Troubleshooting


For Vinmetrica SC-100A, SC-200 and SC300

The topics in our troubleshooting guide refer to the methods in the Vinmetrica manuals. For current versions of the instructions for pH, TA, and SO2 tests refer to your manual, or here: https://vinmetrica.com/manuals/

Please click on the links in the text to navigate our troubleshooting guide step-by-step

 

My problem is related to:

1. pH

2. TA

3. SO2 (sulfite)

4. Other Instrument malfunction

 

pH problems

Whatever issues are occurring, always do the following first:

1. Make sure the batteries are good. If you’re seeing “Lo Bat” on the display, replace the batteries.

 

2. Inspect the pH electrode. The probe’s immersible end has a glass bulb as the pH sensor. Ensure it is intact and not shattered, and free of visible deposits; it should appear as shown below:

pH electrode
How the intact electrode looks

 

If the electrode appears dirty, try to clean and/or recondition it.

 

3. Be sure the pH electrode has been stored at least 24 hours in a proper electrode storage solution (Vinmetrica’s product is 2M potassium chloride in 50 mM potassium hydrogen phthalate; other similar products may be used). The entire bottom 1 inch of the electrode needs to have been submerged for at least 24 hours. If this has NOT happened, it’s best to wait until it has! The reference junction needs to be in contact with the storage solution when not in use in order to maintain its reference potential.

 

NOTE: when calibrating or measuring with a pH electrode, always be sure the solution is gently stirred or swirled constantly!
Stagnant solutions can give pH values that are erroneously high.

 

4. If the instrument has firmware version 3.2.d or E, or 3.0.6 through 3.1.3, perform a pH calibration reset in Test Mode. Exit Test Mode and try to calibrate again if possible.

 

My pH problem is

1. Can’t calibrate properly

A. Calibrates at pH 7.00, but not at 4.01

 

i. 4.01 reference solution reads between 3.5 and 4.5, but Cal LED never flashes

If you haven’t already, do a pH Calibration Reset (pH CalRst) in Test Mode.

Try to repeat calibration at 4.01

If 4.01 now reads above 4.50, there is offset error and calibration will not proceed; go to offset error if you have firmware version 3.1.1 or higher

 

ii. Reads around 7 no matter what solution the probe is in.

     1. Probe pH sensor glass bulb broken? Re-check.

If bulb is broken, replace the pH probe.

2. Electrical short in probe, connector, or instrument

Remove probe. Put instrument in CAL mode.

pH goes low or high (towards 1 or 13)

Do the short out test on the pH connector

Test Passed: instrument OK, probe shorted; replace probe

Test failed: internal Open circuit; contact Tech Support

pH stays at 6.5 to 7.5 with nothing attached:

instrument connector, or internal, short. Contact Tech Support

 

B. Calibrates at neither pH

 

     i. If you haven’t already, do a pH Calibration Reset (pH Cal Rst) in Test Mode.

 

ii. pH 4.01 reads outside the range 3.5 to 4.5 but within the range 3.0 to 5.2 and/or
pH 7.00 reads outside the range 6.5 to 7.5 but within the range 6.0 to 8.2:

1. go to offset error if you have firmware version 3.1.1 or higher.

2. Contact Tech support if you can’t do the above step.

 

2. I can calibrate, but the results are unexpected or varying

A. Check your calibration with the cream of tartar test

       i. If this test passes, your instrument and electrode and solutions are apparently OK

ii. Cream of tartar test gives pH values that way exceed the normal range (less than pH 3.30 or more than pH 3.80): you may have a miscalibration at pH 4.

(1) If you can, check the voltage of the pH electrode in pH 4.01 and 7.00.If 7.00 shows a mV valuein the normal range (Table 1), but 4.01 is coming out more than 195 mV above, or less than 140 mV above the pH 7 value,then your pH 4.01 reference solution is likely contaminated with acid or alkali. Replace it.

(2) If pH reference solutions are giving proper mV values, try a CAL reset followed by another calibration.

B. pH response seems to drift

i. Be sure electrode has been in electrode storage solution 24 hours.

ii. Be sure to re-check calibration, and use stirring when measuring or calibrating

iii. Be sure all reference solutions, samples, and electrode are at the same temperature when calibrating and measuring

iv. Try cleaning the electrode

 

C. pH results differ from those reported by a lab, or measured on a different pH meter

i. pH values determined by two competent, properly calibrated pH meters should agree to within 0.05 pH units or better.

ii. If this is not the case, recheck calibration and cream of tartar test

iii. Remember to keep solutions stirred and at same temperature as reference solutions.

