A load probe is a temperature sensor located in the sterilizer chamber. During use, the probe is inserted into the product being sterilized. This allows for highly accurate monitoring of the product temperature. This feature is particularly useful when sterilizing liquid loads, media, pharmaceutical products, and other temperature-sensitive materials.

A load probe is an available option on all PRIMUS control platforms.

Load Probe Advantages

The primary advantage of a load probe is that the chamber is held at the sterilization setpoint, but the exposure timer does not start until the product reaches the load probe setpoint. This value is typically one degree below the sterilization setpoint but is adjustable to meet the needs of the end user.

Another key advantage is that the load probe takes the guesswork out establishing proper parameters for liquid load sterilization.

Without a load probe, an end user must take into account the following variables for each load – liquid volume, load size and configuration, liquid viscosity, and container type.

Load Probe and Fo Accumulation

Fo is available on the PSS7, PSS8, and PSS9 PRIMUS control systems. It is not available on the PSS5 control system.

The load probe will take into account the relative effectiveness of kill as the product increases in temperature. This cumulative time at temperature will be reflected as a total Fo accumulated. Since microbes have been killed during the heat-up portion of the cycle, less exposure time is required at 121°C to attain total sterilization.


Load probes are very precise and sensitive components. It is very important that the load probe not be dropped or damaged by the operation of the loading equipment.

90%+ of all load probe field issues reported to PRIMUS are due to inadvertent abuse!

When not in use, the load probe should always be placed in the holder that is installed on the inside of the chamber.

Problem Type #1

Most field issues will begin with a complaint that the load probe will not read the correct temperature or is unstable. It is important that the end user realize that the load probe and chamber temperatures will not match until the load is actually at the sterilize temperature.

Typical symptoms of a load probe failure are erratic readings of the chamber temperature, jacket temperature, pressure, and load probe temperature during the Heat Chamber portion of the sterilization cycle.

If the load probe becomes defective, it will generally “go bad” somewhere in its middle range. Therefore, many times the load probe will calibrate at the high end and low end of the range with no signs of trouble.

On a PSS500 controller, the analog inputs of chamber temperature, jacket
temperature, pressure, and load probe temperature are not completely isolated
from one another. If the load probe becomes damaged or defective, it can affect
the readings of the other temperature and pressure devices.

The first thing that must be done is an inspection of the load probe for obvious signs of damage. In several cases recorded in the PRIMUS Customer Service Call Log (CSCL), the load probe has been damaged by the loading equipment.

If nothing adverse is found upon inspection of the load probe, the next step is to do a qualitative check of the RTD with an ohmmeter.

Problem Type #2

Another potential load probe issue is that the sterilizer never goes into the exposure phase. There are two possibilities for this problem:

  • The load probe is out of calibration. It is extremely important that the load probe and chamber drain calibrations are consistent. It is always advisable to calibrate both sensors at the same time using the same heat source.
  • Ensure the load probe setpoint is set 1º C or more below the sterilization setpoint.

Ultimately, if the load probe is unstable and/or will not calibrate, it must be replaced. Please contact PRIMUS Parts at (402) 344-4200 Ext. 1010 to order a replacement.

Have a question or having difficulties? Talk to Dave Schall, our PRIMUS sterilizer expert who is here to assist you anytime at 877.679.7800 extension 1212 or email at dschall@spire-is.com.

Visit us at spire-is.com

Back to Blog