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Transformer %IZ Change

Transformer %IZ Change

Transformer %IZ Change

(OP)
We have two transformers at a hospital that were recently tested by the manufacturer. Both are 5 years old and are 12.47kv-480v delta/wye. the 5000kva unit's Percent Impedance test revealed an average of 9.5 ohms and a calculated X/R of 1.6. Nameplate is 6.07% and typical X/R is closer to 9. The 3000kva unit also tested high, but not to the same extent. The manufacturer recommended replacing both units.

What could have caused this change? Neither has been subjected to a significant "thru-fault".

RE: Transformer %IZ Change

Who carried out the tests?  The manufacturer?  If so what is their reasoning?  Sounds like a Bill Gates Wannabe!

RE: Transformer %IZ Change

Percent Impedance is a factory test and is stamped on the nameplate. This value should never change unless the transformer is rewound.
You have stated "Percent Impedance test revealed an average of 9.5 ohms " where this value should be in percent.
I am also curious as to who did the test and where the results came from.

RE: Transformer %IZ Change

(OP)
The manufacturer did the test and I can't get a real good rationale for the replacement, or why the impedance changed. They say coils could have been physically deformed, as by a heavy secondary short circuit, which has not occurred.

The 9.5 is not ohms as stated: it is the % impedance derived from their test. They tested each coil separately and I'm not sure what device they used.
The secondary coil was shorted and they applied a voltage to the primary side. The results on the center phase of the 5000kva unit is as follows:
pri. volts: 27.41
pri. amps:  3.0
sec. amps:  135.4
watts:      40.0
calc'd Zbase: 93.3 This is on a single phase basis as that's the way it was tested.

Pri impedance is calc'd by   27.41/3.0 = 9.14 ohms
% impedance calc'd by     9.14 X 100/93.3 = 9.793%

Thanks
Jim Wilson

RE: Transformer %IZ Change

I would not consider changing out the transformers until I saw the results of a set of typical maintenance tests(megger, TTR, Doble PF).
The test procedures for the %Z are interesting. I believe it is usually tested by shorting the secondary and applying a low but increasing  voltage on the primary until the secondary winding carries rated current. The percentage of the primary voltage used compared to the full rating is equal to the percent impedance.
I'm  very curious about the test results since they applied just 0.2198% of the primary voltage. (By the way, are you sure the tap setting is on 12470/480V?)
I would expect that excitation and core loss values at low level of input voltage might account for a higher impedance, but I'm not sure from where the test procedure came. Do you have any references for the test procedures?

RE: Transformer %IZ Change

I think you need to see the rational for the test proceedure.  If the transformer is 5 meg there should be a record of the original factrory test.  I would ask to see that as well as the "test" they just did.
Did the same people provide the switchgear downstream from the transfromer?  At 6.07% you have close to 100KA of fault current available.  Prehaps they sold you some 65 KA gear and want to get that transformer out of there.
5,000 KVA is big for a 480 volt transfromer, what kind of switchgear do you have connected to it?

RE: Transformer %IZ Change

The most likely thing to cause a change in impedance is a heavy thru fault which could distort the windings.  There is something wrong with the X/R ratio.  The original X/R of 9 and Z=6.07% would give a resistance (at 12.47 kV) of 0.208 ohms and a reactance of 1.876 ohms.  The new X/R of 1.6 and Z=9.5% would give a resistance of 1.566 ohms and a reactance of 2.505 ohms.  Resistance increased 750%.  Maybe you have a bad connection internally that developed a high resistance.  This high resistance would cause heating could be confirmed by a dissolved gas analysis (DGA).

What was the reason for the manufacturer's test?  Impedance testing is not a normal maintenance test.  DGA is a more common maintenance test that can be done without taking the transformer out of service.

RE: Transformer %IZ Change

Were the tests done on more than one tap?  I suspect that the tapchanger may be part of the problem, as the resistance value appears to have changed radically from the original.  
From the measurements given, the value of resistance is derived as follows -
VA = 27.41*3.0 = 82.23
PF = (W/VA) = 40/82.23 = 0.486
R = Z*PF = 9.14*0.486 = 4.44 Ohms

From nameplate test value, Z = (Zpu)*(kV^2)/MVA = 0.0607*12.47^2/(5.0/3) = 5.66 Ohms
Assume X/R = 9 :
tan(phi) = 9
cos(phi) = 0.110
R = Z*cos(phi) = 5.66*0.110 = 0.623 Ohms

Could contact resistance in the tapchanger be the culprit, at least partly?  Also some questions need to be answered re the test method - where were the voltage drop measurements taken?  How long were the test cables?

