CEM dt9939 Voltage Readings - Page 1
Forum section: electronics / repair

author: dseyst
author_rank: Contributor
timestamp: July 25, 2017, 01:00:15 pm
content: Many times when I measure dc voltage with the CEM dt9939 the voltage displayed is low, but when I reverse the leads the reading is correct. Does anyone have any idea what would cause this?

author: retiredcaps
author_rank: Super Contributor
timestamp: July 25, 2017, 11:39:06 pm
content: Give us an example of the magnitude. For example, you have a 10V reference.reading 1: 9.80reading 2: -10.00orreading 1: 9.92reading 2: -10.00

author: dseyst
author_rank: Contributor
timestamp: July 26, 2017, 12:45:04 am
content: Reading 1 2.4843Reading 2 -2.4998The negative reading is much closer to the actual voltage.

author: retiredcaps
author_rank: Super Contributor
timestamp: July 26, 2017, 01:15:44 am
content: What is the actual reference you are using so we can calculate the %? Your CEM may be in spec, but we don't know the reference voltage.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 26, 2017, 01:22:32 am
content: Quote from: dseyst on July 25, 2017, 01:00:15 pmInteresting. Many times, meaning that some times you don't see this. Strange. Assuming it's not something in your setup, the spread does seem rather high no matter the absolute value. Of the two I have, with 1V I get about a uV spread on both. These meters have never been aligned out side from the factory. Yours may need to be aligned.These two were bought for my own testing and really don't see much use but I have one that is over five years old now and has seen a fair amount of abuse. It still passes cal.

author: dseyst
author_rank: Contributor
timestamp: July 26, 2017, 02:33:54 am
content: As I type, the meter is measuring a 4.99852 voltage at 4.998 volts. This problem doesn't happen all the time but when it does the only way to get a correct reading is to reverse the leads. Every time I reverse the leads I get a correct measurement. It happens often enough to be a problem. I replaced the battery, tried different leads and took a very good look at the circuit board where I saw no apparent problem and the problem persists. The rub is that I never know when the problem will occur. I also tried removing and replacing the fuses and the wireless module to no avail.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 26, 2017, 03:21:46 am
content: That is very strange. I have no idea and have never seen it with the three meters I have. I've actually ran mine for extended times logging data and they are fairly stable. The one Extech that did not work when I bought it had some other strange problems that others were able to replicate. I am assuming based on your description of the problem that this is not what you are seeing. You are always welcome to review the videos I made of mine showing the problems I saw.Did the meter always have this problem? With the meter attached to a stable source and logging the data for say an 10 hours, what's the distribution look like? Is it bimodal?

author: dseyst
author_rank: Contributor
timestamp: July 26, 2017, 10:58:13 am
content: Joe, I've been watching your videos for quite a while and find them interesting. I actually bought this meter after watching your video. The problem occurs only when I first turn on the meter. I'm not going to lose any sleep over this problem and was hoping that someone had run into a similar problem and knew of a fix but it sounds like this might not be the case.

author: The Soulman
author_rank: Super Contributor
timestamp: July 26, 2017, 11:17:41 am
content: Is it the same when measuring lower voltage's? (200mV or so)

author: dseyst
author_rank: Contributor
timestamp: July 26, 2017, 09:30:30 pm
content: I have never tried to measure millivolts when the meter is acting up. I have been trying to duplicate the problem for two days without success. As soon as the problem crops up again I will try measuments on other ranges.

author: dseyst
author_rank: Contributor
timestamp: July 27, 2017, 12:44:30 am
content: Crap, I hope I'm not replying to myself. Was just measuring watts with my Wavetek 52 and had the dt 9939 measuring dc volts on a cheap Chinese voltage standard on the other desk. The 9939 went to sleep and when I powered it back up by pressing the backlight button, the voltage reading was incorrect. I turned the dial to the millivolts scale and measured 200 millivolts at a power supply and the reading was the same as my Agilent u1252b. I returned to the other desk, turned the dial to volts and plugged the meter back into the Chinese voltage reference and the reading was correct. This is driving me nuts.

author: joeqsmith
author_rank: Super Contributor
timestamp: July 27, 2017, 11:51:09 pm
content: I asked a few other people I know who bought them and no one has seen what you describe. I would be looking at vibration, flex, temperature.

author: dseyst
author_rank: Contributor
timestamp: July 28, 2017, 09:33:18 pm
content: Joe, thanks for the hints. I'll try to find/fix this problem and if I can I'll post the fix. By the way, did you race motorcycles? If I go to your channel and scroll down to the first videos there are a few motorcycle related videos. The reason I ask is that we started racing bikes in the 70's through the 90's, ended up running Pro Mod.