Best temp sensor for local application?

Hello all! Small newbie question on my road to control instrumentation knowledge, if you please.

I need to know what temperature sensor/assembly to use with my FL1D-H12RCC IDEC smart-relay nano-PLC (Siemens rebadge, I believe). Datasheet: here (Fl1E-H12RCC but the specs are the same).

Input is 0-10V analog PNP. I need to measure the temperature of the air right near the PLC - maybe two feet away inside a small enclosure (terrarium). There is a large 200W fluorescent bulb with attached ballast about two feet from the point of measurement, I don't know if that qualifies as a source of interference.

The overall application is maintaining a general (+/- 1 degree celcius) air temperature ceiling of 24 degrees celsius, via PLC-contactor activated vent fan. The lamp heats things up a bit, as does the heat pad. This is a terrarium with reptiles and live plants.

I'm kind of new to this stuff. Should I direct wire, use a thermocoupling with some sort of amplifier? Should I use a temperature transmitter? Is there some device I am unaware of that is ideal for this? I'd like to spend as little money as possible.

Thanks for any help, and if I'm missing anything in my description, feel free to ask, I'll reply promptly.

Alan

sounds like an interesting home project with regards to a terranium. Couple of things if the lighting is always on the difference in temperature should be easily compensated. A couple of problems I can see is that your heating system is a dedicated below set point on then when reaches set point off. In this case the type of senser is less important as to how fast you can heat up the enviroment. Pi controllers usually rely upon a heating system that has a reduction of heating levels to gradully decrease the amount of supplied heat dependant on how far away from set point the temperature is. However I am by far not an expert on PID concepts and others here have a far greater understanding of PID than I do. However I'm curious as to what animals/plants require such a stringent control of temperature I would think room temperature to be a greater factor than your lighting, as I use the standard temperature heaters supplied by pet stores to maintain my 3 auqariums and the greatest deviation I have is room temp rather than lighting or any fancy controls.

By the way welcome to the forum

after thinking about your problem for a bit longer I have a couple of questions.

1) is this a glass terranium?
2) is there a lot of variation in room temperature?
3) can you insulate your terranium to reduce heat loss except for your viewable area?
4) Is the terranuim all air or mostly water?
5) If air how sealed is it from heat loss
6) is there a significant differents from room temperature to terrainium temperature?
7) how big a budget on tight controls can you afford to spend?
8) what type of heating system is in place?
9) whats the volume of the area to be heated?

mordred

I guess my requirements do come across as a tad stringent, in terms of the +/-1 degree ceiling. I suppose I can allow for greater tolerance, say 24 degrees +/- 2 degrees.

In terms of ambient air temperature outside the tank, it is usually 15 degrees C, which is fine when the lamp is off, as this simulates a "nighttime" environment.

Essentially, the terrarium heats up a tad quickly because of the large lamp, but this is easily remedied by exhaust fan activation, which is also needed to supply CO2 to the lush jungle-like environment of my terrarium (to be). However, I do not want the terrarium to be constantly ventilated, as 15-17 degrees is a tad low for "daytime." My fan is a 80x80mm fan that runs 24CFM and is more than enough for the terrarium, which is about 8 Cu.ft.

mordred said:

after thinking about your problem for a bit longer I have a couple of questions.

1) is this a glass terranium?

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Yes.

2) is there a lot of variation in room temperature?

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12-15 degrees average

3) can you insulate your terranium to reduce heat loss except for your viewable area?

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Heat loss is the goal, not the problem. I am exceeding my maximum temperature already.

4) Is the terranuim all air or mostly water?

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All air.

5) If air how sealed is it from heat loss

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It's glass.

6) is there a significant differents from room temperature to terrainium temperature?

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10 degrees or so.

7) how big a budget on tight controls can you afford to spend?

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All I need to know is what kind of temperature sensor I should use.

8) what type of heating system is in place?

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None. The lamp and the heat rock combined overheat the tank, which does not have a net top.

9) whats the volume of the area to be heated?

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8 Cu.Ft, though I don't want to heat the tank, I want to control it.

Sorry if I wasn't clear before.

couple of things to look at is heat loss, this can be reduced by insulation. sounds like you want a nighttime mode and a daytime mode. In this case you will want a contoller with a real time clock. Your heating if I understand is through lighting this will be difficult to control to a finite level as their is no way to reduce how much light to scale it down. The type of senser is practically unimportant as long as its accurate the rest is in how to control the temperature. This is tricky as we would need a clear understanding of your system. For example how fast can your light raise the temperature 1 degree?
How much heat loss can we expect?
is there a means of delivering reduced heat levels to gradually raise the heat when its close to set point?
I'm sure others will have better questions perhaps their is a way to time the lighting to gain a gradual heating level when it reaches close to set point but this will be hard on the bulbs and circuitry I would thing that your controllewr will have to shut off the lighting when it reaches setpoint on what we would call a P only control. then turn the lighting on when it reaches set point. The temp senser can only supply readings not control.

Heat loss is the goal, not the problem. I am exceeding my maximum temperature already

this part tells me you may need some form of cooling

Yes, my cooling system is a fan. I want the fan to turn on when it gets too hot, and turn off when I am within acceptable parameters again. I already have a controller, the IDEC PLC I mentioned in my first post.