RE: Transformer %IZ Change

jwilson3 said:

The secondary coil was shorted and they applied a voltage to the primary side. The results on the center phase of the 5000kva unit is as follows:
pri. volts: 27.41
pri. amps:  3.0
sec. amps:  135.4
watts:      40.0
calc'd Zbase: 93.3 This is on a single phase basis as that's the way it was tested.

Pri impedance is calc'd by   27.41/3.0 = 9.14 ohms
% impedance calc'd by     9.14 X 100/93.3 = 9.793%


The impedance test should circulate rated current in the primary (231.5A).  Zbase using 1ø quantities is (kV ø-g)²/(1ø MVA) = 7.2²/(5/3) = 31.1 ohm, same as using 3ø quantities.

When testing one phase at a time, measuring power of P12, P23, P31 and volts of E12, E23, E31,

Impedance watts = 1.5·(P12+P23+P31)/3
Impedance volts = 0.866*(E12+E23+E31)/3

See ANSI/IEEE C57.12.90-1987, IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers, Section 9.2.5.

RE: Transformer %IZ Change

(OP)
Thanks for all the great comments. I'll try to answer the questions you've raised.

To DanBel: I agree with you about not rushing to replace the units. The TTR and PF tests showed not turns ratio change and the PF indicated good insulation. (Note: These are dry type xformers). You're right about how the impedance is tested in the factory and the field tests are done at a fraction of rated values because they use smaller test units I guess. The taps are in fact on the 12470v-480v setting. Unfortunately I did not get any details of the test procedures.
To BJC: The main secondary breaker and transformer are of the same manuf. Current limiting fuses are in series with the breaker to cover the 100KA. You are right about a 5MVA transformer. If I'd designed the system I'd have split the load up and used two transformers.
To jghrist: I don't know the reason for the % Impedance field test. I agree with you. This is a dry xformer so a DGA test can't be done. How about a partial discharge test?
Referencing your calc's, rated primary current in the coil is 133 amp which is line current/1.732 or 231/1.732.
I came up with the same Zbase as you (31) until review of GE Bull GET 3550F. Your equation is correct, but the phase voltage of 12.47kv must be used;  (12.47)^2/5/3 = 93.3
To peterb: test on only one tap; the 12470 tap.

RE: Transformer %IZ Change

Suggestion: Considering the price of transformer replacements and transformer test results, it would be better to have the transformers retested by an experienced firm with some good transformer testing record that would imply a testing procedure in compliance with an industry standards, e.g. ANSI/IEEE C57.12.90 or applicable one to the transformer type (it still needs to be posted).

RE: Transformer %IZ Change

I think they were trying to do a power factor test.  Hence the recommendation to replace the transformer.  
I would need to see a test diagram to determine weather or not "%Z" could be determined the way they went about it.
Were the primary coils tied together in a delta configuration or were they disconnected for the test?  Was the secondary connected L1 to X0 or how.  
Does the transformer have CTs on it and were they in place during the test?
I would like to see the test set up.  I am always eager to learn something.

RE: Transformer %IZ Change

(OP)
To BJC: They did a power factor test separately and the insulation tested OK. The impedance test was done on each phase separately. They shorted the secondary L1 to X0 and applied voltage to the primary. They didn't raise the voltage until they got rated secondary current as is done at the factory. They used lower levels for a field test, which is OK, but probably not as accurate. See my 19 May post that gives the readings for one phase. I think the methodology is OK, I just question the results.
No CT's.

RE: Transformer %IZ Change


Interesting situation.  Have DC winding-resistance and excitation/iron-loss tests been conducted and compared to original to confirm or contradict the tester's recommendation?
  

RE: Transformer %IZ Change

(OP)
To busbar: No DC resistance test, although another person has suggested this. No specific iron loss test, but excitation test was done. Nothing out of the ordinary was noted.

RE: Transformer %IZ Change

What is the manufacturer's "technical" reason for recommending replacement?