 

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Offset error in pH or CAL mode: Electrode or instrument error

If potential mode is available (firmware 3.1.1 and higher), follow the steps below.

Check the voltage of the pH electrode in pH 4 and pH 7 reference solutions. On the newer versions of our instrument’s firmware (version X.1.1, X.1.2 x.1.3, or X.2.d or e, where X = 2 for an SC-200 and X=3 for an SC-300) you can check the raw voltage coming out of the pH probe.You can also check these values on a good laboratory pH meter if it has a potential (mV or millivolt) mode.A good electrode should have +5 to -15 mV at pH 7 and at least an additional 160 mV on top of this pH 7 value at pH 4.01 (e.g., 165 to 145, resp.).

Note: do not check the voltage of a pH electrode with a voltmeter! You will get meaningless results, because even very good voltmeters do not have the necessary high input impedance to accurately measure a pH electrode’s voltage.

 

a) Put the instrument into Potential Mode (push the Mode button once when in pH mode); now the pH LED blinks.

b) Rinse the pH electrode and place it in the pH 7.00 reference solution. Stir or swirl the solution gently and continuously so that the electrode is not sitting stagnant in the liquid.

c) Write down the mV reading on the LCD screen once it stabilizes. On instruments with FW version 3.1.1 through 3.1.3, a negative mV value is indicated by the red STOP LED illuminating.

d) Repeat steps a. and b. using the pH 4.01 reference solution.

On Table 1 below are shown acceptable values for the pH 4.01 responses for a particular pH 7.0 value, assuming an acceptable slope is 59.2 mV/pH +/- 102/95% at 25°C. If your electrode is falling outside of these ranges, and/or if its response time is very slow (>30 seconds to reach within 0.02 of final value), you should try to clean and/or recondition it. If these steps do not restore proper function, replace the electrode.

Table 1. Acceptable mV responses in potential mode for pH electrodes. Values in red are marginal.

 

Measured pH 7 value, mV

Ideal pH 4.01 mV value

Lower OK pH 4.01 mV value (95% slope)

Upper OK pH 4.01 mV value (102% slope)

-20

158

148.

161.

-15

163

153.

166.

-10

168

158.

171.

-5

173

163.

176.

0

178

168.

181.

5

183

173.

186.

10

188

178.

191.

If mV readings are OK, offset error is probably due to bad bias voltage. Try resetting the pH DAC

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pH Cal Reset

How to do a pH Calibration Reset. This resets your instrument’s pH calibration parameters to default settings.

 

1. Enter Test Mode (see Appendix A of your SC-200 or SC-300 manual, or go to https://vinmetrica.com/manuals/ to get the latest version.)

a. turn off the instrument.

b. Now press and hold down the POWER button about 2-3 seconds, then release it. You should now see the firmware version number, like 3.1.2 or 3.2.d. The latest version is 3.2.E, or 3.1.3 on older models.

c. If you’re working on an SC-200, the firmware version number will look like 2.1.2 or 2.2.d; the latest version is 2.2.E, or 2.1.3 on older models.

2. Press the POWER button briefly 12 times to get to module 13, “pH CAL reset”. The message “PrESS EntEr” scrolls across the screen.

 

3. Press the ENTER button. Verify that the instrument beeps and the message “Good cal Rst” scrolls across the screen.

 

4. Exit Test Mode by holding down the POWER button until the instrument shuts off.

 

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Setting the pH DAC

If the electrode appears to be normal per Table 1, there may be an instrument issue known as “pH bias error” that’s preventing calibration. You can try adjusting this bias with the “set pH DAC” section (#14) in Test Mode (firmware versions 3.2.d or higher and 3.1.2 or higher). This changes the pH Bias in an attempt to bring the pH response into reference range. See
Appendix A, Section 14 in the manual for more information.

1. Enter Test Mode: turn on the instrument holding down the POWER button until the firmware version is displayed, then immediately release the button). If you haven’t done so
already, it’s a good idea to do a pH Cal Reset before proceeding.

 

2. Scroll through the sections of Test Mode by briefly pressing the POWER button. Pressing it 13times from the first section should bring you to section 14.

