Author Topic: MTU & Spyder calibration  (Read 2694 times)

Y0tsuya

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MTU & Spyder calibration
« on: December 26, 2014, 08:09:43 AM »
How much variation are people seeing between individual CTs and MTUs?  It seems I'm getting measurement errors of +/- 20% across the board.  I don't know whether I should be calibrating each MTU and spyder input one-by-one if it's CT variation or if I should use a uniform adjustment for all inputs if it's a variation between MTUs and spyders.
« Last Edit: December 26, 2014, 11:54:23 AM by Y0tsuya »

pfletch101

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Re: MTU & Spyder calibration
« Reply #1 on: December 26, 2014, 07:54:56 PM »
I have compared what my (uncalibrated) MTU CTs record with my utility meter records and my solar inverter records and have never seen discrepancies of more than a percent or so. I have no corresponding independent way of checking on my Spyder measurements, but the increments and decrements measured by the main MTUs seem to correspond within no worse than a +/- 5% range with the Spyder readings when their larger monitored loads turn on and off. You should not need to calibrate out errors of the magnitude that you describe. Something is wrong!
Peter R. Fletcher
TED Pro Home - main MTUs monitoring utility and PV Solar feeds; 2 Spyders monitoring selected individual circuits

Y0tsuya

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Re: MTU & Spyder calibration
« Reply #2 on: December 27, 2014, 12:54:05 AM »
Here's what I'm seeing.  For example I have a spyder and a MTU measure the same line and the spyder reading is consistently 20% greater than MTU reading.  Then another example I have a spyder and a kill-a-watt measuring the same water heater and kill-a-watt is 10% greater than spyder reading.

Attached illustration.

pfletch101

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Re: MTU & Spyder calibration
« Reply #3 on: December 27, 2014, 05:49:28 AM »
A couple of thoughts:

MTUs are generally paired, with one CT on each phase of a 240V supply. Spyder CTs are inherently single-phase devices. In your sub-panel supply instance, it is perfectly possible, If the MTU CTs are both installed on the sub-panel supply but only one phase is being monitored by the Spyder CT (and its reading presumably doubled to get the notional total), that both readings are correct - there is simply an imbalance in the draw from the two phases on the sub-panel. Other than this possibility, I cannot think of a way that a Spyder could be reading higher than a MTU on the same balanced circuit without one or both of them either being defective or misinstalled.

In your water heater instance, there is another possibility. If you are using the two devices to look at instantaneous power, then they should read the same. If you are looking at energy consumption over a period, and the water heater draws low-level power some of the time (presumably for something other than heating water - e.g. controls, or a pump), the Spyder may be ignoring that current, but the Kill-a-Watt will be correctly including it in its accumulation. All Spyder channels ignore (treat as zero) current/power below a certain level - the level depending on the rating of the CT.
Peter R. Fletcher
TED Pro Home - main MTUs monitoring utility and PV Solar feeds; 2 Spyders monitoring selected individual circuits

Y0tsuya

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Re: MTU & Spyder calibration
« Reply #4 on: December 27, 2014, 06:52:33 AM »
I forgot to add that I indeed monitor both phases, since they're inherently unbalanced.  The MTU at the subpanel presents one single value to me, and I have to add 2 values on the main breaker spyder and see if they match.

Kill-a-watt has been said to be inaccurate under certain conditions, particularly under inductive loads (though the water heater is a resistive load).  In this case I would like to trust the spyder readings more, but since the spyder readings don't match the MTU reading, I do not have a baseline from which to perform a calibration.

pfletch101

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Re: MTU & Spyder calibration
« Reply #5 on: December 27, 2014, 07:50:09 PM »
I don't know how well Spyders deal with inductive loads, either, which could conceivably be relevant to your sub-panel discrepancies if there is a substantial inductive component to the loads on the sub-panel, but I would otherwise go back to my original feeling that you should not be seeing discrepancies of the magnitude you are reporting - you need to find out what is going on rather that try to compensate for them.
Peter R. Fletcher
TED Pro Home - main MTUs monitoring utility and PV Solar feeds; 2 Spyders monitoring selected individual circuits