Temperature Sensor RTD module

sn.white

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Mar 2013
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california
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I'm having some trouble trying to find what kind of sensor I can use,
I'm going to get an RTD module for a micrologix 1500 to measure temperature,

Can I just use a thermistor like this
http://www.mouser.com/ProductDetail/Honeywell/701-102AAB-B00/?qs=sGAEpiMZZMtdU8v%2fCHkq31lWWXRrQ3Tt

or do I need some type of RTD sensor?

for this project I'm working on, we need to use about 24 temperature sensors total, Any advice on sensors I should look at to try and keep the cost down?

Also Is it best to use the RTD module or should I look at something else?


Thanks for any help you can provide,

steve
 
You will need a RTD. A 100 Ohm Platinum 4 wire is what I use. You can use others. What you choose will depend on the temp range you need.
I get them from Logan Enterprises. Loganent.com is the web site. I have has excellent service from Mac at Logan.
 
Last edited:

2cwqzrm.jpg


or do I need some type of RTD sensor?

The way I read the spec sheet, it is an RTD, not a thermistor.

The problem is that it is a tiny RTD element, totally unprotected from damage or pollution. It is an RTD element designed for an OEM to package into a commercial RTD assembly/probe, meaning into a stainless steel sheath with wires coming out.

Look how small it is:
20zs94x.jpg


10 of 'em fit on your fingernail.

Is this what you expect to use and hold up?

for this project I'm working on
It would help to know what temperature range you're looking at and what accuracy you expect.

we need to use about 24 temperature sensors total, Any advice on sensors I should look at to try and keep the cost down?

Thermocouples cost less than than RTD's. I suggest you avoid thermistors, because replacement is problematic, too many 'curves'. RTD's perform well in their temperature range, are more accurate than thermocouples, but a little more money and RTD's do not hold up well in vibration.

Also Is it best to use the RTD module or should I look at something else?
I suspect that an RTD I/O card and a thermocouple I/O card probably cost about the same.

Using a temperature I/O card (RTD or T/C) is far less expensive than using a temperature transmitter for each sensor element and then using 4-20mA AI cards.
 
The temperature range will be between 60-220 F,
It would need to be fairly accurute, maybe +/- 1 degree

A little more info on the project
Its basically a hot water/solar heat reclaim system, in the past some standard generic controllers were used, now we're using a plc instead.
The only reason I was looking at thermistors is that was what the guys I'm working for are use to using with the previous controller.
I believe it was DALE thermisters that was 10,000 ohms(trying to find some more info on it), though they don't appear compatible with the AB RTD module its outside the resistance range,
so they asked me to find a compatible replacement.


Those thermistors look a little small I dont think there what I want,
Can you recoment an RTD I could use?

thanks steve
 
The PT100 RTD's that I use are good up to 400 Deg F. We use them for Custody Transfer Measurement so they have to be accurate. Most of the time they are within 1 deg with no offset.
Are you going to be putting the Sensor in a Thermowell and installing it in the line? If so then Logan will have what you need.
I don't work for Logan Enterprises I have been using their products for over 20 years and can count on 1 hand the number of failures I have had.
As for price we buy explosion proof models so I am not sure if it compares. We by a kit that has a RTD Probe, Threaded Themowell and Explosion Proof Junction Box. We pay around $180.
Check them out at www.loganent.com and I am sure you will get what you need. If you call Ask for Mac he can help you out.
 
The PT100 RTD's that I use are good up to 400 Deg F. We use them for Custody Transfer Measurement so they have to be accurate. Most of the time they are within 1 deg with no offset.
Are you going to be putting the Sensor in a Thermowell and installing it in the line? If so then Logan will have what you need.
I don't work for Logan Enterprises I have been using their products for over 20 years and can count on 1 hand the number of failures I have had.
As for price we buy explosion proof models so I am not sure if it compares. We by a kit that has a RTD Probe, Threaded Themowell and Explosion Proof Junction Box. We pay around $180.
Check them out at www.loganent.com and I am sure you will get what you need. If you call Ask for Mac he can help you out.

Thanks for the help, I'm a little new to temp sensors, only worked with the plc programming and analog 4-20

I believe it will be going in a thermowell,

So can I just look for PT100 RTD?

would these work fine,
http://www.grainger.com/Grainger/DWYER-INSTRUMENTS-Immersion-Temperature-Probe-6CTY5

http://www.grainger.com/Grainger/DWYER-INSTRUMENTS-Immersion-Temperature-Probe-6CTY4?Pid=search
 
Yes, but depending on how long your wiring is going to be a 3 or 4 wire RTD would be more accurate. 3 and 4 wire RTD's compensate for wire resistance.
 
Thanks for the links,
looks like I have a little extra reading to do tonight,

I'll most likely just go with a 3 or 4 wire to be more accurate,

steve
 
I am not a thermistor advocate for industrial work.
Thermistors have two problems

- the entire thermistor market is aimed at OEM business, embedded temperature sensing in a product. So the packaging tends to be tiny and unprotected. Process mfg like United Electric do offer packaged thermistors, packaged in a stainless sheath, like an industrial RTD or thermocouple, if you really want to go there.

- You have to deal with the non-linearity of a thermistor's curve, and its range. When you're talking 1° that can be an issue and you've already run into the issue as to whether a standard resistance I/O card can handle a given thermistor and even trying to find a thermistor.

Most, if not all PLC's offer an RTD card or a thermocouple card or a universal temp card that routinely handles either RTD or thermocouple, but when it comes to a thermistor, you're on your own: getting a resistance card with a fixed current output and then doing a polynomial or look-up table to convert to temperature. While that design effort can be amortized over thousands of units for an embedded thermistor in a consumer production item, in most industrial projects, it can't. It's a whole lot of work to implement a thermistor for an industrial job, but it isn't for an RTD.

Process instrumentation distributors can handle RTD's for you. Someone's recommended one, there are dozens, if not hundreds of others. Personally, I'd stay away from Grainger, because no one there knows an RTD from a pipe wrench, but that's my experience.

You definitely want a 3 wire, 100 ohm, DIN standard, platinum RTD. On the PLC end, all modern RTD I/O cards do 3 wire with compensation. There are very FEW RTD inputs that handle true 4 wire RTD's that are not priced for lab work; besides, the practical difference in accuracy between 3 wire and 4 wire outside of a laboratory environment is negligible so it isn't worth shooting for 4 wire accuracy. You do not want a 2 wire RTD because the offset error over 100' of copper wire is substantial. A 3 wire RTD is the perfect avenue with its compensated lead resistance.

If they're indoors, you might not need a 'head' (although a head serves as a junction box), you might be able to use the leads coming out of a stainless steel sheath.

2rgma1u.jpg

above: RTD with a 'head' that's used as a junction box (terminal strip inside).

u7ou0.jpg


above: An RTD in the sheath with some lead wire attached. A coiled spring serves as a strain relief

Sometimes you need a compression fitting to hold it in place:

Either buy RTD extension wire (color coded) from the distributor who sells you the RTDs or use dual twisted pair cable and leave one wire open. Do not use twisted pair and the shield, because the 3rd wire compensation is out the door.
 
RTD is more precise, buy always 4 wire cables, sometimes only 3 are used.
your original is fine, solder a cable on it and put them in this heatshrinktube with paint put them in AMP ring. this way easy and cheap sensor you can mount anywhere.
as analog inputs are costly better to ue some small relays to switch the inputs. system is slow(same principle is being used for many years in temprecorders.
 

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