Thermistor help?

[mvh_2000]

I’m new to the forum so not sure where to post this.

I need some basic help with a control circuit that has an electric solenoid, thermistor and a temperature switch. I need basic on/off control of the solenoid by temperature sensing with the thermistor. I want to maintain 70 Deg +/- 2 deg. Here is what I have so far.
I would like to use this Love On/Off controller.
http://www.coleparmer.com/catalog/pr...9352012&pfx=EW

Questions?
With this controller I have to use 1000 ohm@25C thermistors only?
I can only use PTC thermistors?
I would like to use a surface mount thermistor but cant find many with the above specs?
Could I use a thermocouple to hold this temp? Easy to find the equipment.

Thanks
MVH

[m411b]

Welcome to the forums!

[DetroitAC]

Welcome, please repost your link, or give a Cole Parmer part number, that link takes me to cole parmer, but to page not found. Why that controller? Do you have it in hand, thus it's free?



With this controller I have to use 1000 ohm@25C thermistors only?
Dunno, can't view the controller specs


I can only use PTC thermistors?
Usually NTC thermistors are for temp measurement, PTC is for temperature protection, self regulating resistance heaters, etc., but maybe that controller uses a PTC? that would be a bit odd, but as long as the controller is calibrated for it...


Could I use a thermocouple to hold this temp? Easy to find the equipment.
Yes, of course you could. I'd bet 75% of industrial applications in that range would use a thermocouple

[mvh_2000]

Thanks DetoritAC,

I too am from the Detroit area, Macomb County.

Full Story:
I’m making wine in a 30 gallon Stainless Steel conical fermentor, 20 gallons at time. The first 4-5 days the temp will go up to 90 Deg due to active fermentation. I want to install a Stainless coil in the tank and circulate some of the 50-55 degree Michigan winter water to keep the temp down around 70 +/- 2 deg.

My plan was to monitor the temp with a thermistor or thermocouple to open a water solenoid to run water thru the coil. I thought a thermistor would be more accurate with the temp???

Parts:
The only reason I choose the controller was price and not many controllers I found are usable with thermistors. If I can use a thermocouple I have many more choices of controllers. I would like to try a surface mount device first so I don’t have to weld in fitting into the tank. I’m not sure how accurate a surface mount will be? I plan on having fittings welding in for the coil so 1 more for a probe won’t be a big deal if need be.

The controller was a Love DTS -83010 Temperature Switch. Try this link.
http://www.dwyer-inst.com/HTDOCS/temperature/SeriesDTSPrice.CFM

Thanks Again!
MVH

[DetroitAC]

Ah, a fellow Detroiter!

I guess what you're doing makes sense to me. Thermistors are more accurate once calibrated, but PTC thermistors (it looks to me like that's what this uses) have a very shallow characteristic curve compared to NTC. A PTC does not have much resistance change as the temp changes, an NTC has a better curve. I guess if you go with this controller, you should get the matching probe. I couldn't see any details on the probe, my guess is that (for the $5 price) it's just a thermistor with two wires, with the sensing thermistor potted. Not the type of thing you could insert into a vessel. I suppose you could stick it to the outside wall of the vessel, and insulate over it and the surrounding area.

It's better to use an immersion thermocouple, how close to 70 you need to keep the wine. If you must be +/-2 deg, and it must be 70 deg (would 65 +/-2 work??), In any case if you need that kind of accuracy, you'll want to calibrate whatever you have.

The controller is very cheap, but also very simple (not much you can adjust, and only in whole integer values). I've found some top quality PID process controllers like Honeywell, Omega, Powers on ebay for $30 or so.
My Honeywell controller has a 250 page manual! and i suppose I'm only using about 10 pages worth of it's capability. I'm using it for a simple on-off control like you are talking about with a thermocouple. I do have open thermocouple protection (switches the process off), hi alarm, low alarm, adjustable time base, hysteresis, and the ability to override into manual mode.

