Watlow 8LS Controller - Manual (Page 119)

Appendix B: 8LS-CP
8LS User’s Guide 111
Carbon Probe
In general practice, follow the probe manufacturer’s recommendations
as to installation and maintenance schedule.
General Guidelines
1. The probe must be exposed to the same atmosphere as the work load.
2. Be sure of the physical location as to length so the work load will not
hit the probe and — for free atmosphere — flow around the probe.
3. Reference air flow should be in the range of .2-1.0 SCFH. Air flow
below .2 SCFH will cause incorrect readings.
4. Useful service life of 12 months is normal. Probes should routinely be
rebuilt by the probe manufacturer.
5. Furnace temperature should be 300ºF or less when installing the Zir-
conia probe.
6. Do not install probes in new or rebricked furnaces until they have
been dried out for at least 8 hours at 1700ºF with a reducing atmo-
sphere.
The use of the Zirconia probe in carburizing applications is trouble free
except for two common problems:
• Zirconia probes are sensitive to thermal shock. rapid temperatures
changes will cause the Zirconia probe to crack.
To minimize thermal shock to the probe:
a. When inserting the probe, the recommended rate of insertion is
1"/minute. Too slow is better than too fast.
b. To avoid thermal shock upon opening the furnace door to ambi-
ent air, correct placement of the probe inside the furnace is a
must.
• Carbon build-up on the probe is a very common problem and is one
of the reasons for the probe to measure the oxygen level incorrectly.
To minimize carbon build-up:
a. Avoid high carbon sooting if at all possible.
b. Clean off the carbon on the probe with a carbon burn-off.