Does anyone know any good substitution for Fluke 845AB Null Detector?
I think Tegam has a replacement meter.
Also, the HP 3458A (And Several other Meters) have a null measurement function.
Mike.
Check out Fluke article "Self‐Calibration of the Fluke 752A Reference Divider using the Fluke 8508A Reference DMM in place of the Fluke 845A Null Detector".
I would not recommend getting the Tegam AVM-2000 Null Meter.
Do you have a link to that? I can't seem to find it.
I agree with beadwork, we have shifted that workload to a Fluke 8508A Reference Multi-meter as well.
This link has many great resources, you will see the articles you require listed under Electrical Calibrations http://us.flukecal.com/category/literature-type/articles-and-education.
Quote from: beadwork on 05-29-2013 -- 06:33:41
Check out Fluke article "Self‐Calibration of the Fluke 752A Reference Divider using the Fluke 8508A Reference DMM in place of the Fluke 845A Null Detector".
I would not recommend getting the Tegam AVM-2000 Null Meter.
The idea of using any digital multimeter (DMM) as a null detector is flawed by the fact that DMMs contribute input bias current and pump-out noise to the measurement, not to mention how frustrating it can be attempting to null a manual device, such as a Fluke 752A, while watching the digital readout bounce around like a ping-pong ball. The Fluke application note mentioned is a good one, but merely offers users a band-aid workaround when what the industry really needs is a good, decent null detector.
I have no specific idea what the poster meant by his comments regarding the Tegam AVM-2000, but would add that in my limited experience with same, I found it somewhat trying when attempting to use it to perform the 752A Self-Calibration routine.
Hi I have used the 3458A in a differencial mode with excellent results.
In a pinch, I've used a HP 34420A nanovolt meter in place of an analog null meter, though I have heard from others that a Keithley 2182A works better for this application...
The AVM-2000 is about as rugged and reliable as a wet matchbox... Everyone I have ever worked with failed.
Quote from: Squidley on 06-25-2013 -- 08:23:46
The AVM-2000 is about as rugged and reliable as a wet matchbox... Everyone I have ever worked with failed.
RUN..., don't walk away from them!
34420A works but you have to do a coarse and fine setup
as you get towards a null. We use it with metcal to capture the readings from the 752A
cal.
Karl
Karl, believe me or not, but if you're using a 34420A as a detector with a Fluke 752A, your not getting the true picture (I'm in a position to know!). The 752A MUST work into a high impedance, and the 34420A, along with some other design oversights, does not satisfy that requirement. If you don't believe me, borrow a good working Fluke 845AB (if you can find one) and make comparative measurements. I'm certain you'll find differences, especially on the 100:1 ratio and in varying amounts, based on the individual characteristics of the 34420A. Trust the readings from the 845AB and shuck the rest.
My Fluke 845AR randomly wanders approximately +/- 1/4 microvolt on the most sensitive ranges. This happens with the meter in either zero or operate mode. I suspect this is a common problem. Has anyone heard of an effective fix?
Thanks,
Joe
Ex-Navy,
My first thought is Thermoelectric Voltages.
http://www.callabmag.com/?p=2180
Better cables and connections
Thanks for the replies.
Some additional information:
The meter wanders when the input switch is in zero position as well as in operate with a copper wire across the input terminals. No external thermals or cables are involved.
A HP 419A null meter does not wander like the 845AR does.
I have read the article by Martin Kidd about thermal voltages.
I suspect the problem is in the 845A input chopper or amplifier, but I have no experience repairing these types of circuits. The Fluke manual is for an early version of the 845AR; mine appears to be an exceptionally clean, late version.
The best of season's greetings to all,
Joe