3. Section 14 will show a messageset pH DAC”, then will cycle between two numbers, a voltage number like 2.23, followed by an integer whose default value is 16. This integer is the “pH DAC Index”.

4. Pressing “Enter” will raise the pH DAC Index value which will lower the apparent pH value. Pressing the “Mode” button does the opposite. So if, as is most common, your calibration values are too high (like 4.6 in pH 4 reference solution), you’ll want to press ENTER. You’ll see the pH DAC Index increase to 17.

5. Exit Test Mode: hold down the POWER button again until the instrument shuts off.

6. Now try your normal calibration.

If adjusting the pH DAC Index up or down two steps (i.e. from 16 to 18 or to 14) does not bring the calibration values into line, there is a likely an “impedance problem”. Contact Tech Support.

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Slow response or drifting values – cleaning and/or reconditioning

First, just a reminder: Be sure that you always stir or swirl the electrode; don’t let it sit stagnant.
Second, remember that your electrode needs to have been in proper storage solution for at least 24 hours for proper equilibration of the reference junction.

Note: Avoid touching the electrode’s glass bulb! Use a vigorous stream of water to rinse any residues off.

Cleaning the pH electrode: An electrode that is difficult to calibrate, or that is very slow in response time (or “drifting”) can sometimes be reinvigorated by
cleaning. Try the following steps in the order given:

1. Shock treatment in acid and alkali. Place the electrode in 2 M HCl (this is the“Acid Solution” of the SO2 reagents set) and let it sit for 15 minutes with
occasional stirring. Then remove it and rinse with water, followed by 15 minutes in 0.1 to 1 M NaOH (the TA Titrant or 1N NaOH) also with some stirring. Remove and rinse, then place in electrode storage solution for at least 1 hour before trying to use it again. If this isn’t showing improvement, go to the next step.

2. Soak electrode in hot (NOT boiling!) water, about 60 °C, for 10 mins. Allow to cool to room temperature then rinse and place in pH 4 reference solution for 5 minutes. Try to recalibrate. If this does not work, try the next step.

3. Place electrode in electrode storage solution (from Vinmetrica, or 2M KCl with optionally added 0.01M potassium acid phthalate, KHP) at 60 °C and allow electrode and solution to cool to room temperature, then rinse and place in pH 4 reference solution for 5 minutes. Try to recalibrate. If this does not work, try the next step.

4. Soak electrode for 15 minutes in a 1:10 dilution of bleach in a 0.1 % solution of lab (“Alconox” ) or liquid detergent in hot water with occasional vigorous stirring. Rinse with DI water, then place in pH 4.01 reference solution for 5 minutes. Try to recalibrate.

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Cream of tartar calibration test:

A quick way to check your calibration and pH accuracy is to measure the pH of a saturated solution of cream of tartar which has a pH of 3.56 at 25 degrees Celsius:

1. Get pure cream of tartar (grocery store stuff is fine, provided it’s pure), or reagent grade potassium hydrogen tartrate, also known as potassium acid tartrate or potassium bitartrate. Call it KHT for short.

2. Place about 1/8 teaspoon of KHT in 20 mL of distilled water. Mix well for about 30 seconds. You want to be sure the solution is saturated, i.e., everything that can dissolve, has dissolved. There should be some undissolved solid left.

3. Decant or filter the solution off the solids if possible.

4. This solution has a standard pH of 3.56 at 25 degrees C (78 degrees F). It should be within 0.02 pH of this value at temperatures from 20 to 30 degrees Celsius. We usually are OK with a value between 3.50 and 3.60. Discard after 24 hours

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TA problems

Most TA problems have to do with pH performance problems, so be sure you are getting good calibration settings with your pH function. Assuming that is the case, the remaining issues are largely around expected values and have to do with operating errors in the sampling or in the titration part of the test.

