Lying to your centrifuge to keep it warm: Via resistors!

[cyclopath]

Please demonstrate you have followed scientific procedure before asking for resistors…

What IS the scientific procedure for lying to a centrifuge?

Let’s explore:

I’ve got a client who picked up a used Beckman Allegra X-12 table-top centrifuge so they could be one of the (sp)in crowd

It has some neat features, including a self-balancing rotor. It also has a flaw, at least for spinning sugar….it wants to maintain a constant 20C.

That’s right, it has refrigeration, but only wants to aim at 20C. We want 35-40C, but it has been told to quit if it hits 40C.

What to do?

Client simply disconnected the compressor, which works, until it hits 40C.

Adding a separate PID control will get the job done, but I think there is a more elegant solution: lie to the damn thing!!!

Using a $0.95 potentiometer

Fine manual https://www.kimforest.com/upload/2016/08/20160822101023kso.pdf

I first suggested a potentiometer across P7 to “set” the temp at 20C. Then OEM RTD could be used on pid control to achieve desired temp, while Fuge happily thinks it’s running at 20C.

But it seems like adding the right resistor (in series) could be used to shift “20C” to 35C.

Which is similar but not identical to the trick suggested over here

Overriding Lower Set Limit on Mini Split Heat Pump

just in case you feel the need to : RTFM ac012mnadchaa.pdf (4.8 MB) Resistance is futile? [image] Nope: you could replace that glass of water with one… And if you’re clever about it, you can use another resistor across the temp sensor (parallel) to spoof it in a controlled manner: Coolant Temperature Sensor (CTS) Resistor Trick You want 13C to read as 20C. So you need to lower 18000ohm to 12000ohm 1/R = 1/R1 + 1/R2 1/12000 = 1/18000 + 1/R2 => R2 = 36000 Most flexible solution would …

Any of you hardware geeks wanna chime in?

@SidViscous @greenbuggy @Lincoln20XX

How would you go about this trick?

[TJMON]

this is a fun one that has me thinking, only issue i see with modding the thermistor is how would it know when it reaches desired temp when it thinks its 20c… im racking my brain now on how to bypass the cutoff itself

[Lincoln20XX]

20°C is 12.26 kΩ
35°C is 6.752 kΩ
40°C is 5.592 kΩ

Adding a 6668 Ω resistor in series would tell it that 40 is actually 20, and likely ensure it never turns off from over temp.

[cyclopath]

Yep, that’ll get it done (simpler than the last “let’s baffle it with a resistor”), but we’re lying here…sometimes that gets messy.

I submit that it’s nice to be able to change your story just a smidge, or even pretend like nothing ever happened…which is why I like the 10k pot.

[Lincoln20XX]

True. However, one concern that crops up: are all of the humans who will put hands on the thing likely to be calibrated to check and put the pot in the right position on each run?

I know that I forget stuff like that all the time. If I have to break out a soldering iron to fuck something up, I’m a small bit less likely to.

Perhaps pot+readout of current pot value, or something of the sort?

[cyclopath]

@Lincoln20XX

If I have to break out a soldering iron to fuck something up, I’m a small bit less likely to.

Very good point.

Even without involving the operator, putting a pot in there could conceivably give it the ability to lie to itself (or at least change its story)…I don’t like that one at all.

[Lincoln20XX]

If you wanted to get fancy about it, a digital potentiometer would do the trick.

[pdxcanna]

A rotary selector switch with pre-selected resistors would fit the bill, pretty budget friendly as well

[cyclopath]

Love it!!

Vive la resistors!

[Lincoln20XX]

Make the final selector position a bit of wire instead of a resistor, and label it god mode.

[cyclopath]

Another cute response.

Not what I expected the all Gnowing one to offer when asked for “digital potentiometer”

@Lincoln20XX

and label it god mode.

DigiKey wants $66.33 for this in a 10k which makes dialing 667 for 40C just a matter of poking the pot with an appropriate stick.

Now I know the part exists, I feel the need to haz…

[greenbuggy]

@cyclopath because I’m a tight SOB, I went looking on eBay for a cheaper option. Assuming the pictured part is the one you need, here’s on for thirty bucks shipped, with a make offer option

BOURNS 3683S-1-502L POTENTIOMETER | eBay

[cyclopath]

Thank you!

That IS the one pictured above, but for this project we need 6668 ohms to achieve our goal, and that is the 5k model.

The 10k model is also available used.

Conceptually, this started out as a replacement for a glass of water…

@cyclopath

When making “chillers” out of AC units, the low tech version is to place the sensor in a glass of warm water…

@GroovyOctopusLabs now wants to know if we actually turn the glass of water into the required resistor (conductor)?

Maybe by getting a little salty with it?!?

[cyclopath]

Different trick.

Wiped Film Parameters

I’m now gonna have to learn of swamping resistors so the PID controlling the heat tape on our new wiper can use average temps. [image] @NitroCat … don’t neglect the links under the top post.

Also “Via Resistance!”