Hi all,
I bought a Fluke 80K-40 HV probe recently because it was cheap and I wanted one for a long time, and had a close look at the manual. Being a regular voltage divider network, the HV probe depends on a 10MOhms input impedance of the meter. I have a Fluke 289 and had a close look at the impedances, and found that the impedance changes as a function of the range:
50mVDC: 10MOhms
500mVDC: 10MOhms
5VDC: 11MOhms
50VDC: 10,11MOhms>= 500VDC: 10MOhms
Volts AC is AC-coupled, I didn't check these ranges yet. More important:
The Fluke 289 manual specifies 10MOhms at all (!) ranges (page 77).Input Characteristics
V 10 MOhm <100 pF
mV 10 MOhm <100 pF
V~ 10 MOhm <100 pF (ac-coupled)
However, the manual of the HV probe states that this is normal:
Specifications
The 80K-40 will achieve rated accuracy when used with a
voltmeter (ac or dc) having an input impedance of 10 MOhm±1.0%.*
Specifications for the probe are as follows:
(...)
Input Resistance: 1000 MOhm
Division Ratio: 1000: 1 (1000X attenuator)
(...)
* The input impedance of Autoranging Fluke handheld digital
multimeters varies as a function of range. The only range that
deviates significantly from 10 MOhm is the 3V (Models 21, 23, 25,
27, 70, 73, 75, 77) or 4V (Models 10, 11, 12, 29, 79, 83, 85, 86,
87, 88) range where the impedance is 11.11 MOhm
. To enhance themeasurement accuracy when using this range, apply a correction
factor of 0.99, i.e. multiply the displayed reading by .99.
My question at this point (because google did not help): Why is that the case?
- If it is a feature, I cannot see why anybody would want that. That 10% increase in input impedance does nothing but throw my readings off. If you give me high impedance, give me proper >1GOhms.
- If it is a bug, I do not understand how a company that has been building handheld multimeters for decades cannot overcome this problem.
- Saying "just multiply by 0.99 sometimes and you'll be fine lol" is a bit weak for a $1000 equipment combination in my opinion.
I would be happy if anybody has some insights to share.
Best regards,
Andreas