AC vs. DC ohm meter

[Delbert]

I?ll try to make this short. Hopefully it?s worthy of this forum. I teach aircraft systems to aircraft mechanics. My question revolves around the heat sensors installed along the ducts carrying hot air off the engines used for anti-icing ans such. I totally understand how they work, and to quote the manual:

The leak detection sensing elements consist of two coaxial wires with the space between them filled with an electrically insulating salt. In case of local temperature increase, the salt becomes conductive and the resistance between the wires drops suddenly. The outer conductor is grounded while the inner conductor is supplied with AC voltage by the AILC for resistance measurement. The controller detects the change in insulation resistance. When loop resistance between the outer and inner conductor decreases below the setting point, the controller posts an EICAS message. When the temperature decreases below the element setting point, the resistance returns to the normal valve and the message is removed.

Pic

Now for my my question. When trouble shooting these sensors the manual says:

CAUTION: Improper testing can cause permanent damage to the heat sensing system and could void the manufacturer?s warranty (Fenwal). When testing sensing elements, an AC Ohm meter MUST be used to avoid any damage to the Element. A DC meter might alter the conductance of the conductive salt inside the element and change the Temperature Set point.

So much for keeping it short, but if anyone can explain why using my good ole Fluke VOM will change the properties of these eutectic sensors I?d appreciate it. This has bothered me for years.

 

[Navid]

I can only speculate that applying a DC voltage can have an electrolysis effect and cause chemical change to the material.

If you apply a DC voltage to two electrodes inside a glass of water, you will get Oxygen and Hydrogen from the two electrodes. This is because water is made up of those two elements. The DC voltage forces the positive element to move to one electrode and the negative element to the other. This modifies the water. In this particular example, the two elements are gas and move away and the remaining water is unchanged. However, in case of the substance you are working with the change may have a damaging effect on it.

Edit:
An Ohm meter applies a voltage to the two terminals and measures the current flowing through. The resistance is then calculated by finding the ratio of the applied voltage to the flowing current.

 

[Delbert]

I will admit electrolysis is not something I had considered. However typically we would only be applying the dc meter to the sensing elements for a few seconds at a time trying to find the one section that was shorted. I could understand how using a megger could damage the semiconductor perhaps, but I don?t see how a couple seconds of small DC current is so different than the small (5V) AC current that is applied to the center conductor all day long. The salt reaches a ?melting? point and conducts. When it cools it crystallizes and stops conducting. The average technician wants to find the fault and return it to service. Instead they want us to use an LCR meter, do some math and see how many microsiemens of conductivity there is in the isolated segment. I would just like to give them a valid explanation.

edit: Wow, after almost 5 years I am about to reach 1000 posts!

 

[Navid]

I hope my post does not imply that I have experience with this particular material! I said I speculate, didn't I?

What is the frequency of the AC voltage used on this sensor? I mean when the sensor is operating. Not, when you are testing it.

What is the voltage applied to the operating sensor?

What is the brand name of the sensor? Who is the manufacturer?

Do you have a datasheet?
Do you have a link to the warning about the requirement for using AC Ohm meter? I saw the text in your original post. Can you provide a direct link to it instead of the cut and paste of the warning?

Do you know the formulae for the salt used inside the sensor?

Do you have any other information that can help us figure this out?

Edit:
In school, they told us that when you use a magnet to magnetize a piece of metal, you practically arrange the molecules so that they all face the same direction. As a result, their magnetic forces add up and the piece acts like a magnet.

If you wanted to take the property away, you could use a hammer to bang on it, which resulted in the arrangement of the molecules to break down.

Applying a DC Voltage to a substance may have a similar effect. Not magnetically; but, electrically.

As you said, as far as the passage of current through the substance goes and the energy transferred to it, there is no difference between AC and DC (of the same RMS voltage).

But, there may be a difference in the arrangement of the molecules. An AC current changes direction several times per second depending on frequency (hammer) while a DC current always is in the same direction.
Note: I am thinking aloud again! Do you have any more info you can provide about the sensors?

 

[Delbert]

The magnetizing effect differences between AC and DC is something I wondered about.
This ?Fenwal? sensor has been used on aircraft for probably close to 50 years. Maybe more. I am off to teach class as we speak. Here is a link to a manufacture of some test equipment approved for our use. It has a small description of their properties.
link
edit:
<-Golden!
Oh and I want to make sure you know I appreciate any ideas and discussion Navid.

 

[CycloWizard]

I'm also speculating here, but I would have to think it has something to do with migration of the salt. It will tend to move in one direction due to the flow of current. If you only tested it for a few seconds with a DC meter once, it would probably have no ill effects. However, if you do it repeatedly, then the migration would continue each time and eventually become significant. With AC current, the alternating current would negate this effect.

 

[bobsmith1492]

When you apply an AC voltage, it is measuring the impedance due to the capacitance of the system; a DC checker just measures the "regular" resistance. AC tests it as a dialectric, while DC tests it as a resistor.

 

[Navid]

Originally posted by: bobsmith1492
When you apply an AC voltage, it is measuring the impedance due to the capacitance of the system; a DC checker just measures the "regular" resistance. AC tests it as a dialectric, while DC tests it as a resistor.

The question is why DC voltage is destructive and AC voltage is not.