Temperature Sensors


1. What can I use to extend my RTD wire?

You should use regular copper wire for the RTD sensor connection. The only requirement is that the wire should not be thinner than 26 Ga.

 

2. What can I use to extend my thermocouple wire?

You must maintain the same dissimilar metals to the instrument. You need thermocouple wire or thermocouple extension wire to extend the thermocouple wire. The B type thermocouple is the only one that can be extended with copper wire.

 

3. How to join the thermocouple extension wire?

You can use regular copper wire splicer to crimp them together. As long as the splicer is short, it is fine.

 

4. The measurement on the controller is not consistent with other standard, is my controller or thermocouple (TC) defective? 

It is not necessarily in the case.

1) Expected Accuracy

All temperature controllers that Auber sells have an accuracy of 0.2% of the range. For a K type thermocouple, that translates to +/-5 °F. Most of time, the error is within 2-3 °F. If the error you get is more than 5 °F, something is wrong.

2) Statistic of Auber Equipments

  • The probability of having a defective controller that could not read the K thermocouple within specification is less than 1%.
  • The probability of having a defective K thermocouple is less than 0.1%. For more than tens of thousand of thermocouple we sold, we had less than 5 that can’t read correctly.
  • For the customer that had problem of inaccuracy, the probability that customer didn’t set the controller correctly is about 1-2%.
  • For the customer that had problem of inaccuracy, the probability that customer didn’t use the thermocouple correctly is >95%.

So, you should have good confidence that the Auber equipment is most likely not the cause of the problem.

3) Isolating the Problem

a. The first thing to do is to verify the controller is OK. This can be done very easily. Just use a copper wire to short the terminals for the TC input (e.g connect terminal 4 and 5 for 2342 or 2352; connect terminal 6 and 7 for 1512, and connect terminal 9 and 10 for 2363). The reading of the controller should be about the ambient temperature, where the controller is located.

If that is the case, the controller is fine. To be 100% sure the controller is fine; check the parameter setting for the input type. Make sure it is set to K type.

If reading does not show ambient temperature, change all the parameter to the initial setting listed in the instruction manual. If it still not shows ambient temperature, the controller is defective.

If “a” passes, go to next step.

b. The next thing to do is to verify the system (the controller and the sensor) is OK. You need to remove the TC from your installation, connect it directly to the controller (without extension and connectors). Dip the TC in ice/water mixture (70%: 30%). The sensor body need to be complete immersed. Read the temperature in 5-10 minutes. The reading should be close to 0 °C or 32 °F. You can also do this in boiling water. The reading should be close to 100 °C or 212 °F at sea level. Important, you can not calibrate the system in the air because some of the sensors may take 30 minutes to response in the air.

If the system fails to pass “b”, then the TC may be defective.

If the system passed test “a” and “b” then, the problem is the set up. Here is the list of most common problems reported.

a. wrong type of sensor style is used.

For air measurement, the most common problem is a wrong style sensor is used. To measure the temperature of the air, the thermal mass of the sensor has to be small. e.g. the sensor that comes with ebay auction, the TC-K3 is designed to measure metal temperature such as plastic injection mold. In the air, the response time is about 15 to 20 minutes. It is not suitable for control the temperature of air. For air measurement, the sensor should be thin with low thermal mass. Normally, for an oven, the sensor should not be more than 5/32” (or 4 mm) in diameter. A 1/8” (or 3 mm) diameter probe is preferred. For very fast response, you will need to use the “bare” wire thermocouple.

For liquid and air measurement in a container, the length of the probe is also important factor if the sensor is mounted on the wall of the container. Normally, the temperature by the wall is lower than in the center. If the sensor is very short, the mounting on wall will function as a heat sink, it will drain more heater than the probe tip can pick up, making the reading constant lower. Normally, the probe should be at least 4 in (100mm) from the wall. If possible, an insulated mounting, such as plastic mount instead of metal mount, will help.

b. wrong extension wire is used.

If the TC is not directly connected to the controller, the extension between the controller and TC is a common problem.

a) The most common problem is the wrong polarity used. The main reason is that there are too many color codes for the K type thermocouple. Most people think the red wire is positive. For the US market (including the Auber product), there are several different color codes used on K type TC and TC extension wire. The US color code (some imported product also use US color code) is red for negative and yellow for positive. The DIN color code is red for positive, green (or blue or black) for negative. Very often the extension wire has different color code than the thermocouple itself. If you extend the TC with wrong polarity, the common syndrome is that reading is 40 to 60 °F off. When you use a panel mount connector, there will be three pairs of connections: TC to male connector; extension cable to the jack connector; and extension cable to the controller. If you make a mistake in one of these three, the reading will be off. To prevent any mistake, just make sure positive always connect to the positive, no matter what color is used. If you are not sure which is positive, just remember that unless it is red and yellow pair, red is always positive. For red and yellow pair, red is negative.

This is problem is very difficult to figure out unless you are experienced with TC. Many brewing plant need long TC cable for the extension. We had several customers that had hard time to figure out the problem.

b) The second common problem is the wrong type of wire. The extension wire used has to be same K type TC wire or K type TC extension wire. You can’t use regular copper wire.

c. Ground loop and interference.

TC signal is a mill volt level high impedance signal. It is very susceptible to the RF and ground loop interference. The heating element, pump and fan can all produce significant interference to the controller reading. If the reading is accurate until you turn on the heater or pump, you know that is a ground problem. The grounding of a system is very tricky issue, each system might have different problem. Here is a list of some common problems.

a) Ground loop. You should put all the ground wire to a single point, not all over the cabinet.

b) Unshielded TC. The circuit ground of the controller is normally floating. If the TC is unshielded (or not well shielded) and tip is touching a large electrolyte object, the TC can become an antenna that picks up all kind of noise. Normally, the negative input of the TC on the controller circuit board is the circuit ground. You can connect the negative input terminal to a sable metal chassis to function as a stable electric field reference.

5. How to use a switch to select sensor input.


Wiring Example 1. Selecting from two PT100 RTD sensors by a toggle switch – switching all three wires.

Wiring Example 2. Selecting from two PT100 RTD sensors by a toggle switch  – switching two wires.


Wiring Example 3. Selecting from two thermocouple sensors by a toggle switch.

 

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