Request for Advice on Choosing the Appropriate PT100 Sensor

[Ciaci]

Dear all,

I am planning to replicate a relatively simple experiment, but I have no prior experience in temperature measurement and control. I would like to manage it on my own, but I would appreciate any advice you can provide (even just to avoid unnecessary expenses) on selecting the most suitable PT100 sensor.

As you can see from the attached image, I need to immerse the PT100 in distilled water or glycerin, which can reach temperatures up to 300°C. The required accuracy is around ±1°C. Additionally, it is specified that the PT100 should be calibrated for an immersion depth of 40 mm.

I have started reading through several datasheets, but I feel like I am missing something in understanding the specific requirements that the PT100 needs to meet. Could you kindly share any advice and perhaps suggest a website where I can find some cost-effective options?

Thank you in advance for your help, and I look forward to hearing from you.

Best regards,

 

[danw2]

>the specific requirements that the PT100 needs to meet
assuming key #2 is the RTD:

your diagram seems to show a reduced tip RTD.
- reduced tip, main section 1/4" diameter,
- stainless steel sheath, length? depends on the setup
- 3 wire
- presumably liquid boils off so you might get an RTD that is bent 90° in the heavy section of the 1/4" shank so the wiring end with the transition (to lead wire) isn't getting exposed and damp from the boiled off liquid.
Class B accuracy at 300°C is only ±1.8°C; you need Class A for ±1°C.
Lead wire is cheap, get enough to work with.

The diagram does not show how you fasten the RTD to whatever.
Consider a compression fitting that slips over the 1/4" shank. The compression fitting (typically 1/4" NPT threads) can thread into a hole in metal bracket.

 

[Ciaci]

"Bent 90° in the heavy section" means what I highlighted in the attached documents?


In this catalogue https://cascade.net/wp-content/uploads/2020/05/Thermokoppel-catalogus.pdf I did not find any reduced tip PT 100. Can you help me in the selection?

 

[danw2]

No. The straight RTD element, along with its connection head on the right, fit into the thermowell and the protection head shown on the left. So, no, that is not a bent element.

A thermowell like that is chunk of stainless steel that completely misses the concept of an immersed, reduced tip RTD. The RTD is not immersed, the thermowell is. The reduced tip is a lower mass that provides for faster response whereas the heavy thermowell adds a lot of mass that reduces the response time.

The graphic below shows an RTD with a 90° bend.

 

[MintJulep]

distilled water or glycerin, which can reach temperatures up to 300°C

Your image shows an open container.

The boiling point of glycerine (glycerol) at 1 atm pressure is 290°C

The boiling point of water is even lower.

 

[Compositepro]

300C requires a silicone fluid that costs about $500 per gallon.

 

[Ciaci]

>the specific requirements that the PT100 needs to meet
assuming key #2 is the RTD:

your diagram seems to show a reduced tip RTD.
- reduced tip, main section 1/4" diameter,
- stainless steel sheath, length? depends on the setup
- 3 wire
- presumably liquid boils off so you might get an RTD that is bent 90° in the heavy section of the 1/4" shank so the wiring end with the transition (to lead wire) isn't getting exposed and damp from the boiled off liquid.
Class B accuracy at 300°C is only ±1.8°C; you need Class A for ±1°C.
Lead wire is cheap, get enough to work with.

The diagram does not show how you fasten the RTD to whatever.
Consider a compression fitting that slips over the 1/4" shank. The compression fitting (typically 1/4" NPT threads) can thread into a hole in metal bracket.

Dear danw2,
reading the standard that I am trying to follow in this paragraph "
"Laboratory temperature measuring device, having an accuracy of +0.25% of the nominal rating, calibrated to a depth of 40 mm immersion, for determining temperatures of liquids in bath tests and operating temperatures. The thermally sensitive part of the sensor (e.g., bulb of a thermometer) shall be held level with the centre of the sprinkler operating parts (glass bulb or fusible element). To control the temperature in the thermal bath, a PT100 sensor conforming with EN 60751 or equivalent shall be used."
I have realized that :
1. It can be implicitly requested that a control system and a independent measuring system are necessary;
2. If the +-0.25% of accuracy of nominal rating (the minimum is 57°C is required) then an accuracy of 0.14°C is required. This means that A pt100 1/10 is required. Furthermore, the temperature reported by the PID controllor can be affected by the inaccuracy of the PID controller, which is around 0.3%.
Which instrument is usually used for reading temperature values and reporting them to a PC?

 

[danw2]

1. I'm lost on your distinction of a controller and an independent measuring system. idk.

2. I don't know what 'nominal rating' means.

Generally, temperature systems are accuracy rated on their span.

Specifically, accuracy statements for digital PID controllers are always based on "full scale" range of the input, not percentage of reading.

Sometimes temperature controllers have multiple ranges for a given sensor. The smaller the range, the tighter the accuracy because the range is smaller.

For instance, a Honeywell UDC 2800 with a factory accuracy spec of 0.15% (Honeywell says that this model can be field calibrated to 'typically' 0.05%) has two Pt100 ranges:

Low Range: -184°C to 149°C The span from -184 to 149 is 333°C.
0.15% of 333°C = ±0.5°C at reference conditions
0.05% of 333°C = 0.2°C
High Range: -184°C to 644°C The span from -184 to 644 is 828°C.
0.15% of 828°C = ±1.2°C
0.05% of 828°C = ±0.4°C

Your requirement to reach 300°C means that your system could not use the 'low range' because the device won't read above 149°C. The factory accuracy spec for the high range is a little over your ±1°C requirement so a field calibration at your maximum operating temperature would very likely provide the ±1°C needed, when combined with an offset for whatever the certified RTD error is at your maximum operating temp.

Post #1 says 300°C. Post #7 says 57°C. You can pick and choose numbers to your hearts content but whoever audits your system for compliance might not agreed with the number you picked.

>Which instrument is usually used for reading temperature values and reporting them to a PC?
A software app on a PC can connect to a digital temperature indicator with a digital interface or a digital controller with a digital interface to get temperature values.

 

[hacksaw]

Per the original post:
Your project requires precision laboratory measurements. That will not be possible with your current design approach

 

[Ciaci]

Per the original post:
Your project requires precision laboratory measurements. That will not be possible with your current design approach

Thank you hacksaw. I already realized that in the meanwhile. The accuracy of the PID controller is by itself already rougher than the requested accuracy.
I was thinking to use two different system, one to control, one to measure (where it is requested the real accuracy).
Do you have any advice for where I can buy these precision instruments? Thanks