Why don't Digital Ohmmeters have a zero initialization? - Page 1
Forum section: electronics / beginners

author: Hobby73
author_rank: Regular Contributor
timestamp: December 29, 2018, 12:10:20 am
content: My old analog multimeter had a control knob to zero the meter before measuring resistance. My understanding is that this adjustment would account for resistance in the test leads/probes, as well as changes in battery life.Why don't DMM's have this function? I can imagine switching out test leads with different resistance that would seem to impact the reading, especially for very low resistance ranges.

author: IanB
author_rank: Super Contributor
timestamp: December 29, 2018, 12:12:14 am
content: Many of them do have this function. It is usually on the button marked with Δ or Rel.

author: glarsson
author_rank: Frequent Contributor
timestamp: December 29, 2018, 12:27:09 am
content: Common analog multimeters didn't have the resolution needed to see the test probe resistance. What they absolutely needed was a way to adjust the zero point as it varied substantially with battery voltage.

author: David Hess
author_rank: Super Contributor
timestamp: December 29, 2018, 12:28:25 am
content: The state of the battery had a major effect on the ohms calibration so analog meters had to have an adjustment. But the constant current source used for ohms conversion in a digital meter is so accurate that this is not required.As IanB wrote, the relative function can be used to remove the lead resistance if necessary.

author: wasyoungonce
author_rank: Frequent Contributor
timestamp: December 29, 2018, 01:38:11 am
content: And old analog meter used moving coil as the meter which needed adjustment to zero.We used to put them on amps and short across the probe inputs to make the meter coil form a reactive magnetic field if it moved during transport. This field would dampen the coil bouncing around. They were/are sensitive instruments.Moving coils are still the easiest best way to measure pots etc for contact IMHOSent from my iPhone using Tapatalk

author: IanB
author_rank: Super Contributor
timestamp: December 29, 2018, 01:51:23 am
content: Quote from: wasyoungonce on December 29, 2018, 01:38:11 amTrue. When you switch an analog multimeter to the off position it puts a dead short across the coil to damp the pointer. You should always turn such a meter to off before putting it away.

author: Brumby
author_rank: Unknown
timestamp: December 29, 2018, 03:31:29 am
content: Quote from: IanB on December 29, 2018, 01:51:23 am+1If you want to see the difference this makes, get your old analogue meter and put it on a volts range and (gently) twist it back and forth sideways (in the plane of the needle movement). Take note of the range of movement of the needle. Then switch it to the off position and repeat the twisting action. The needle movement will be greatly reduced in a properly configured meter.You can do this same experiment with a bare meter movement. If there is a shunt across the terminals (for higher current measurement) disconnect it, then perform the twist test and note the needle movement. Put a short across the contacts and twist again. Much less movement.This is an electromagnetic brake and it helps prevent the needle from being flung across the movement range and getting damaged - all without any external mechanical restraint.

author: bsfeechannel
author_rank: Super Contributor
timestamp: December 29, 2018, 04:43:02 am
content: The reason why you have to adjust for zero on analog ohmmeters is related with the way they measure resistances. Below we have a simplified schematic.For you to measure RU, you need to know VBand RK. However, batteries change their voltages as they discharge, so they are unreliable references. To mitigate that you have to calibrate your meter to the actual voltage. You do that by shorting out the leads so that VB= VR. Then you adjust to zero ohms which is in fact the full scale of the galvanometer.RKchanges with the range, which means a different load. Since the battery has internal resistance, it will show a different voltage for every range. That's why you always have to adjust for zero every time you change the range. This ends up taking into account the resistance of the leads, but that's not the main reason why they need that calibration.Digital meters on the other hand use precision reference currents and voltages, and perform automatic zeroing for each sample, so you don't need any kind of manual calibration, just an occasional compensation for the resistance of the leads.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 29, 2018, 10:01:53 am
content: Quote from: IanB on December 29, 2018, 01:51:23 amIt depends on the meter - quite a few don't even have an off position. The extra damping of the meter is usually not a big deal: the damping is not a big deal when it comes to possible damages from shock - electric damping can be important to make the reading stable and not overshoot too much. But this would be more by having an extra capacitor in parallel to the coil, of an extra shorted turn / conductive frame.The Ohms adjustment usually found in analog DDMs is due to the batteries, though in some ranges it could also compensate for lead resistance.