Fluke 115 problems?

[Nightcruiser]

Friends of mine arrived this week with a nice birthday surprise

A brand new Fluke 115 with carry case et all but in use I cannot seem to get it to settle down properly before taking any measurements.

For years I used to my old AVOmeter but when using the Fluke as soon as the probes are inserted on the milivolt setting i'm getting wild reading on the screen from 2.0 up to 26 just by my hand capacitance alone!

Even on the D.C. voltage setting the display goes wild as soon as i go near the probes.

Unplugging them gets the meter to zero - mostly- but surely this sensitivity isn't correct?

I don't want to mention it to them as they were very kind to buy it for me but to my mind, this isn't right and others with greater experience of this model may be able to let me know.

John

[ionburn]

If you are not used to a digital meter it will be seem very different but if it measures OK when connected in circuit it is probably OK. If you are sure there is an issue, erhaps a few basic checks. I would try shorting the input using a link connector with plugs. This should obviously always show zero. My next step would be to use known zero resistance leads and touch probes to give zero. Try the Fluke leads on another meter.

When I am looking to find meter resistance (it would have to be high impedance to readily react when not connected) I would use a battery and resistor setup to enable calculation of approximate input resistance.

Compared with an AVO which is so low resistance such to not react in general at all a digital meter will give some wild response, especially if a probe is touched with a finger. A 10MOhm input is far more reactive than 20KOhm per volt.

They are different instruments and each have their fields of advantage, but you will find a digital meter takes a bit of getting used to. Do a bit of experiments to get used to it before you write it off. Due to higher imedance it will show far higher voltages in many situations (back to Ohms law etc).

[Nightcruiser]

Yes it does show zero when linking the inputs but for a meter like this i would not have expected it to be succeptable to stray capacitance or for the leads to be suspect. It does seem very lively.

[high_vacuum_house]

On the millivolt range with a very high input impedance it is most likely the leads are picking up a 50 Hz hum which can easily be induced into the test leads giving phantom readings. Shorting them together should give you 0 millivolts.

To prove that it is working correctly you could create some basic 2 resistor potential dividers just with a battery and some resistors to give say 1mV, 10mV, 100mV volt drop across one of the resistors. With the test leads connected across the resistor the measurement should be stable and repeatable.

Christopher Capener

[Nightcruiser]

Thank you Christopher.

The display was not settling down at all so that is why I questioned whether in operation this unit was actually going to be accurate.

To my mind is no point owning a Fluke ( or any digital meter) if the calibration is way off from new.

I'll try what you suggest and let you know.

Best

[julie_m]

Digital meters have such a high input impedance, they will give false readings unless connected to something. As the input is continuously changing, and multimeter ADCs are slow by nature, it will pick up random amounts.

It's nothing to worry about; random readings can be taken as a sign of an open-circuit input.

[G4_Pete]

Hi Fluke do a training leaflet on this as some of their meters can be switched to low impedance to drain off the "ghost voltages" ( Fluke terminology). Think of it like picking up an insulated wire to an amp input , lots of hum but no "real " circuit.

Fluke training leaflet.
http://support.fluke.com/find-sales/...105317_A_w.pdf

The other one that crops up especially in industry is measuring Ohms on circuits that have noise or have got damp and are generating small voltages galvanically or have residue voltage like that generated by electrolytic capacitors. The low Ohms measuring current of the DVM can also get swamped and give false readings the check here is to measure with the probes one way and then the other if you suspect small voltages are skewing the reading and of course the reading should be steady and the same both ways round.

Hope that helps with the transistion from Analogue to digital.

Pete

[The Philpott]

As suggested this behaviour is only questionable if it persists with the test leads shorted together. The low millivolt settings on digi meters can be somewhat prone to phantom or wandering readings- one of mine will give a small reading when placed next to a mirror.

[Biggles]

A lot of digital meters can be prone to this in my experience. There are stray fields in most locations. Think about the buzz on the speakers when you touch the input of an amplifier. The important thing is that it reads zero when the leads are shorted. It should settle down once the meter probes are connected to something. I would expect this effect to be worse when AC is selected, but I have seen it happen on DC too. I did once have a meter (which may have been a Fluke?) at work which did this after an awful lot of use but it was down to the rotary switch PCB contacts tracking across between pads. It definitely wasn't a new one.
Alan.

[julie_m]

It's the slow ADC that is the problem; it gets confused by rapidly-changing voltages such as random noise. Once there is a steady voltage on the input, the ADC will be able to read it correctly and the display will stabilise.