProton
author_rank: Frequent Contributor
timestamp: March 29, 2013, 04:50:20 am
content: You could simply do a stepper motor mounted to a pump of some kind. Depending on your volumes, you could use syringes, pipettes, the pump things that work by squishing the tube, and scales with mechanical feedbackwww.GarrettBaldwin.com

author: IanB
author_rank: Super Contributor
timestamp: March 29, 2013, 04:55:59 am
content: Quote from: smashedProton on March 29, 2013, 04:50:20 amThat would be peristaltic pumpsBut yes, some kind of metering pump would most likely be the way to go...

author: MacAttak
author_rank: Unknown
timestamp: March 29, 2013, 05:41:23 am
content: I'm not sure if it would work for you, but there are certainly peristaltic pumps (essentially a motor that squeezes a tube) and flow meters that should be OK with most viscosities. You would just run the pump until the desired amount has been measured (as opposed to trying to time the pumping action or something like that).Adafruit sells a pump and meter that might work. I'm sure you could find better / cheaper / more appropriate parts if you looked around though.http://www.adafruit.com/products/1150http://www.adafruit.com/products/828

author: SeanB
author_rank: Super Contributor
timestamp: March 29, 2013, 06:26:32 am
content: Normally gear pumps are used, especially if you want to blend large varying volumes. For fixed volumes use a piston pump to do the fixed volume dispensing. You can get a repeatability of under 1% with that. For the valving pneumatically actuated ball valves would be best, as you have such a viscosity range. Best will be to look for a used ointment/ gel filler, as it will be strippable to clean out the interior parts in contact with the material, as well as being all stainless steel and PTFE or polypropylene construction. You will need 2 though, probably with varying maximum volumes to get the ratio right. No electrics or anything on the simple manual machines, all air operated.

author: Corporate666
author_rank: Unknown
timestamp: March 29, 2013, 07:06:43 am
content: Thanks for the info guys - the only thing that's coming to mind is that I was hoping to do away with a pump and just have gravity dispense the chemicals - that would simplify things a lot and would be less tubing/pump to clean out, plus they would not need to be primed.Sort of like putting an electrically actuated valve on water cooler, if that makes sense. But it sounds like the metering would be a problem with that setup, especially for substances that would need cleaned out between uses from the mechanism.

author: SeanB
author_rank: Super Contributor
timestamp: March 29, 2013, 10:53:57 am
content: I have modified a simple manual one to air power, as it made working it easier and was cheaper than the air operated version ( by a considerable amount a lot more than a cylinder and a speed control cost) plus I was able to make the footprint a lot smaller, just by using the space under it's new dedicated table. I reused a lot of parts from the gear unit it replaced, mostly the custom made table and the nozzle that fitted the bottle we use. Foot operated switch, 2 air regulators to control speed of each direction of stroke and speed control fitting on the cylinder along with some new stainless steel sheet and other work made up a small increase in cost.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 29, 2013, 03:04:27 pm
content: Surely the easiest way to measure would be weight change of the thing you're dispensing into?As mentioned, a peristaltic pump is probably good choice as the pump volume is well-defined, and the only thing in contact with the substance is and easily replaceable tube, avoiding cleaning & cross-contamination issues.

author: smashedProton
author_rank: Frequent Contributor
timestamp: April 02, 2013, 02:28:45 pm
content: You should get hardware hacking a scale!

author: Rufus
author_rank: Super Contributor
timestamp: April 02, 2013, 05:17:47 pm
content: Quote from: mikeselectricstuff on March 29, 2013, 03:04:27 pmWeigh filling can be very accurate. If you want fast and accurate you usually need to reduce the fill rate as you approach the cut off point. For gravity feed a pinch valve (where a pipe is pinched closed, usually with a solenoid) has benefits similar to the peristaltic pump.I should have noted if your requirement is to fill a volume product density variation can screw weigh filling accuracy.

