Digital multimeter setting time - Page 1
Forum section: electronics / beginners

author: vasilistzo
author_rank: Newbie
timestamp: April 17, 2015, 04:12:07 pm
content: HiyaI hope someone in here can help me out a bit.i m using a yokogawa 7562 digital multimeter to measure the resistance of a linear potentiometer that provides the displacement information of a motorized linear stage.My question is why the multimeter needs a few minutes to give me a stable readout. It starts from a value a few ohm higher than the final and slowly decreases for 3-4 minutes.Cheerz

author: cyr
author_rank: Frequent Contributor
timestamp: April 17, 2015, 04:27:37 pm
content: How large part of the actual value is "a few ohms" ?It could be thermal effects (pot heating up and changing value, and/or offset voltages).

author: Lightages
author_rank: Unknown
timestamp: April 17, 2015, 04:29:11 pm
content: When a multimeter is measuring the value of the device under test (DUT), it is doing this many times a second. Better meters make more measurements per second than lower quality meters. The problem with these individual measurements is that each of them can be slightly different from each because of ambient electrical nose, unsteady hands on the probes making intermittent contact, and the inherent characteristics of the device which also might be a bit noisy.To help give a more stable and easier to read value on the display, the meter will average many of the measurements before displaying the value. In the case of the ohms function, when the probes are not contacting anything the measurement is infinity. The meter will accumulate a number of these infinity measurements and then display them. When you touch the probes to the DUT it see first one measurement of the DUT plus the other averages of infinity. So the first displayed value will be just under the maximum range of the meter, then with each successive measured value being added to the average the displayed value drops to the real value. When all the averages are actually from the DUT, that is when you get the stable reading you expect.

author: vasilistzo
author_rank: Newbie
timestamp: April 17, 2015, 04:40:56 pm
content: Thanks for the answers. To give some more details on this:The Pot is 1k, the multimeter is constantly connected to the pot leads with alligator clips. The pot looks sealed and everything takes place in an air conditioned room without windows and no direct contact with the environment. The DUT is not even touched.The range of the pot is 1000 ohm.A typical example of a measurement goes like this : First measurement is around 395,6 ohm, within a few seconds drops down to 395 and then after 4-5 minutes i get a stable reading of around 393. In all the measurements it follows the same pattern, it starts from a higher value going down to a smaller.

author: Lightages
author_rank: Unknown
timestamp: April 17, 2015, 04:49:08 pm
content: Potentiometers are sensitive to temperature changes and also can creep after being set to a position. The contact point in the pot is small and any current flowing through it can cause a little bit of local heating. This can cause the value to change a little. Your hands can also supply heat to the pot and when you let go the temperature slowly drops back to ambient and can cause the value to change too. Even after you stop touching the pot, the wiper for the resistance track can still creep a little too and this can sometimes show up as a value change over time.

author: vasilistzo
author_rank: Newbie
timestamp: April 17, 2015, 04:55:09 pm
content: Ok then, so it all seems normal.Best thing i can do is to wait a bit before making every measurement.Many many thanks for the enlightenmentCheerz

author: Wytnucls
author_rank: Super Contributor
timestamp: April 17, 2015, 05:38:59 pm
content: Doesn't happen with the 6 1/2 digit Keithley 2000. Immediate steady reading with or without filtering, measuring a Bourns potentiometer at 350 Ohms.Your 6 1/2 digit Yokogawa should behave the same way.Try measuring a resistor of a known value. If the reading takes time to stabilize, contact Yokogawa or attempt to locate someone who has the same meter to find out if it is a normal behavior.