Anyone work with process control, specifically water temp?

[Kayos]

Hi all, I know a few on here work with machinery and stuff and wondered if anyone had experience with this.

Basically its for developing films, and I need to keep several bottles at a set temp, either 37.8 deg or 20 deg, depending on what im doing

I currently have a cheap temp controller, thermistor type, set the temp and it heats 2 modified aquarium heaters (300w each) until the temp is reached, but im finding that altho the water bath of about 30l is staying stable, the temp of the developer etc varies wildly.
I generally have the water bath at 40deg+ and just keep an eye on the developer temp, but this defeats the object of an automated setup.

It is set to come on when the temp drops 0.2 deg so it doesn't keep switching around the set point.

Im thinking a thermocouple one may be more accurate, drop it in the developer bottle and let it control the water bath, is this a good idea?

I only need the dev to be accurate, ive +/- 5 deg on the others

Also any better (but still cheap) ideas for this? I tried cartridge heaters instead of the aquarium ones, but found they were ok if I wanted boiling water and not warm

 

[voipio]

The ideal sort of thing is this:

http://www.bibby-shop.com/product.aspx?pid=196

It keeps the water circulated to combat thermal stratification in the heating tank, but the price is prohibitive, unless you can find one from a lab surplus auction house.

At the very least, a simple motor/propeller combination for stirring will go a long way to getting the developer to where it needs to be temperature-wise. I suspect stratification is part of the problem, so bottles with less devloper in will tend to sit in the colder water lower in the tank.

 

[Kayos]

The ideal sort of thing is this:

http://www.bibby-shop.com/product.aspx?pid=196

It keeps the water circulated to combat thermal stratification in the heating tank, but the price is prohibitive, unless you can find one from a lab surplus auction house.

At the very least, a simple motor/propeller combination for stirring will go a long way to getting the developer to where it needs to be temperature-wise. I suspect stratification is part of the problem, so bottles with less devloper in will tend to sit in the colder water lower in the tank.

Ouch! it looks good but is about twice the price of a Nova heating system, specifically for developing

Stratification is a problem, I expected it and set the probe 1/3 way up the water level (which matches the depth of chems in the bottles so they don't float away) but my idea of set the temp, let it warm a few hours and come back hasn't happened

Do you thing a small pump taking water from the bottom and mixing it in the top would help to even the temps out?

 

[daleyd]

I work for a company called eurotherm and deal predominantly in temp and process control.

A thermistor will act as a switch and simply turn on or off at a set temp, you are correct in that you would ideally want a thermocouple and some means of regulating the output rather than just being on/off.

You can do this a couple of ways, but if you are using a heating element then then you would generally use either a thyristor unit to control the amount of power going to the element or a cheaper alternative is to use a solid state relay and a "time proportional output" control - the SSR will always provide full power but the output will turn it on and off at varying intervals to regulate the temp.

It can be expensive (I know ours are, but they are proper industrial units that are designed for harsh environments and are built really solidly) but ebay may provide some cheap controllers which will do what you want.

I may even have something here that would be suitable if you don't find anything, though I'm not sure without looking at what I have lying around! (Office full of bits of projects at the moment!!)

 

[Kayos]

Thanks for that, the thermocouple and ssr ones are pretty cheap, about £15 so not gonna break the bank.

My initial reason for going with thermocouple type was that it was all in one, however I'm happy for a bit more complexity if it works.

My idea is that I can set the chems warming, go out shooting and develop when I get home, with hopefully minimal fuss

 

[daleyd]

if you can get an ssR one for that price I'd give it a punt, it should be a lot more stable than a thermistor type.

I've just done some ssR heaters for manchester uni on a vacuum furnace and they control to +\- 0.1 degree when set up correctly.

 

[voipio]

Ouch! it looks good but is about twice the price of a Nova heating system, specifically for developing

Stratification is a problem, I expected it and set the probe 1/3 way up the water level (which matches the depth of chems in the bottles so they don't float away) but my idea of set the temp, let it warm a few hours and come back hasn't happened

Do you thing a small pump taking water from the bottom and mixing it in the top would help to even the temps out?