Is your vessel wall thick enough to tap pipe threads into, or do you need to weld bosses? In industry it's common to use a sheathed thermocouple, and either a thermowell
http://www.omega.com/toc_asp/subsectionSC.asp?subsection=B08&book=Temperature
or a swagelok and pipe thread, bored through fitting with a sheathed thermocouple immersed directly in the fluid

or just a sheathed thermocouple with an integral pipe fitting
http://www.omega.com/ppt/pptsc.asp?ref=NB1-RP_REPLACE_PROBE&Nav=tema09

However you are measuring the temperature, I suppose you've figured out that you will need to put an adjusting valve in your water line so that you can adjust the water flow to a reasonable rate: too fast will chill quickly near the coils and the sensor will not detect it until you have chilled far too much (a bad undershoot), and then there will be a long off time. Too slow will have too short of an off time, and not much ability to cool down during an overshoot and bring the process back under control. I would think a 50% duty cycle of the solenoid valve is a good starting place.

[mvh_2000]

I have been to the Omega web site many times! It sounds like a immersion probe or a dry well would be the way to go. The vessel is not thick (1.5mm) enough to tap, I need to weld in fittings. 65 Deg +/-2 is a little cold. The optimal temp would be 68 deg but I would rather be a little warm then cold. If I’m to cold the fermentation could stop. I have seen many controllers on EBay by Omega, I'm open to use any type of controller/probe at this time. What type of thermocouple would you suggest for this setup? (t,k,s,r,etc) Could you give me a ball park number on how accurate I could get with a thermocouple? I have planned for flow control on the system.

Thanks
MVH

[DetroitAC]

What type of thermocouple would you suggest for this setup? (t,k,s,r,etc) Could you give me a ball park number on how accurate I could get with a thermocouple?

An immersion probe will be very quick response and more accurate. A dry well is so that you can replace the thermocouple (if it should fail in the middle of a "batch") without emptying the tank.

For you application, personally I'd use a type T or K. These are very common and easy to get. Type K is the most popular in the world. Type T is similar to K, but the standard limits of error of the wire are smaller (they are more accurate "right out of the box"). For your application you will need to calibrate whatever you use, since a 2 degree error is not uncommon for brand new equipment. You'll be only as accurate as your calibration :D

[mvh_2000]

Thank you very much for all the help!!

Would this controller work with one of the omega probes?

Ebay item number = 190061305365

I would buy an omega (t) immersion probe and use this controller?

http://www.omega.com/ppt/pptsc.asp?ref=NB1-RP_REPLACE_PROBE&Nav=tema09

[SexyMF]

Hello mhv, welcome to this forum. But I have to say this isn't the place to be discussing control solutions. If you were using a refrigeration system to cool things down, yes.

There is a lot more to this than just how accurate a probe is. You need to do a lot more experiments first to see how well the water will cool the exothermic reaction. The temperature spread from next to the coil and the furthest away from the coil. You are not even starting with coolant at a known temperature.

[wdrzal]

Be careful how you bring the temperature down as not to stop fermentation. Tap water out of the ground can get down in the 45 degree range +/-. you have to be carefull 90 degree grape juice is not in contact with 45 degree heat exchanger , make the design so the heat exchanger does not get that cold. You sure don't want to cool any portion of the juice to 50 degrees then cool the rest of the tank when mixing occurs (cold with warm)

It makes more sense to me to be able to alter the temperature of room the container is in. and if the juice gets to hot just circulate thru a external coil using convection (line out the top,thru coil ,in the bottom). until the temperature comes down.

[DetroitAC]

Thank you very much for all the help!!

Would this controller work with one of the omega probes?

Ebay item number = 190061305365

I would buy an omega (t) immersion probe and use this controller?

http://www.omega.com/ppt/pptsc.asp?ref=NB1-RP_REPLACE_PROBE&Nav=tema09

I don't know anything about that PID, looks like Chinese if it's selling new for that price. I prefer American or European equipment (look at a honeywell UDC3000 on ebay 180064126876), but that controller might be just fine. It says it will work with a type T, so if it does what it claims you should be fine.

[jinu117]

The one mentioned would do just fine (chinese one). Bit lacking on documentation but it isn't rocket science to figure out really. So far the similar type I've been using works just fine. YOu might want to do bit of caliberation at start and it will work with any thermocouple, RTD... :)

[mvh_2000]

I think I have it!! I'll keep looking for a better quality controller on Ebay.

Thanks for all the help.

MVH