1. TA values don’t agree with expectations or with a reference lab’s values.

2. Getting highly variable TA numbers on the same sample.

3. Can one use different sodium hydroxide solutions rather than the Vinmetrica TA Titrant?

 

  1. TA values don’t agree with expectations or with a reference lab’s values:

a. Be sure to de-gas your sample, because dissolved CO2 will increase the apparent TA value, sometimes significantly, and this is not want you want in your TA value.
Follow instructions in the manual for taking and preparing the wine sample.

b. By default, Vinmetrica’s equipment titrates to pH 8.2 for a TA value. Some labs may use a titration to pH 7.0, which will result in a slightly lower number. [You can set your instrument to indicate the TA endpoint at pH 7.0 by going into Test Mode; see Appendix A in the instrument manual]

c. Be sure you are using Vinmetrica’s TA Titrant (0.133N NaOH). If you are using something else, see below

2. Getting highly variable TA numbers for the same sample:

a. Be sure that you are operating and reading your TA Titrant dispenser (syringe or buret) correctly. If you’re using a 5 mL syringe, be sure that
bubbles aren’t present in the filled syringe before you start. Expel these. If you’re using a buret, be sure there’s no air bubble in the buret’s
tip, as this can suddenly dislodge and cause error in the measurement of the volume. For either dispenser, be sure you read the starting and ending volume numbers correctly. Refer to your manual for directions.

b. Be sure you aren’t overrunning the titrationendpoint. You should slow down your titration to one drop at a time when you reach near 1 pH unit of the final value (i.e. 8.2, so slow down at 7.0). If your final pH endpoint is more than 0.3 above the correct value (i.e., you ended up at pH 8.5 vs the 8.2 you were shooting for), then you will get a higher than expected TA value and you may want to repeat that test.

3. Can one use different sodium hydroxide solutions rather than the Vinmetrica TA Titrant?

Yes. Vinmetrica’s TA Titrant is 0.133 M (molar, or N normal) sodium hydroxide (NaOH). Commonly, one finds 0.10 or 0.20 N sodium hydroxide available. If you use one of these, then correct your TA value by the factor N/0.133. For example, if you use 0.20N NaOH and it takes 2.5 mL to titrate 5 mL of wine to the endpoint, then you would multiply our usual calculation (2 x vol) by the factor 0.20/0.133:

Corrected TA value = 2 x 2.5 x (0.20/0.133) = 7.5 g/L as tartaric

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SO2 Problems

Whatever problems you are having, check these items FIRST

1. Make sure the batteries are good. If you’re seeing “Lo Bat” on the display, replace the batteries.

2. Make sure the SO2 electrode is being plugged into the correct receptacle on the instrument. Occasionally someone puts the electrode in the wrong place. This is possible on some units because we have gone through three iterations of SO2 electrode plugs over the years. Check this link to get a view of the correct plug/receptacle combinations:

https://vinmetrica.com/wp-content/uploads/ManualsDataSheets/Replacement_SO2_Electrode_Purchasing_Guide.pdf

 

My SO2 problem is

1. No apparent response – the instrument stays low, or on 0.0 all the time during a titration

a. Double check that you used the reagents & instrument correctly:

 i.  Mix 25 mL wine, 2 mL SO2 Reactant (yellow dot), 2 mL SO2 acid solution (red dot)

ii. Attach SO2 electrode to proper connector; put instrument in SO2 mode

iii. Titrate with SO2 Titrant (blue dot)

b. Do the liquid SO2 test

Liquid test is OK: Perhaps you have high free SO2 in your wine?

1. run the SO2 test again, this time on diluted wine:

2. take 10 mL wine and add 15 DI water and re-test.

3. If a response is seen within 5 mL of Titrant, you have high free SO2 (calculate value in usual way then multiply by 2.5)

ii. Liquid test fails: do the short out test on the SO2 connector; check plug connection; check electrode wires for cleanliness.

1. Short out test passed: There is an apparent problem with SO2 electrode

a. Check cleanliness of SO2 electrode

b. Check continuity of electrode: attach electrode, put instrument in SO2 mode and carefully short out the platinum wires with metal paper clip or other metal device. You should see a strong response, up to 1999 and STOP conditions

c. If a. and b. fail to restore or demonstrate function, replace SO2 electrode

2. Short out test failed: there is an instrument malfunction; contact Tech Support

2. Response occurs right away, before starting the titration in SO2 mode

a. I get 1999 and STOP before even connecting the SO2 electrode!

Instrument short: contact Tech Support

b. I get 1999 and STOP after connecting the electrode but before putting it in a solution!

