Power from post to EMS remote readout is
Ln1 126; Ln2 125;
Read out on interior panel is
Ln1 111; Ln2 110
Where am I losing the voltage between the two?
Nothing on in the coach except the chassis battery trickle charger.
Is the discrepancy a new condition? Is the 110 Vac an unusual reading for the current coach external power source?
Do you have an autoformer?
Craig
My surge protector EMS showes a different number then the interior panel too. It is higher inside then at the readout by a few points.
Our EMS readout is different than the original analog Powerwatch in the coach, lower by 4 to 5 volts. When checking at an outlet with my Fluke meter, it agreed with the Progressive HW50C EMS. That is, until just now when I checked it again. Now they are all in agreement. The only thing different is that I have been installing a SeeLevel 714 Display up in the are where the Powerwatch is. In that process, I did undo the wired feeding the Powerwatc and retightened them. Maybe the connections were a wee bit oxidized??? Don't know, but I will be replacing it with the second EMS display.
Don
Not sure, but measuring AC takes a calculation as AC is a moving voltage changing from minus to plus voltage 60 times a second. To create a standard for consistency an RMS formula (Root Mean Squared) is used in some metering devices, but there are other ways to measure AC voltage. Progressive Industries EMS does not use RMS and your Foretravel panel may use RMS. Progressive Industries told me that by not using a RMS calculation, they can react faster to an out of line voltage.
I think different meters can measure differently also because of a load on the wire being measured.
When we think nothing is on, we may be surprised of how many devices use current without being "on". Satellite receivers use about the same current on or off, TVs look like they are not on, but some of their electronic circuits are powered watching for the push of the on/off button. Any device with a momentary on/off power button is never 'off', that's why some little two way radios are found with dead AA or AAA batteries even if not used for months. 'Old fashioned' devices with a hard click off switch (separate switch or part of volume control) usually turns off all power and will not use current if left plugged in.
True RMS meters are expensive, and not needed if the AC waveform is reasonably close to a sine wave. Most meters approximate RMS using the assumption that the waveform being measured is probably sinusoidal.
My guess is the different voltage measurements are due to one of the meters being out of calibration.
As an electrical engineer I have long been mystified by a normal 5 to 10 volt difference between my EMS readings and the panel readings with the EMS always being the lower. The differences sometimes are very small. Just from casual observations I have been unable to correlate the differences with what's on/off or other situations.
I have had on my do to list making a break out box to facilitate making measurements between all the legs at the pedestal. I suspect it may have something to do with something in the ground & neutral circuits. Also I would like to understand what both systems use as the reference (ground) when making these measurements.
My interior voltage panel started reading 130-140 volts on both legs one day on both land line and generator. Big scare! Testing voltage at outlets gave me 117 or so. So i got two nice panel volt meters and added them into the space where the incumbent meters are which are simply plugged into a pair of duplex outlets behind the panel, one for each leg. So the new volt meters just plug into the other side of the duplex outlet. I had to unplug the incumbent meters to get all of this in. Plugged it all back in, closed it up, turned on the power and the incumbent meters went back to reading as expected. (?) The new meters read 3 or 4 volts lower on each leg. EMS and my meter in any outlet are usually in the middle. The incumbent meters have a potentiometer to adjust the readings.
As long as they are close, I am a happy camper.
Roger
Harvey, and ALL,
AC voltage and current measurements are notoriously inaccurate and/or misinterpreted for a wide variety of reasons. It is not surprising at all that some owners have higher and others have lower "NORMAL" EMS comparison readings.
First of all, I think that the EMS readout is a Digital Multimeter Avg. readout and most handheld DMM's give Root Mean Square (
RMS) approximations of True AC Voltage and Current values.
But the potential disagreement between the two types of meters goes far beyond that difference.
AC measurement is variously affected by (just to name a few):
- Meter impedance (how much the measuring device "loads down" the source. High impedance paths [oxidized connections, for instance] induce large errors.
- Analog vs. Digital readout
- Resolution and readout settling time
- Accuracy, Repeatability and Uncertainty
- Calibration of the two meters
- RMS vs. True RMS vs. Avg.
- Waveform of the source (1 VDC superimposed on AC source will induce up to 40% of full scale error), (backfed or superimposed 6% spike (harmonic distortion) on a 120VAC source [like from a level measuring digital circuit] will induced an error of 45% of full scale ).
- Crest Factor of the source (ratio of peak to RMS of the source)
- Form Factor of the source (sine vs. triangle vs. square = up to 11% of full scale error)
- Environmental influences upon different measuring devices (a few degrees of difference between inside and outside measurements of the same source can give 2% of full scale, or more, difference in readings)
- Electromagnetic Compatibility of measurement instrument with nearby sources
A good, true RMS (TRMS) meter is the first level of defense (Fluke, DANA, Tektronix, Radio Shack, Simpson, etc.). Few FT owners have one, from my experience. But, one still needs to know much more about the purity of the source wave form, when one starts looking for a few AC volts difference between meters.
The POWERWATCH panel, for instance is a (+/-) 5% of full scale reading analog instrument. So, with a perfectly calibrated meter and a perfectly shaped sine wave source it has only (+/-) 6.5 VAC accuracy. One certainly can calibrate it to be more accurate,
on any given power source, but change to a different power source (different campground, different source utility power factor, different source waveform (gen set doesn't generate a pure sinusoidal waveform for instance), superimpose a one
volt DC or backfeed or induce some electronic noise and the (+/-) 6.5VAC accuracy goes out the window).Second line of defense is comparison (what did it read yesterday or in the last campground?). Changes from "NORMAL" are likely much more important than absolute values.
Measuring AC may not be "rocket science", but it is more complex than many may imagine. Chasing a few volts difference, between various measuring instruments, requires much more knowledge and a much more expensive array of test equipment than the average FT owner generally wants to acquire or pay for. DC is much easier. AC is not for the faint of heart.
Neal
Thanks Neal, since I don't have all the fancy equipment to measure VAC any better than what I have now I will do as you suggest as we all should, look for the unexpected.
Roger
I have a quality true rms meter, and it agrees within half a volt with my kill-a-watt. The ems panel reads 4 volts high. One of these days I plan to play with the cal potentiometers.