Have you made inqueries regarding the manufacturer's other customers and like installations?

RE: Transformer %IZ Change

Question: What is the transformer cooling system?

RE: Transformer %IZ Change

Hi jwilson3

I don't know who has to pay for the transformers replacement?Probably you.If it was under warranty,I doubt it,if the vendor would be so quick in condemning the transformers.(from the info you provided)
They are 5 years old,they gave you no problems until the tests and it appears from your information the Transformer (Z) has changed from I don't know what to what.The 6.9% seems pretty normal.
I can't possibly understand what did the Vendor saw or measure, that calls for the changing of 1 Txr never mind 2 transformers.
Unless you have  more information on the tests,than the info you provided on your post,I can't see the rational for the replacement.

GusD

RE: Transformer %IZ Change

Hi Wilson, (to your answer to jghrist)
The base impedance per phase of a balanced 3phase system is
Zb=(KVb/sqrt3)*1000/sqrt3*Ib
  =KVb^2/MVAb
where KVb is line to line base KV and MVAb is a 3phase MVA.
Zb is always a per phase value (if the numerator is per phase KV then the denominator is per phase MVA, if it is phase to phase volt in KV then the denominator is 3phase MVA)>In both the cases the result is perphase base impedance.
Hence
Zb=12.47^2/5=31.1 ohmsperphase always.

Test results ase per your report:

The secondary coil was shorted and they applied a voltage to the primary side. The results on the center phase of the 5000kva unit is as follows:
pri. volts: 27.41
pri. amps:  3.0
sec. amps:  135.4
watts:      40.0
calc'd Zbase: 93.3 This is on a single phase basis as that's the way it was tested.

Pri impedance is calc'd by   27.41/3.0 = 9.14 ohms
% impedance calc'd by     9.14 X 100/93.3 = 9.793%

My correction is Zbase should be 31.1 only. Otherwise other part of the calculation looks good.
One more question. While conducting the test was the transformer secondary earthed through some impedance? If that is the case centre phase shorted with neutral point and connected to the ground through an impedance, sec. current will be limited by the impedance value isnt it?
The turns ratio being constant the magnitude of the current through the primary should also be less.


Ramesh.

RE: Transformer %IZ Change

yenrameshn,
I also thought the Zbase calc was wrong, but they tested each winding separately.  The rated voltage of the primary delta winding is 12.47 kV and the rating is 5/3 MVA.  Their calculation is correct on this basis.  Works out to be the same, I think, as the ANSI/IEEE C57.12.90-1987 equation which includes a 0.866 factor when using single-phase winding tests.

RE: Transformer %IZ Change

(OP)
To yenrameshn: Each coil was tested separately and the secondary winding was "shorted" by connecting X1 and X0; no impedance. If the test had been done on all three phases simultaneously, the line current would be used, which would be 5.19a (3a X 1.732). The phase to neutral Z would still have to be calc.d on a phase basis by using L-N voltage.
Z = 27.41/1.732/5.196 = 3.045 ohm
Zbase = 31.1  on a three phase basis.
%Z = 3.045x100/31.1 = 9.79  same as calc'd on single phase basis.
I hope this helps.
jim wilson
To jghrist: I need to review the Standard you refer to. I'm not sure what the 0.866 factor is; probably sqrt3/2.

RE: Transformer %IZ Change


Aside:  A poorly connected or undersized jumper on the secondary may have significantly and undesirably jacked up the in-the-field impedance calculation.  Also, the tester {nee, manufacturer} may have inadvertently “overlooked” accuracy in the original test-report measurements.

Be careful.  Given perceptions of reliability need, the transformer folks may be banking on a hospital being an easily railroaded client.
  
Table of contents for IEEE C57.12.90-1993 is http://standards.ieee.org/reading/ieee/std_public/description/dtransformers/C57.12.90-1993_desc.html §9 and Part II are probably the salient text.  It is not exactly “breezy” reading.
  

RE: Transformer %IZ Change

Jwilson3
is this transfromer a 3 or 5 leg core?
Were all six leads on the high side isolated except for the two being tested.  Were there six terminals isolalated on the high side?  H1-H2 for A,B & C?