My question is this: which sensor is the best to wire into my IDEC PLC (model FL1D-H12RCC)? Should I use a thermocouple, or thermistor, or RTD? It has to be analog, 0-10V, and PNP.

http://www.idec.com/language/english/manual/FL1AManual.pdf

looks like you need a thermocouple within range of -30 to 70 degree I can look this up with my thermocpouple specification sheets on Monday.
However I don't have this info handy till then as to what type of thermocouple the range wioll be within.

I posted the PDF that I located to help others help you

Ahhh a thermocouple then. I've been reading up and I suppose I will need a thermocouple transmitter or input module of some kind?

Also, sorry if I wasn't clear before about what I wanted. Reading my first post, it's a bit long and obfuscated.

Yes, if your input is a 0-10, you'll need some sort of thermocouple signal conditioner. One that I've used in the past is Action Instruments WV428. It's basically a "universal" -- nearly ANY thermocouple to nearly ANY analog signal.

Also, from experience, if your heat source is at all "oversized" (if it is able to heat up your vessel in a short amount of time), it is extremely difficult to control the temperature based on a thermocouple in air. The temperature of the air does not change as quickly as you're pumping in energy -- and by the time your thermocouple tells you the temp is too high, you've already contributed so much energy into the system that you're going to overshoot, quite possibly by a lot. (Which you've already discovered -- "heat loss is the goal")

Turkey B said:

I guess my requirements do come across as a tad stringent, in terms of the +/-1 degree ceiling. I suppose I can allow for greater tolerance, say 24 degrees +/- 2 degrees.

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You may need to back off from this spec also.

Turkey B said:

My fan is a 80x80mm fan that runs 24CFM and is more than enough for the terrarium, which is about 8 Cu.ft.

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You have stated that your energy input and your cooling are over sized. Your controller's analog is only 10 bit. If you used the signal conditioner OZEE suggested and a type J thermocouple and had the default scaling and range this would only give you a 1/2 degree resolution. I know this "sounds" like it is within your spec. But consider OZEE's comment.

OZEE said:

Also, from experience, if your heat source is at all "oversized" (if it is able to heat up your vessel in a short amount of time), it is extremely difficult to control the temperature based on a thermocouple in air. The temperature of the air does not change as quickly as you're pumping in energy -- and by the time your thermocouple tells you the temp is too high, you've already contributed so much energy into the system that you're going to overshoot, quite possibly by a lot. (Which you've already discovered -- "heat loss is the goal")

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To make matters worse your controller has PI control and what you need is PD control. Your system is like a "weight on the end of a spring".

It sounds like the only purpose of the PLC is to control temperature through a contactor. The PLC's 0-10V analog input needs a transmitter signal conditioner to take an RTD or Thermocouple.

I have to ask, what's wrong with a relay output temperature controller that comes with a thermocouple input? Turning things on and off is what those things are designed to do.

And there are hundreds of "temperture controller" on ebay at any given time.

Typically their deadbands are adjustable in ON-OFF mode, which will be critical in maintaining 1°C. I suspect short contact life . . .

The difficulty with Ebay is having to decode the model number to find out if it has a relay output (don't want current output) and you don't want the kind with a capillary bulb for sensing temperature.

You can probably buy a temp controller for less than a signal conditioner/transmitter (which typically need a 24vdc power supply also, not a radio shack item.)

You don't have an analog output, so you can't ramp the fan speed, but if you have a spare output (or more), you might want to try multiple fan speeds. This assumes you're using a DC fan.

Select the fan speed based on how far away from the setpoint you are (i.e. larger offset = higher fan speed). This will help avoid overshoot.



-Eric

OZEE and milldrone, that's what I was afraid of, actually. I had this fear that I would be playing catch-up with my temps once my fan was activated. Considering that it is not a critical application, it looks like I may just have to establish a lower target setpoint to overcompensate as an interim solution, though I suspect this may be clumsy in action, i.e. too cold... I think I have to find out how quickly my tank heats up really to determine possible overshoot (experiment time ).

danw, do you mean instead of feeding my controller a range of 0-10V and defining a single setpoint along that range, instead just sending it an on or off signal? Would this device match your description, or is there something simpler (cheaper)?

Eric Nelson, would your solution involve the fan being on constantly, then? I think that could work if I tweaked my fanspeeds in such a way as to have fanspeed options that remove far less, less, more, and far more heat than is generated inside the tank by the lamp and the heat rock, given typical (external air temperature) conditions. The actual logic escapes me a the moment, though, but I have only just read your post.

I am actively researching what I've learned here. First, I am going to time how long it takes to heat up my tank to intolerable levels, to determine how "oversized" my heat source is - I assume that the faster the system heats up then the clumsier the response of my controller, and all I know right now is that it will get "too hot" by general observation. Second, I will research relay output controllers. Third, I will attempt to determine if I can use multiple fan speeds to give my system a more controlled response to rising temperatures, especially if my system heats up very quickly.

Thank You all for your replies so far! I have a much clearer picture of how this stuff works now, and I am much closer to getting a system together that is optimal.