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Technical Tips

Inner Body Check

What do you do if your ammonia probe is not working properly? Symptoms are low slopes, erratic readings, error messages during calibrations, and impossible answers.

Whether you just received the probe, or if you have had it for several years, the first thing you need to do is run an "Inner Body Check". This test will tell whether the sensor system itself is operating correctly. If you just received the probe and are having trouble and call the manufacturer or your dealer, the first thing they will ask for are your results from the Inner Body Check before they consider an in-warranty replacement.

Even if the probe is several years old, and therefore out of warranty, this simple test will save you a lot of headaches. If the probe fails this test, it will not work regardless of how many times you re-calibrate or remake your standards.

This applies to both Thermo Scientific Ammonia Probes.

Note: Before you run this test: If the probe has been dry for more than a few days, you should disassemble the probe as per step #2 below and soak the inner body in regular pH 7.00 buffer overnight. If you neglect to do this, the probe will probably fail the test regardless of whether it is actually good or not!

You will need two reagents to run this test:
pH 7.00 reference buffer with 0.1 M NaCl (Sodium Chloride)
NCL# B-47A
Price: 24.95/500 ml

pH 4.00 reference buffer with 0.1 M NaCl (Sodium Chloride)
NCL# B-44A
Price: 24.95/500 ml

Then you need to:

  1. Pour 50-100 ml of each into a beaker and label each beaker.
  2. Disassemble your Ammonia probe. If you have a Pinnacle ammonia probe, simply unscrew the cap at the bottom. If you have an Orion ammonia probe, unscrew the top and pull the inner body out of the top of the probe.
  3. Once you have disassembled the probe, be careful with it because the glass at the bottom of the inner body is very fragile. If you break it, you WILL be buying a new probe.
  4. Make sure the probe is connected to the meter and put the meter into the millivolt (mv) mode.
  5. Place the beaker containing the pH 7.00 buffer with NaCl on a stir plate. Add a 1" stir bar and stir at slow to medium speed.
  6. Clamp the inner body into a probe holder and immerse the probe in the beaker on the stir plate, being careful not to lower it so far that it is hit by the stir bar.
  7. The reading should stabilize within two minutes. A "stable" reading is a reading which drifts 0.1 mv/minute or less. Record this reading, paying attention to the minus sign (-), if present.
  8. Remove the inner body from the solution, rinse it with DI water, and repeat steps 5-7 above using pH 4.00 buffer with NaCl.
  9. From the two mv readings obtained, calculate the difference in millivolts. Remember to take any minus signs into account. For example, if the reading in the 7 buffer was -25 mv, and the reading of the 4 buffer was 145 mv, the difference is 160 mv.

The ideal difference is 175 mv. Normally we consider any difference over 150 mv to be satisfactory. Usually, if the sensor is bad, the difference will be 2 mv or less, so it should be pretty obvious whether the sensor is good or not.

If the probe fails the Inner Body Check, there is NOTHING you can do to make it work. If you just received it, call your dealer for a replacement as an out-of-box failure. If you have used the probe for some period of time, but it is still under warranty, contact the manufacturer for an in-warranty replacement. If it is out of warranty, you still MUST replace it. You are wasting your time if you continue to play with it.

If the probe passes the Inner Body Check, chances are at least 99% that the problem lies elsewhere: bad membranes, bad filling solution, or improper lab technique.

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