Using a Paristaltic Pump for the Precipitation Stage

[Nodda Duma]

As I prepare to make my second batch of emulsion, one of the details I wanted to gain control of is the precipitation stage. For my first attempt, I used a syringe and winged it after a couple minutes' worth of practice, so I'm sure my precipitation rate was all over the place.

At the suggestion of my wife (now a Physician Assistant, formerly a Mechanical Engineer), I acquired two paristaltic pump heads (1 and a spare) which are capable of the 40 ml / minute drip rate that I need. The recipe I'm using calls for adding silver nitrate at a rate of 20 ml every 30 seconds for 10 minutes while stirring vigorously.

I selected an OEM paristaltic pump head off of eBay (item # 131273566619) which provides a maximum of 100 ml / min pump rate at 12V.

Shown with wires that I soldered on for power

The pump head had a short piece of tubing already attached, which I replaced with a 1m length of the tube that it uses (2.5mm I.D. x 4.7 mm O.D. silicon tubing, or 3/32" ID x 3/16" OD silicone tubing, eBay item # 331786745681).

Initial testing with water dyed blue-green.

That went well.

 

[Nodda Duma]

The pump rate depends on voltage applied to the leads. This particular model provides 100 ml / minute at 12 Vdc (maximum voltage for the pump), or 1 ml / minute per 0.12 Volts. That's what the manufacturer's specification says, but if you're an engineer you know that doesn't necessarily correlate with what it actually does. There will be non-linearity of response at the maximum and minimum voltages that the pump operates at (which is why I selected one with about the double the required flow rate), and the actual pump rate will be slightly different from the specification even in the linear region.

So I characterized the pump rate vs. voltage, adjusting the output at the power supply and letting it pump for 2 minutes. Then I simply measure what has accumulated in the fill cup and divide by two:

Multi-meter measuring the meter output:

Starting a run. I flip power on to the power supply when I hit start on the timer (I used my iPhone):

Accumulating during a test run:

Once I had collected this data, I plotted in MS Excel and added a trendline to provide an equation that I can use to determine required voltage for a given pump flow rate:

NOTE: This curve is only valid for this particular pump (although I would expect it to be similar for the same model # and length of tubing).

This graph of the raw data tells me to expect an increase in flow rate of 1 ml/minute for every 0.1332 V increase in the supply voltage. This is different from spec, and enough of a difference to be wildly off. It also indicates that the pump will stop pumping at 3.0346 V. Based on this characterization, the voltage required for a 40 ml / minute flow rate is about 8.36 Volts (0.1332 * 40 + 3.0346). If you just relied on the specification, you might set your voltage at 4.8V and only get a flow rate of 13.3 ml/min. That's why this type of characterization is important. It also allows me to experiment in the future with different flow rates.

The next step is to determine if the rate (which I measured over 2 minutes) is valid for the whole 10 minute run that I will require for the precipitation step of the process. As motors warm up the resistance in the coil tends to drop and they become slightly more efficient. So I set the voltage to 8.36 V and let it go for 10 minutes. When I finished, I was slightly under the required flow rate at a measured 39.6 ml/minute (total of 396 ml / 10 minutes), but that's close enough for me.

Picture during this run:

Another aspect of determining the required voltage for 40ml/min flow rate is to determine the sensitivity of the flow rate to a change in the supply voltage...if the flow rate changes "a lot" for a small change in voltage, then I might have issues with the accuracy of the power supply. Analog knobs on old analog supplies are touchy. So, from the data curve I generated, I calculate the required supply voltage tolerance for a flow rate tolerance of +/- 1 ml/min (i.e. what range of voltage gives me a range of 39 to 41 ml/minute). This turns out to be 8.36 +/- 0.135 V. So this is fairly easy to set the supply to and the power supply when adjusted is stable to within 0.01 V. So I know I'm good on this front. I didn't expect this to be a problem but it is worth checking.



So next step is to get it set up in my the darkroom where I will be mixing the emulsion. I'll need to hold the tubing above the mixing pot so that the tubing doesn't slip and so that it drips in correctly. I'll also need to set up the motor and supply as well. I'll post an update when I get that done.

-Jason

 

[Prof_Pixel]

Nick Brandreth at the George Eastman Museum has been using a Peristaltic pump since last fall (using a regulated, adjustable power supply I put together for him) and has found that the consistent addition rate it offers has greatly improved emulsion repeatability. See https://www.facebook.com/photo.php?....682422647625.2250666.24402756&type=3&theater

 

[Photo Engineer]

The pump delivery rate will vary with tubing usage and tubing ID. Be careful that you choose the right tubing type. As you use it the tubing becomes more flaccid and delivery rates can vary by quite a bit.