RE: Transformer %IZ Change

Suggestion: Term "Single phase" can become ambiguous since it may mean the single phase between two phase conductors or single phase between the phase conductor and the neutral.
Notice, that the Zan=Zbn related to Zab is over cos30=0.866, namely, set
Zan=1 p.u.
Zbn=1 p.u.
Zab=Zan x cos30 + Zbn x cos30=(1+1)cos30=1.732 p.u.
and 1.732/2=0.866
Therefore, if one measures Zan and Zbn one gets 2pu, and if one measures Zab, one obtains Zab=0.866(Zan+Zbn)=0.866(1+1)=1.732 pu.

RE: Transformer %IZ Change

The calculation of %Z is correct if the secondary of the energized winding was shorted to neutral (X0).  This is not the procedure for 1ø testing in ANSI/IEEE C57.12.90.  In ANSI/IEEE C57.12.90, the 3 secondary terminals are shorted together and neutral is not connected.  This eliminates zero-sequence currents.  BJC's question on the core construction is very relevant if there are zero-sequence currents.  With a 3-legged core, there is no return path for zero-sequence flux except in the insulating the air or metallic connections other than the core.  This may have influenced the impedance measurements.  You are actually calculating (Z1+Z2+Z0)/3.  With a 3-legged core, Z0 does not equal Z1.

I suggest that you get the transformers retested, along with a dc resistance test, according to industry standards before deciding to replace them.

RE: Transformer %IZ Change

(OP)
To busbar: I agree. We're holding off replacement since other test results don't indicate a problem, but I'm still puzzled why the test showed an impedance change and what could have caused this if the test was correct.
To BJC: Three legged core.I think the other leads were open circuit or isolated. If they were not, but were all shorted, I believe this would appear to the leg under test as an increase in secondary load, which would show up as a reduced impedance.
To jbartos: It appears you are referring to a wye-delts transformation, therefore if Zan=Zbn=Zcn=1 pu, then the delta phase impedances Zab=Zbc=Zca=3.
When you measure the Zab of the wye circuit you get 2. When you do the same for the delta circuit you also get 2
(1/Z = 1/3+1/6 and Z = 3x6/3+6 = 2)

RE: Transformer %IZ Change

(OP)
To jghrist: I'm sure the test was done individually, wth X1-X0 shorted. This is a 3 legged core. You've made a good comment. Thanks. I need to review C57 to learn more about the standard test methodology.

RE: Transformer %IZ Change

yenrameshn,
Do you mean to say that if I connect the X1 terminal to the X0 terminal of an energized Delta-wye transformer, I won't get any zero-sequence current unless I also connect to something that I call "ground"?  If I were to also make a connection to a ground rod, no current would flow in the connection to the ground rod (where would it go?).  All current would flow from X1 to X0 directly.  How would you analyze a fault from X1 to X0 with no "ground" connection using symmetrical components?

RE: Transformer %IZ Change

Suggestion to jwilson3 (Electrical) May 22, 2003 marked ///\\\
To jbartos: It appears you are referring to a wye-delts transformation, therefore if Zan=Zbn=Zcn=1 pu, then the delta phase impedances Zab=Zbc=Zca=3.
///Actually, I referred to two wye windings, e.g. Zan+Zbn connected in series and measured from the transformer terminals A and B. Then, considering the one delta connection winding between A and B as Zab only since the delta connections can be opened for convenience of testing.
I agree that the connected delta windings would lead to:
Zabdelta=(1.732)x(1.732+1.732)/(1.732+1.732+1.732)=1.1547pu in terms of Zan=Zbn=Zcn=1pu.\\\

RE: Transformer %IZ Change

Hi jghrist,
Firstly, zero sequence current exist only when there is an unsymmetrical fault and returns through the ground.
In a power system distribution there could be several grounding electrodes which could be interconnected in a earthing ring and may be connected to the star point(neutral point) of the transformer secondary.
If it is connected to the starpoint the zero sequence current will circulate in the transformer winding through the neutral impedance(could also be solidly earthed).
Say when a single line to ground fault occurs at the equipment(say stator ground fault, the current flowing through the fault "I(fault)is=3(Ia0)",here Ia0=Ia1=Ia2. Here the positive,negative,zero sequence flows into the faulted circuit. If the equipment earth is connected to the transformer star point through earthing ring then the zero sequence current returns through neutral impedance and circulates in the transformer winding. For positive and negative sequence currents there is no return current and so they will not pass through neutral reactance.
If the neutral of the transformer is not grounded the zero sequence network is open circuited and Z0 is infinite. under these condition Ia0=Ia1=Ia2=0.

