- December 24, 2009, Kelly Robinson
I received an interesting email question. What is the difference between an electrostatic fieldmeter and a non-contacting electrostatic voltmeter? And, if I had to pick either one or the other, which one is more useful for solving static problems?
I suggest using an electrostatic fieldmeter to begin an investigation of a static problem. Voltmeter measurements are complementary and can be added to the work stream if needed to help sort things out.
Electrostatic fieldmeters are good, workhorse instruments for detecting static charge. Readings should be taken on free spans between rollers and give a good measure of static charge averaged over a relatively large area. Fieldmeter readings are very good for relative comparisons, though it is a bit cumbersome to estimate actual web charge given a fieldmeter reading.
Also, electrostatic fieldmeters are ‘blind’ to a common charge pattern on insulating webs. Often, webs have a positive charge on the top surface and an equal amount of negative charge on the bottom surface. In this case, the field meter will read zero because the sum of the charge is zero. So, fieldmeter can give a false sense of security … reading ‘low static’ even when there is a significant amount of charge … enough to cause static problems.
Electrostatic voltmeters are excellent instruments for getting specific and detailed information on web charge. Voltmeter measurements are somewhat more tedious to set-up and make because readings should be taken on the surface of the web where it is wrapped around a grounded metal roller. The voltmeter probe should be located as close to the web surface as is practical. Usually, I mount the probe using a bracket that is attached either to a rod clamped to the machine frame, or attached to a magnetic base for indicators. One vendor recommends spacing the probe 2mm plus or minus 1mm from the film surface. There is a theoretical basis for this that I will discuss in a future edition of Static Beat, my column in PFFC.
Voltmeters read surface potential over a small area … a spot with a radius that is roughly equal to the probe to film surface spacing. So, voltmeters are especially useful for detecting spots or stripes of charge with a characteristic dimension on the order of a couple of millimeters. For measurements on moving webs or films, it is important to select a voltmeter that is fast enough to see a spot of charge. For example, a voltmeter with a 50 mS rise time (about 20 Hz bandwidth) would be somewhat limited in detecting charge on a moving web. Depending on the web speed, a spot of charge can come and go before it could respond.
A voltmeter detects the relatively common charge pattern where a film has positive charge on the top surface and negative charge on the bottom surface. Since a fieldmeter does not respond to this patter of charge, voltmeter measurements are complementary to fieldmeter measurements.
Fieldmeter readings are easy to make and relatively stable because the average the web charge over a large area. And, while voltmeter readings are somewhat more difficult to make, they give detailed information of the web surface charge distribution and can separately measure the charge on the top and bottom surfaces of the web.
Thanks for the great question. Keep asking! … Kelly R.