Depends what small is exactly. For a 30 litre tank, a pump delivering few litres a minute should be enough. A washing machine pump would probably be overkill.

 

[brewdexta]

I use PT100 SS temp probes (a couple of quid) and a PID from ebay (around 5 to 10 quid) to control a heater via an SSR for my brewery.

in terms of stratification, my requirements aren't so critical so I use a big spoon However I did try recirculating via a small pump, or if the volume is small sit it on a box with a computer fan it it, each fan blade has a small magnet glued to it then use a standard lab stir bar.

 

[brewdexta]

Something like this, you would have to look at the fine details to get the exact type you need. This has a K type thermocouple however you can usually reprogramme them for several different types and temp ranges.

 

[daleyd]

Something like this, you would have to look at the fine details to get the exact type you need. This has a K type thermocouple however you can usually reprogramme them for several different types and temp ranges.

That might be a bit big for his tank!!

 

[brewdexta]

40 amps should do it

I've got a link to a company that sell nothing but probes and thermowells etc. at home, I got some nice long 450mm SS probes for my fermentation vessel from them.

There's also a little pre-built unit that brewers use to make beer fridges or kegerators, STC1000. Not as flexible on the sensor front but can control cooling and heating from one box.

 

[daleyd]

40 amps should do it

I've got a link to a company that sell nothing but probes and thermowells etc. at home, I got some nice long 450mm SS probes for my fermentation vessel from them.

There's also a little pre-built unit that brewers use to make beer fridges or kegerators, STC1000. Not as flexible on the sensor front but can control cooling and heating from one box.

Your link was for a Colchester triumph lathe! But yes, 40a should be more than enough ha ha

 

[brewdexta]

Aaagh - I have updated the link

 

[voipio]

Try Googling "Sous Vide Thermal Circulator" - a bit more affordable and about the right flow rate.

 

[Anguz]

anything to put a swirl in the water would make things better.... search magnetic stirrer

 

[Kayos]

The STC1000 is pretty much what I have now
The main problems are stratification affecting the sensor, and maintaining a stable even temp in the bottles once the bath is up to temp, as it just switches on and off I get too much variation and end up watching a thermometer, thus defeating the object of pid control

Im leaning towards this http://www.ebay.co.uk/itm/121481831049 A case, some other bits and bats plus a small aquarium pump should see me set for less than £50

 

[daleyd]

The STC1000 is pretty much what I have now
The main problems are stratification affecting the sensor, and maintaining a stable even temp in the bottles once the bath is up to temp, as it just switches on and off I get too much variation and end up watching a thermometer, thus defeating the object of pid control

Im leaning towards this http://www.ebay.co.uk/itm/121481831049 A case, some other bits and bats plus a small aquarium pump should see me set for less than £50

I'd say something like that would be ideal for your needs (never used that exact type but as long as it has the correct range of adjustments it should allow you to regulate much better than on-off control).

 

[brewdexta]

I use these PIDs and similar SSRs with 2.5kW elements in boilers and indirect heating using a plant room tube heater in an insulated fermentation cupboard. They pulse the SSR and hence the heater is turned on and off quickly to control the temp. You can change the range of the temp and adjust several parameters including the pulsing frequency, if it does it too quickly it can destroy some heaters, I killed two tube heaters that way.

Here's the control panel in the centre with 5 Rex C100 PIDs, 4 in use and a spare. The sixth unit is a Sestos timer, it has 2 relay outputs but I just use it as a visual timer at the moment.

 

[Kayos]

That's a fantastic setup, I need to keep about 30l within 0.3 deg, seems simple in comparison

 

[brewdexta]

Thanks
I use PT100 RTDs as they are typically more accurate than thermocouples, if you want to keep with 0.3 degree I think you will struggle with a K type, however the PIDs generally work with PT100 too, just a config change.

Here's a thread I started on a home brew forum I used to frequent about using PIDs, the site got sold to a US outfit for profit and spoiled it so I go elsewhere now.