author: C
author_rank: Super Contributor
timestamp: April 02, 2013, 06:14:13 pm
content: A lot of good idea(s) here.But not seeing much on the down side, What could go wrong.In my past, had to maintain something similar.One of the liquid's was water. Just needed to add _ of a second liquid.Sounds simple right.The water was for a drinking water supply and the second liquid was Chlorine.None equaled people getting sick, to much equaled people being harmed or dead.Chlorine is not hard to pump, But if you do not what to have to pump a large amount, it needs to get stronger. The bleach you can buy at the store can treat a large amount of water, but even it will eat on the pump. The pumps built for the job all had problems.Things like pump stopped, water stopped, pump leaking, pump slow, pump fast,. Actually the last two should read "to little Chlorine" or" to much Chlorine". A test for amount of Chlorine in water is easy, just not continuous. Easy Testing is a batch process with mixing.So a good solution should also cover what could go wrong, and how long to fix,Just a thought,C

author: SeanB
author_rank: Super Contributor
timestamp: April 02, 2013, 06:47:18 pm
content: Chlorine pump is easy, you go to Watson Marlow, and give them a big pile of money, and get a chlorine dosing pump that is reliable, and works for decades with regular maintenance ( changing peristaltic pipes every few months with new), or you use a nickel gas bottle and gaseous chlorine ( and a blending pump to dissolve it as you flow water through) to do it. Works well on continuous flow ( water plant for a town) or on a batch basis, though then you are talking of batches of 100kl or more.You really want problems look at a municipal swimming pool with continuous dosing, as often it is being operated by untrained people ( most lifeguards are not going to be trained in water treatment) and operated often with insufficient and incorrect chemicals and mixes.

author: C
author_rank: Super Contributor
timestamp: April 02, 2013, 08:47:25 pm
content: SeenB,I agree, but with out that pump, what sounds simple can have a whole lot of "GOT YOU" in it.and You can easily have water side broke, good pump fine.C

author: SeanB
author_rank: Super Contributor
timestamp: April 03, 2013, 04:24:35 pm
content: My father designed and installed a few water purification plants. Biggest cost in that was the chlorine generator plant, it was around 60% of the equipment cost. He did a modular one, and specced the chlorine plant to run the complete unit from the get go. When it came time for stage 2 ( a few years later) they came in at the same price, and at stages 3 and 4 were making a good profit, as all the civils had been done at stage 1 and 2 already, just needed the tanking built and piped in, nothing else needed.

author: C
author_rank: Super Contributor
timestamp: April 03, 2013, 10:47:06 pm
content: SeanBYour father Sounds like quite the MAN, My hat's off to him.Could the powers at be, see the need for the four stages or just your father?C

author: Paul Moir
author_rank: Frequent Contributor
timestamp: April 04, 2013, 05:46:22 am
content: Peristaltic pumps are a bit funny in their absolute accuracy. Viscosity matters a lot obviously but even if that's not variable they don't all pump at the exact same rate. Some "manipulations" of the tubing affect flow quite a bit.If I needed high accuracy and weighing the deposited sample is possible, that would be the way I would go. Second is a piston pump with some form of control (screw/stepper or servo, pneumatic cylinder with scale, etc) all assuming you need electronic control of course. To see what I mean, google "piston filler pump".

author: SeanB
author_rank: Super Contributor
timestamp: April 04, 2013, 03:52:16 pm
content: C, the original spec made mention for expansion, but the submitted design had the full chlorine plant, as it was not a lot more than the smaller one, the biggest cost was the metering and controls, a small part was the actual differing size pumps. As the company took a bet on getting the rest of the contract in time it was a small cut in immediate profit, but a huge payoff in time, as the town grew a lot faster than the planned predictions, so the 3 extra phases came in about 15 years ahead of schedule. He did the same with the sewage treatment plant, but that was basically just duplicating existing plant. I drive past one that he built in the 1970's regularly, still in use, and still looking good.Paul Moir, peristaltic pumps are good provided you keep the required volume as a large multiple of the volume pumped with each revolution, as there can be a 5% or so variation per cycle ( I have experience of this pumping a slurry) in most cases, and a gradual change as the supply tank level varies. If you want to reduce this you need to have a buffer vessel on both input and output, to provide constant inlet and back pressures. As well long term drift as the pump tube ages and loses flexibility. I still have some of the $200/m Watson Marlow silicone tubing around, and some of the cheaper tubing we used to make up the line, just using enough to get in and out of the pump, with a coupling at inlet and outlet. The tubing is easy to burst just with the pump if it blocks on the output, that was messy.

author: mickpah
author_rank: Regular Contributor
timestamp: April 05, 2013, 11:08:26 am
content: I'm in the process of building a Ph level controller for my pool. Couldn't find a suitable pump in Australia ( as a reasonable price) so I got it from these guys.http://www.clarksol.com/html/liquidPumps.cfmI was impressed, most companies like this don't generally deal with low volume or "hobbyists" but they were really helpful to the point of special ordering a unit for meI have no interest other than being a happy customer.The pump I bought was this onehttp://www.clarksol.com/html/prodspecsM500Pump.cfm( just happens to be the oem for a commercial pool dosing pump)