PE

 

[Nodda Duma]

That's very cool. I'm betting this characterization effort would be familiar to you and/or Nick then.

There's one little resourceful detail in the photo that I'm going to take full advantage of: the "horse syringe" which appears to be the supply reservoir. I was trying to figure out what kind of bottom-feed container I could use rather than just sucking it out of the jar that I prepare the silver nitrate in.

 

[Nodda Duma]

The pump delivery rate will vary with tubing usage and tubing ID. Be careful that you choose the right tubing type. As you use it the tubing becomes more flaccid and delivery rates can vary by quite a bit.

PE

Yep. The motor ad provided the tubing specification required for use with the pump, so I was able to get the (almost) exact tubing required. This is a big part of why I characterized the actual arrangement I will be using tonite: tubing dimensional and material property variances would affect flow rate. I also intend to do a test characterization immediately prior to precipitations in future batches as well to account for aging and use.

My anal-ness is based on years of learning engineering lessons the hard way

 

[Jerevan]

Out of my depth here ... but can one use some other sort of tubing that does not go soft? (stainless steel, glass or something else)

Or is it a good thing that the tubing is expendable?

 

[Nodda Duma]

You actually need soft tubing for the pump to work properly. Same principle as milking a cow.

The tubing is very inexpensive, so it is easily replaced.

 

[Jerevan]

Thanks - just goes to show how much I know about milking the cows!

 

[AgX]

I rather would consider a surplus peristaltic pump assembly.

 

[Nodda Duma]

When I looked, a surplus pump was flipping expensive for not much gain...like $100 or more. The cost of this pump head and tubing (about $10 total) plus the ~2 hours of labor devoted so far is much more cost effective.

In a hobby (emulsion making) that is all about DIY, there's that additional satisfying aspect to this project.

 

[Prof_Pixel]

The pump Nick ended up using cost a lot more than your $10 pump, but then, he makes a lot of emulsions.

 

[Nodda Duma]

I'm sure it is. Acquire what you need for the job. I'd expect this pump to last through at least 20 batches of emulsions but not much more. That'll get me through about 5-10 years of dry plate photography.

 

[dwross]

Congratulations on a VERY cool accomplishment!! Fun!

 

[Prof_Pixel]

Yup. BTW, a great job of documenting your work.

 

[Nodda Duma]

Thanks!

Preparing the emulsion now so I should be able to provide an update soon.

 

[Nodda Duma]

Pump worked perfectly. Timed it at exactly 10 minutes to start pumping air, then a few extra seconds to pump out the remainder of the silver nitrate solution into the emulsion.

I suspended the tubing over my mixing jar using a combination of a clothespin and a 2nd jar (happened to be what I had handy).

 

[EdoNork]

For repeatability you can change the tubing after each use. You can use a piece of the exact tube for the pump assembly (about 10-15mm?) and another type of tube for the IN and OUT sections, so you don't need to change the full tube.

 

[Photo Engineer]

I do the same because that tubing can get expensive. Home Depot sells quick disconnects for this type of setup.

PE

 

[Nodda Duma]

Good tip guys, makes sense. I'll look for the quick connects.

 

[Prof_Pixel]

The pump Nick is using came with several short pieces of tubing of different diameters for different flow rates. All of them had quick connect fittings on the ends.

 

[Kirk Keyes]

I'm a little late, but a few years ago I set up a peristaltic pump for adding reagents to emulsions. Instead of retyping a bunch of test, you can read and see my set up on the post here:
(there was a url link here which no longer exists)
Here's a few (but not all) of the photos from the link above:

A digitally controlled, multiline peristaltic pump (with two lines in use - max of 5 possible.) Differing rates can be added by using different diameters of pump tubing.

Perhaps a nicer shot of the pump. Only one line is shown connected to the pump at this point.

The end of the two lines that are put into the emulsion for dispensing solutions. Small diameter Teflon tubing allows a very fine jet of reagents to be added.

Here's the setup in the dark.

Not shown was the vAg probe I made and used.

 

[dwross]

Kirk! Great to see you here again! Tell me you have time to be back in the game . d

 

[Photo Engineer]

Great to see you again Kirk. Hope things are going well. Send me a note.

Ron

 

[HiHoSilver]

Neat work, Nodda. Most medical tests using an analyser employ (or used to) a free-dried control - assayed cocktail of body fluid/chemicals, for the relevant tests. Those assayed controls were filled by peristaltic pump - whose tubing was changed for each lot/run of controls. 'Seems a natural application for photochems, though maybe favors larger production runs. 'Neat to see your work here. Thx for posting.