SO2 electrode short: probably replace electrode, but contact Tech Support

c. I added Acid and Reactant reagents to the wine sample; when I put the electrode in the sample, it went to a STOP condition without adding any SO2 Titrant

i. Most likely this is indicating that your free SO2 is less than 1 ppm

ii. Rule out hot electrode

3. I only added 1 or 2 drops of SO2 Titrant before the STOP condition occurred

a. Most likely this is indicating that your free SO2 is less than 1 ppm

b. See “SO2 questions” [I added the right amount of sulfite to my wine but the numbers are still low!] under

https://vinmetrica.com/faqs/

4. SO2 values don’t agree with expectations or with a reference lab’s values:

a. Make sure your equipment passes the Liquid SO2 Test

b. If your numbers appear lower than expected, see “SO2 questions” [I added the right amount of sulfite to my wine but the numbers are still low!] under

https://vinmetrica.com/faqs/

c. If the numbers are higher than expected, double check the titration on a smaller volume of wine

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Liquid SO2 Test

1. Place 25 mL distilled water in beaker

2. Add 1 mL SO2 Acid Solution and 1 mL SO2 Reactant

3. Attach SO2 electrode, turn on instrument in SO2 mode, press ENTER if requested.

4. Place SO2 electrode in beaker and stir/swirl

a. Instrument should read 0.0 or a low value (less than 50); green PROCEED light is on.

b. If STOP light is on, go to Hot SO2 electrode

5. Add 1 or 2 drops of the SO2 Titrant. Solution should turn slightly yellow. If not, add 1-2 more drops. If still no yellow, check your reagents.

a. Instrument should read above 100; red STOP light on, beeper sounding

b. If not, add 1-2 more drops SO2 Titrant

c. If still not, go back and try the next step (liquid test fails)

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Hot SO2 electrode test:

1. add 1 drop 10% KMBS, or a single small crystal of KMBS, to the solution that is showing a high response (the red STOP LED is on, signal on the LCD screen is above 50, beeper is on). Stir continuously.

a. If now, the red STOP light goes off, green PROCEED LED is on, the beeper stops, signal drops below 50: Electrode is OK

i. Note: STOP signal was probably due to small amounts of free iodine in the Reactant

ii. unless the Reactant is strongly yellowed, this is OK

b. If the high response continues, Electrode is “HOT” and needs conditioning.

i. Leave in this solution with the instrument attached and turned on, in SO2 mode.

ii. Let run for 1-2 hours. (Note the instrument will shut off after 30 minutes so you will have to turn it back on from time to time).

iii. Repeat the Liquid SO2 Test. If probe is still not passing, contact Tech Support.

 

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Cleaning the SO2 electrode:

The platinum wires at the immersible end of your SO2 electrode should be bright and shiny. Often you can see reddish-brown or sometimes white accumulation on them. Here’s how to clean them:

1. Soak the SO2 electrode in your SO2 Acid Solution for about 10 minutes and rinse with DI water.

2. Using the back edge of a pocket knife or something similar, remove any accumulation by gently scraping the two platinum wires, being sure not to bend or break them.

3. Thoroughly rinse with DI water and try your test again.

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Short out test:

To do the short out test for pH or SO2, remove the electrode from the instrument. Short out the center pole of the connector to its outer edge using a metal paper clip or similar metal device. Figures 1-4 below show the test on pH and SO2 connectors on the latest version of the SC-300. Figures 5 and 6 show connectors on older models of SC-300 and SC-100A

Click the result you observed:

pH short out test passed        SO2 short-out test passed

pH short out test failed           SO2 short-out test failed

 

Shorting out the pH connector
Figure 1. Shorting out the pH connector

pH Shorted good
Figure 2. Successful pH short-out test: reading is between 6.5 and 7.5

 

SO2 short out test
Figure 3. Shorting out the SO2 connector

 

Good SO2 short-out
Figure 4. Successful SO2 short-out test: reading is 1999

 

Old connector short-out
Figure 5. Shorting out the SO2 or pH connector on original models of SC-100A or SC-300


old RCA SO2 short-out
Figure 6. Shorting out the SO2 connector on some older models of SC-100A or SC-300 that have an RCA (phono-plug) connector for SO2. Note: the center hole’s contact is about 0.5 inch deep, so insert the wire thoroughly.

 

Other Instrument malfunction

If you bought your instrument less than 2 years ago, it is still under warranty. Contact us for advice on any repairs.

1. Instrument won’t power on.

a. Check and replace batteries if needed

b. Check battery terminals for broken wires or corrosion; clean if needed

2. LCD display demonstrates missing segments

a. Contact Tech Support – usually replacement of the LCD screen is needed.

