About the Radio-Shack (Tandy) 22-175b multimeter

On the lower capacitance scales, it _shouldn't_ be zero. The test leads have a certain amount of capacitance.

If you've removed the leads, a residual reading might be due to internal capacitance, or the converter's LSB flopping.

In any case, if you're reading microfarads, 0.1nF (100pF) is hardly of any concern.

Hello

Even without tests leads there is a .1 nf reading, I want to use it for low value cap testing (5 to 50 pf), so it's kind of problematic.

Thank

Bye

Gaetan

"William Sommerwerck" ( snipped-for-privacy@comcast.net) writes:

Okay. Does it have a capacitance scale _intended_ to read 5 to 50 pF? I doubt it.

Do you have a basic understanding of the terms "resolution" and "accuracy", and how they relate to measurements?

measuring,

is

have

any

My Fluke 189 is .065 without leads. Of course I can zero it by pushing a button. The leads add 20-30 pf depending on the leads. You can't use leads for that low of a value. You need to insert the caps directly into a stable fixture. Make thing to plug into the meter and have a clip or something.

greg

1. Don't top post.

1. You cannot use a Radio Shack meter for any precision measurement. Get a proper meter and you'll not have this problem.

Hi For such low capacitances, there are is only one method that I've found that works well. Find or make a coil that when used with you caps to be measure, will resonate in the 2 to 10 MHz range some place. Use a few reference caps of known value and tolerance. These should be close to the values being checked. Use a grid dip meter to measure the frequency and back calculate the values of the caps. Do remember that smaller values can be made by stacking ( or series wiring ). If you wind your own coil, try to leave at lease one or two wire widths between each turn. This will minimize the effects of internal capacitance of the coil. This is easiest done by winding wire two or three wires and then removing the extras while a fixing glue is setting. I do have a bridge that I get reasonable measurements for around 50 pf but anything in the 5pf range is not reliable.

This is not true. It has nothing to do with "precision".

Using a simple digital meter, you can't make accurate measurements of capacitors whose values are comparable to the lead capacitance. You need more-specialized test equipment.

Huh? No RS product is going to be noted for accuracy. Low price, maybe, low quality probably, but not high accuracy.

I'll agree with that... The OP is lookng for the impossible.

A chain is only as strong as its weakest link... That grid dip meter needs to be accurate, and you need to know the inductor's value accurately as well.

is

have

As long as your reference parts are on the same scale of the grid dip meter, you should be able to ignore the coils actual value. One should be able to interpolate the grid dip scale. If more accuracy is needed, a frequency counter can be used. In which case, your readings would most likely be as good as the reference parts. Your not depending on the absolute accuracy of the meter or knowing the actual value of the coil, that is why you use reference parts. Dwight

Hello

No it's an auto ranging.

Why ?

Thank

Gaetan

An auto-ranging meter still has ranges. What is the lowest capacitance range?

"accuracy",

If you understood these things, you probably wouldn't be asking this question.

is

have

any

With that I am sure we are well beyond the capabilities of the OP!

What is the spec for measuring capacitance?

With a decent ;-) general purpose meter like say a Fluke 110 the maximum resolution on capacitance is 1 nF, so a 0.1 nF error is neither here nor there.

--
*Why is it that most nudists are people you don't want to see naked?*

    Dave Plowman        dave@davenoise.co.uk           London SW

Hello

They say in the booklet a resulution of 1 pf, well it seem that it's not true.

I will look for another meter for real 1 pf resolution.

Thank everybody.

Bye

Gaetan

That spec means that on the lowest range, the least significant digit is pF. Even if the meter reads 0.1 nF with no capacitor connected, you may still be able to measure capacitors down to 10 pF or so. Just note the "no capacitor" reading, and subtract that from the reading with a capacitor connected to get the actual value of the capacitor.

--
Peter Bennett, VE7CEI  
peterbb4 (at) interchange.ubc.ca  

This is normal for your meter. There will always be some residue capacitance.

Take the reading with the leads opened up. Then when taking a reading subtract the difference. The problem will be that even the physical position of the leads will change the readings a little.

For capacitors in the value range that you want to read, you really need a high end cap meter. Check out the higher end of the Fluke DVM's with the capacitor option. With these meters, you will have a much higher resolving and stability. But, when reading caps below 200 pF, you have to consider the wires. It is best to use very short wires something less than 4 inches with small clips in them.

For example, with a Fluke 89 series, they have excellent capacitance performance. There are also dedicated capacitor meters which are the best.

You can also set up a cap tester using a signal generator and a scope. If you want to do the math, you can work out a an RC network where you feed in a frequency and amplitude calibrated signal. You then use a dual trace scope and plot out the phase and amplitude differential. With some calculations you can then have a very precise reading of the cap.

--
JANA
_____

What you don't understand is that resolution has nothing to do with accuracy.

One can also make a capacitance bridge from easily obtained components; as a kid I made one in a cookie tin that served me well even for values in the

10s of pf for many years. Now I rely on a Sprague Tel-Ohm-Mike for the single digit pf measurements (a better capacitance bridge).

Regards,

Michael