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Need SMF eletrical help

post #1 of 6
Thread Starter 

I've just finished my fridge build (see photo below) and I gave it a test run only to find out my PID controller is off by 25 degrees. I checked it with two other temp gages which i checked in boiling water they read (213F). My question how do I calibrate my PID. I have enclosed a photo of the type I have. The instruction that were sent with it are as hard to understand as Chinese arithmetic  so if any knows how to calibrate this type I would greatly appreciate some help......

PID.jpg

DSCN1272.jpg

post #2 of 6

Scrappynads, morning....   The first thing that comes to mind is the type of thermocouple you have hooked up to the controller... 

Is the thermocouple you have hooked up to the controller compatible with it ?? 

Each "type" has it's own resistance and is accurate only with a circuit designed for that "type"....  All components used must be of a "type" compatible also so the resistance measured and converted to a temp reading is correct...

If all is correct then there might be a "pot" to tweak the calibration inside the unit.... 

I'm just not an expert by any stretch of the imagination about this subject.... just thinking outloud....  Dave

 

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Types

Certain combinations of alloys have become popular as industry standards. Selection of the combination is driven by cost, availability, convenience, melting point, chemical properties, stability, and output. Different types are best suited for different applications. They are usually selected based on the temperature range and sensitivity needed. Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions. Other selection criteria include the inertness of the thermocouple material, and whether it is magnetic or not. Standard thermocouple types are listed below with the positive electrode first, followed by the negative electrode.

[edit] K

Type K (chromel{90 percent nickel and 10 percent chromium}–alumel)(Alumel consisting of 95% nickel, 2% manganese, 2% aluminium and 1% silicon) is the most common general purpose thermocouple with a sensitivity of approximately 41 µV/°C, chromel positive relative to alumel.[7] It is inexpensive, and a wide variety of probes are available in its −200 °C to +1350 °C / -328 °F to +2462 °F range. Type K was specified at a time when metallurgy was less advanced than it is today, and consequently characteristics may vary considerably between samples. One of the constituent metals, nickel, is magnetic; a characteristic of thermocouples made with magnetic material is that they undergo a deviation in output when the material reaches its Curie point; this occurs for type K thermocouples at around 350 °C .

[edit] E

Type E (chromelconstantan)[5] has a high output (68 µV/°C) which makes it well suited to cryogenic use. Additionally, it is non-magnetic.

[edit] J

Type J (ironconstantan) has a more restricted range than type K (−40 to +750 °C), but higher sensitivity of about 55 µV/°C.[2] The Curie point of the iron (770 °C)[8] causes an abrupt change in the characteristic, which determines the upper temperature limit.

[edit] N

Type N (NicrosilNisil) (Nickel-Chromium-Silicon/Nickel-Silicon) thermocouples are suitable for use at high temperatures, exceeding 1200 °C, due to their stability and ability to resist high temperature oxidation. Sensitivity is about 39 µV/°C at 900 °C, slightly lower than type K. Designed to be an improved type K due to increased stability at higher temperatures, it is becoming more popular, though the differences may or may not be substantial enough to warrant a change.

[edit] Platinum types B, R, and S

Types B, R, and S thermocouples use platinum or a platinum–rhodium alloy for each conductor. These are among the most stable thermocouples, but have lower sensitivity than other types, approximately 10 µV/°C. Type B, R, and S thermocouples are usually used only for high temperature measurements due to their high cost and low sensitivity.

B

Type B thermocouples use a platinum–rhodium alloy for each conductor. One conductor contains 30% rhodium while the other conductor contains 6% rhodium. These thermocouples are suited for use at up to 1800 °C. Type B thermocouples produce the same output at 0 °C and 42 °C, limiting their use below about 50 °C.

R

Type R thermocouples use a platinum–rhodium alloy containing 13% rhodium for one conductor and pure platinum for the other conductor. Type R thermocouples are used up to 1600 °C.

S

Type S thermocouples are constructed using one wire of 90% Platinum and 10% Rhodium (the positive or "+" wire) and a second wire of 100% platinum (the negative or "-" wire). Like type R, type S thermocouples are used up to 1600 °C. In particular, type S is used as the standard of calibration for the melting point of gold (1064.43 °C).

[edit] T

Type T (copperconstantan) thermocouples are suited for measurements in the −200 to 350 °C range. Often used as a differential measurement since only copper wire touches the probes. Since both conductors are non-magnetic, there is no Curie point and thus no abrupt change in characteristics. Type T thermocouples have a sensitivity of about 43 µV/°C.

[edit] C

Type C (tungsten 5% rhenium – tungsten 26% rhenium) thermocouples are suited for measurements in the 0 °C to 2320 °C range. This thermocouple is well-suited for vacuum furnaces at extremely high temperatures. It must never be used in the presence of oxygen at temperatures above 260 °C.

[edit] M

Type M thermocouples use a nickel alloy for each wire. The positive wire (20 Alloy) contains 18% molybdenum while the negative wire (19 Alloy) contains 0.8% cobalt. These thermocouples are used in vacuum furnaces for the same reasons as with type C. Upper temperature is limited to 1400 °C. It is less commonly used than other types.

[edit] Chromel-gold/iron

In chromel-gold/iron thermocouples, the positive wire is chromel and the negative wire is gold with a small fraction (0.03–0.15 atom percent) of iron. It can be used for cryogenic applications (1.2–300 K and even up to 600 K). Both the sensitivity and the temperature range depends on the iron concentration. The sensitivity is typically around 15 µV/K at low temperatures and the lowest usable temperature varies between 1.2 and 4.2 K.

 

 

post #3 of 6
Thread Starter 

. I got the PID controller build instructions from SMF so I assume they are correct.

post #4 of 6
Thread Starter 

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post #5 of 6

SN,,,  I'm  just saying, check the components..... 

Could you post the instructions you used and maybe we can find the discrepancy..... Also the parts you used with their part #'s etc.....  

Someone who knows more than I do will jump in here to help you out..... 

We need to get this problem solved....  Dave

post #6 of 6

Some PID controllers also use a temperature sensor called and RTD. The purpose and function is the same as a thermocouple though, just a different type of technology.

 

Often the sensor will have a small tag on the cable or sensor that identifies what type it is (J, K, etc).

 

Does your sensor have a tag or any marking on it? If you don't find a tag, go to this link and see if you can match the color code of your sensor to those shown in the chart.

 

http://www.omega.com/techref/colorcodes.html

 

Also, check this link and see if the model number of the instructions matches any markings on your controller. If these are the correct make/model info, some of us can use it to help you diagnose your controller's issues.

 

http://auberins.com/images/Manual/Manual%20version%203.4.pdf

 

The PID controller usually has to be configured to tell it what type of thermocouple or RTD is connected to it.

 

Have you ever unwired the sensor from the controller since you have owned it?

 

The controllers and thermocouples are  polarity sensitive, so if you reversed the sensor wires to the controller terminals, it will make it read incorrectly or not at all. You can try reversing the connections as a quick check.

 

Most controllers can be adjusted to calibrate for sensor error, but its unusual to have on that is 25 degrees off.

 

Check this stuff out and let us know what you find out. Usually its something simple.

 

 


Edited by processhead - 4/19/12 at 11:13am
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