3. Buttons don’t operate properly

a. If a button appears to be stuck in the “pressed” position, sometimes you can free it by putting a small piece of adhesive tape on the button face and pulling up on it.

b. Otherwise, call Tech Support for repair information

4. Probe connectors on the instrument are loose

a. If instrument is off of its 2-year warranty, these can usually be fixed by the user. Contact Tech Support.

 

Tech Support: 760-494-0597 x102 or info@vinmetrica.com

 

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10 thoughts on “Troubleshooting

  1. I have an SC-300, it appears the digital readout when I do a PH reading is malfunctioning. I can not see all of the numbers, they are cut off. Can this be repaired and if so how do I get it repaired. I am in Sterling, CT 06377

    1. Ross,
      It sounds like you LCD screen needs to be replaced. The cost for this repair is $75 plus shipping. If you want to send in your instrument, please call our tech support line and they can set you up with an RMA number so you can send us your equipment. Let me know if you have any other questions.
      Taylor

  2. Can pre-fermented juice be checked for SO2 ?

    1. Yes, it can. No change in the usual procedure is needed.
      -Rich

  3. Some of the segments on the LCD of my SC-300 have stopped working. Is this a known issue with worksaround? Do you offer a repair service (we are in Spain)?

    Thanks

    1. Andy,
      It sounds like your LCD screen needs to be replaced. I am sorry for the issue. The only workarounds are in SO2 mode, CAL and maybe in TA mode. You don’t really need to see the digits on the screen since you are only counting the beeps at or near the end of the titration. You should be able to still perform those three tests without the screen functioning. In TA and CAL mode, it is obviously very helpful to have the screen but they can be done without it. The only mode that will be an issue is pH mode since you cannot read the full numbers.

      We do have a repair service but this does get tricky since you are in Spain. We have a distributor in Denmark that has made repairs for us on our instruments. I can check with them to see if this is something they can do. Or you can always ship it back to us for the repair. We would recommend insuring it and getting tracking on the package as we have been seeing some issues with our postal delivery services lately.
      Let me know what you decide and Ill reach out to our distributor.
      Thank you
      Taylor

  4. I absolutely LOVE my Vinmetrica SC-200 & SC-300 meters! Accurate, reliable information every time!

    But now, they need a little maintenance love from me!

    I would like to purchase 4 of the bottles with the cap & rubber gasket used for storing the probes in the buffering solution when not in use. One has a little split in the plastic side wall. It is not leaking, but 2 others are showing “stress lines” also. I want a 4th for a spare.

    I do not have a part number, so I appeal to you for assistance.

    1. Roger,
      Thank you for your inquiry. We are glad to hear that you love your Vinmetrica units. We always recommend doing some basic maintenance on them every year. The replacement storage solution vials can be found here: https://vinmetrica.mystagingwebsite.com/product/replacement-storage-solution-vial-for-ph-electrode/ . They do not come with fresh solution so you may want to get some of that too. We recommend replacing the solution at least once a year or if your solution gets cloudy or has debris in it.
      Let me know if you need any additional help.
      Thanks
      Taylor

  5. I have sc300 that is giving wacky readings. I seem to remember there were two test to do that would tell me if a problem, but cant find them. I remember one was to ground the probe. Thanks

    1. Tracy,
      Thanks for contacting us. One of the tests is to ground out the instrument. You are correct. To test the functioning of the instrument:

      1. Be sure the battery is good per the manual’s instructions.

      2. Remove the electrode to expose the BNC connector at the back of the instrument. Turn on the instrument and select SO2 mode. Short out the terminals on the connector, using a paper clip or similar metal piece to touch the center pin of the connector to its outer metal sheath. The device should indicate “STOP” with its red LED and buzzer or beeper. If this does not happen there may be a problem with the instrument; contact us for more information. Make sure you are making a good connection here.

      3. Connect the electrode and put it in about 20 mL of distilled water; add about 1 ml (half a bulb squeeze) of each of the acid solution and the reactant and swirl in the usual way keeping constant motion. The instrument may or may not indicate STOP as above. If it does not, add a drop of the SO2 Titrant solution. This should make the STOP condition occur. [If it doesn’t you may have an electrode problem read in the next section below how to fix this.] Now add one drop of a concentrated sulfite solution (1-10% is fine) and verify that the STOP signal ends and the PROCEED light illuminates. If this happens, your electrode is probably OK as well.

      4. Finally, you can check your SO2 reagents with the Ascorbic Acid Test, this is found on the trouble shooting page.

      Let us know how it goes.
      Taylor

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