Now coming back to the point, what I meant is there will be no zero sequence current in the secondary of the transformer since there is no condition of a ground fault in the circuit to be short circuit tested. Once the neutral is earthed  and by shorting x1 and x0 then atleast we can assume it as a ground fault condition!!! and have the zero sequence component in the circuit.
Ramesh

RE: Transformer %IZ Change

yenrameshn,
My point is: X1 (Aø) was shorted to X0 during the impedance test.  From the standpoint of current flow, it does not matter one bit if X0 was connected to ground.  If there are no connections to X2 (Bø) and X3 (Cø), there will be zero current in these phases.  There will be current in Aø.  Therefore Ia0 = (Ia+Ib+Ic)/3 will not be zero.  You can call the wire connecting X1 to X0 "ground" if you want or even connect it to a ground rod.  It makes no difference to the flow of current.

RE: Transformer %IZ Change

If the Manufacture's test showed that the transformer is bad I would start making plans to replace these units since the transformers are being used at a hospital and these type of transformers can not be obtained quickly. Remember- transformers do fail, you should be ready. More testing by another test outfit would be good for a second opinion. Don't delay, if those transformers go out people lives may be effected.
A.D.Vidana
5-28-03

RE: Transformer %IZ Change

Hello Gentlemen,

I am a new comer for this forum. I saw this thread on transformer impedance change, but decided to not to comment, as I also cannot agree in anyway with the so called “impedance test” and also due to the fact that JWILSON3 has not witnessed such an important test.

Since YENRAMESHN had pushed this posting to a different way and made some comments on the zero sequence current components, which has no relevance to this thread as he himself says, I too got some interest to get involved.

YENRAMESHN
I cannot agree with your comment on May 23 rd which says “ For a zero sequence current to flow in the secondary winding, the star point (neutral point) has to be grounded”. IT IS NOT NECESSARILY TO BE. If there is a 4 th conductor on a three phase system then zero sequence components will exit. In that way JGHRIST is fully correct and I agree with his immediate reply on May 23 rd. Load side of a Delta/Star transformer is not grounded. Then how do you analyze a Phase to neutral fault ( X1 to X0)on the load side using sym. components where zero seq. components are involved in the calculation? In a 3-phase 4-wire distribution system if a fault occurred between a phase and the neutral, will the residual earth fault protection (51N) or zero sequence protection (51GS) scheme respond? The answer is No. The transformer will trip from overcurrent (50/51) and not from any of the ground fault schemes (51N or 50GS). The reason is that the zero sequence currents DO exist but they are balanced by the 3I0 flowing in the neutral which is the 4 th conductor. Similarly, take a 30 A, 3 phase, 4-wire household ELCB (RCCB) and if we bridge one phase and the neutral terminals using a test lamp, it will not trip on earth leakage but depending on the fault the main or branch MCCB/MCB will trip on overcurrent. Same thing will happen for a single phase RCCB too. But if we bridge a phase terminal and “ground”, then  definitely it will trip. Reason is even though the zero sequence currents DO exist in the first case they are all balanced in all 4 conductors. Nothing exist in the earth path. In the second case, yes they DO exist and flow in the earth return path, but this time nothing in the earth path. I hope this will help you to think about it again.

JWILSON3
With regard to impedance change, I can reasonably claim that the manufacturer had done a test which is not correct neither as per ANSI/IEEE C57.12.90 nor per IEC 76. Normally the impedance test is done along with the test for load losses so that we can estimate transformer load losses too. If we are doing the impedance test

1)Using a three phase supply then a VARIABLE 3-phase supply has to be connected to H1, H2 & H3 while short circuiting X1, X2 & X3. (no connection with X0). Supply is raised till we get the primary current equal to 138 Amps corresponds to 3 MVA (AA rating) and not to 5 MVA(AFA) and then to be corrected to 75 deg C for accurate results to compare with original test results.

2)Using a single phase supply then it has to be connected to H1 (H2 or H3) while H2(H3 or H1) is earthed while short circuiting X1, X2 & X3. (no relevance with X0). Supply is raised till we get the primary current equal to 138 Amps correspond to 3 MVA (AA rating) and not to 5 MVA(AFA) and then to be corrected to 75 deg C for accurate results to compare with original test results.

