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Paul, Electrician
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Experience:  23 yrs as an electrician, self employed contractor, municipal inspector
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how do I use a digital ohm tester to take resistance readings

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how do I use a digital ohm tester to take resistance readings of 120 VAC / 60 Hz Superior Pro-Heat radiant floor heating cable? The directions require out of the box before installation testing of cable black to white, black to green, white to green, and sensor wire. How do I do this? I have a Sperry DM-210A digital multimeter. Thanks.
To use the OHM setting on your meter, insert one lead in the slot with the icon that looks like a horeshoe, and the other into the COM slot. Then turn the dial to the OHM (again, horeshoe icon) setting, 20K should work ok for your particular reading. Then place one lead on black and the other on the white. You'll get a reading in "ohms" which is resistance. Repeat for each set of wires as the manual requests.. black to green, white to green, etc.

Regards,
Mike
Customer: replied 7 years ago.
Hi Mike - Thanks. XXXXX touch the leads to the black and white I get a reading of .05. Does this sound correct? When I touch the leads to black and green or white and green it just displays "1" (i.e., no change in the reading). Does this sound correct?
There is no "correct" reading other than what the manual for your product specifies the resistance should be. Is the cord you are testing connected to the unit? Does your manual specify what resistance reading you should be getting?
Customer: replied 7 years ago.
Hi Mike - The cord is not connected to the unit. It is still on the spool right out of the box. I'm supposed to take resistance readings of the cable out of the box before installation and then take resistance readings of the cable after the cable and sensor are fastened to the floor and then after the floor covering is installed. The manual is not clear on what the resistance readings should be. I'm using a spool to cover 25 square feet so there is 118 feet of wire. A chart in the manual indicates that the resistance range is 41-51. The label on the cable indicates that the resistance range is 44-55. Does this help?
Is it specifically saying that you should take a resistance reading from "black to white", "white to green", and "black to green"? Typically, you would want 0 (some meters indicate 1 as "infinite") or "infinite" resistance between those conductors unless they are connected to equipment. I'm wondering if it is asking to you check the resistance of each wire individually by placing one lead on one end of the black wire and the other lead on the other end of the black wire and repeat for each wire. Does this sound like what it's asking you do perhaps?

Try turning the dial to the "200" setting since we are looking for a low range number here and test for resistance both ways if you're unclear and tell me what you get.

Customer: replied 7 years ago.

Hi Mike - The manual says to use a quality digital ohm multimeter "able to measure up to 20,000 ohms. To check for breaks, it says "Measure resistance between the black and white cable leads. This measurement should be within the cable resistance range shown on the nameplate label. Measure between the lead wires of the floor sensor. This resistance varies according to the temperature sensed in the tip. A cut or break in the wire is indicated by a resistance of "infinite" ohms (no continuity)."

 

To check for breaks, it says "Measure resistance between the black and green leads and between the white and green leads. These measurements should be "infinite" ohms (no continuity). A cut or pinch in the wire is typically indicated by a resistance value less than the cable resistance range."

 

So, when I touch the ohm meter leads to the black and white wire with the ohm meter set at 20K I get a reading of .05. When I touch the ohm meter leads to the black and green and white and green wires I get "1". "1" is what is displayed in the ohm meter window before I touch the leads to any wire.

 

Am I getting the readings I should get with the cable fresh out of the box?

Frankly, I am confused as to what the manual is asking you to do. You should have 0 ohms from black to white and black to green if you are just measuring resistance from an unconnected cable. Another potential issue here is the meter you are using. While Sperry meters are certainly acceptable for typcial home use, I can't speak for their accracy for resistance tests like you are attempting to perform. That being said, I will "opt out" in hopes that maybe another expert will better understand what it is you need to do here; perhaps they will have a better working knowledge of your particular meter as well. Good luck!

Regards,
Mike

Edited by Mike Pettinato on 11/23/2009 at 4:59 PM EST

Hi

 

Mike asked me to help you out.

 

What the manufacturer wants you to do, is verify that the cabling is in good shape before it gets installed and then to recheck it before you install your tile or flooring.

 

So the first step will be, to make sure there not any readings from black to ground or white to ground and that you have a reading between black and white.

 

Does the reading between black and white equal the resistance marked on the product lable?

Customer: replied 7 years ago.
Hi Paul - Thanks. It appears that I'm not getting any readings from black to ground or white to ground. So this appears correct. The reading I get between black and white says ".05" ohms. The product label says the resistance range (ohms) of the cable is 44-55. I have the ohm meter set to 20K, so is ".05" an ok reading? Maybe there isn't any acceptable reading just as long as I maintain a .05 reading throughout the installation and I don't exceed 44-55 ohms once it is connect to power?

Yes

 

If you set the meter to 1K or auto you will see the reading is closer to the tag.

 

After you install the cable check it again to make sure the readings did not change.

 

Let me know if you need more help.

 

Paul

Paul, Electrician
Category: Electrical
Satisfied Customers: 805
Experience: 23 yrs as an electrician, self employed contractor, municipal inspector
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