As far as I know this is the standard routine test suitable for measuring the impedance voltage irrespective of whether the secondary is delta or star. That means, for your
3/ 5 MVA(AA/AFA) dry type transformer having a primary rated voltage of 12470 Volts with a name plate impedance of 6.07%, we should apply at least 756 Volts to the primary so that 138 Amps is flowing in the primary while X1,X2,X3 terminals are short circuited. Therefore the results of 27.41 V. 3.0 Amps, 135.4 Amps does not sound good for me.
Since the manufacturer has not followed the standard method, I think before any replacement you should ask them to repeat the test so that you also can witness. If all the other routine tests ( specified by NETA-MTS-2002) are OK and if the impedance test results is then become “SENSIBLE”, my advice is to energize the transformer without a second thought.

Regards!




















RE: Transformer %IZ Change

Suggestion to jwilson3 (Electrical) May 22, 2003 marked ///\\\
To jbartos: It appears you are referring to a wye-delts transformation, therefore if Zan=Zbn=Zcn=1 pu, then the delta phase impedances Zab=Zbc=Zca=3.
When you measure the Zab of the wye circuit you get 2. When you do the same for the delta circuit you also get 2
(1/Z = 1/3+1/6 and Z = 3x6/3+6 = 2)
///Yes, I agree with the delta-wye transformation (transfiguration). I was addressing the test of two windings connected in wye with Ean=1pu, Zan=1pu, Ia=Ib=1pu, Ebn=1pu, Zbn=1pu
versus
Eab=1.732pu, Ia=Ib=1pu and Zab=1.732pu.
To comply with the transfiguration relationships between Zan=Zbn=Zcn=1pu and Zab=(Zan x Zbn + Zbn x Zcn + Zcn x Zan)/Zcn=(1 x 1 + 1 x 1 + 1 x 1)/1 = 3pu
current would have to be 1/1.732pu to obtain:
Eab=Zab x Ia = 3 x 1/1.732 = 1.732 pu = 1.732 x Ean, in pu
For line-to-line fault:
Eab=Isc x (Zan + Zbn)
or
Zan + Zbn = Eab/Isc = 1.732pu/0.866pu = 2pu since Zan + Zbn = 1pu + 1pu
Therefore, the line-to-line short circuit current Isc is 0.866 x Iline-to-ground since
Iline-to-ground=Ia=Ib=Ic=1pu for Ean/Zan=Ebn/Zbn=Ecn/Zcn=1pu/1pu\\\  

RE: Transformer %IZ Change

(OP)
I've been in the Carribean on a 60 ft schooner for the last two weeks, so I've not been able to participate in the "thread" nor give it much thought.
A generous amount of interesting discussion has been posted and it has forced me to think thru the testing procedure in relation to ANSI/IEEE C57.12.91 for dry transformers.
The field test was done on each HV/LV coil pair separately: that is with H1-H2 energized and X1-X0 shorted for the first test with all other leads opened. Then the other coils were similarly tested. They tested at lower amperage than the normal full load values and got an impedance value of 9.14 ohms for one phase, which when divided by a Zbase of .933 gives a % Impedance of 9.79%. If they had raised the primary voltage from 27.41v to 1221v, the pri current would have been at full load of 133a(phase current). The 1221 impedance volts is 9.79% of full voltage.
The single phase test procedure in ANSI/IEEE C57.12.91 is with all primary connections between the phases intact, and X1-X2-X3 connected together on the secondary. If 27.41v is applied to H1-H2, 3a flows in the H1-H2 leg and 1.5a in the parallel legs. The line current is 3a+1.5a = 4.5a. If the voltage was raised to 1410v, the pri. line current would be at full load of 231a(line current). Each leg is similarly tested, averaged, and multiplied by 0.866 per ANSI/IEEE. Assuming each phase is at 1410v, the % impedance volts is 1410 X .866 = 1221v or % IZ of 9.79.
So, I feel the field test procedures, done on a single phase/single coil basis gives the same result as the ANSI/IEEE procedure, which is done without disconnecting internal connections.
The big question is, assuming their readings were accurate, is why the results varied from the factory results?

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