Steam "Wetness" in Benchtop Steriliser- Measure?
Steam "Wetness" in Benchtop Steriliser- Measure?
(OP)
Hi,
Excuse the ignorance, but as my job is mainly electrical & its 20 years since my thermodynamics subject, I need help. The Standards Committee in Australia is proposing that steam in benchtop sterilisers (often made by boiling water in chamber, or external small boiler, or by dry heated block with water injected into a "maze") be a maximum of 3% wet. I think that one would expect about 5% by boiling it in the chamber?... QUESTION:Is there a readilly acessible & practical method for measuring this????(Chamber is generally about 20Litres & has a 1/4 threaded test port BTW). Calibration usually involves measurement of pressure & temp. Temp to +/- 0.5C (at 134-135C) & Pressure to (supposedly 0.5% @ about 2.0-2.3 BAR). I think wetness is going to be impractical to measure , & would prefer them to allocate a pressure range by which it could vary from sat. conditions, as our main problem is insufficient air removal giving high pressures, or radiant heat off the wall giving high temp measurements...
Excuse the ignorance, but as my job is mainly electrical & its 20 years since my thermodynamics subject, I need help. The Standards Committee in Australia is proposing that steam in benchtop sterilisers (often made by boiling water in chamber, or external small boiler, or by dry heated block with water injected into a "maze") be a maximum of 3% wet. I think that one would expect about 5% by boiling it in the chamber?... QUESTION:Is there a readilly acessible & practical method for measuring this????(Chamber is generally about 20Litres & has a 1/4 threaded test port BTW). Calibration usually involves measurement of pressure & temp. Temp to +/- 0.5C (at 134-135C) & Pressure to (supposedly 0.5% @ about 2.0-2.3 BAR). I think wetness is going to be impractical to measure , & would prefer them to allocate a pressure range by which it could vary from sat. conditions, as our main problem is insufficient air removal giving high pressures, or radiant heat off the wall giving high temp measurements...





RE: Steam "Wetness" in Benchtop Steriliser- Measure?
I think rather you need to turn the problem around and 'ensure that the quality is greater than 97%' by ensuring that the quality is greater than 100%. Keep your system slightly in the superheat region. Or is this not possible with your equipment?
Use a reflective shield on your temperature sensor.
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
I2I
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
"Sterilisers shall be calibrated to the correct point on the phase boundary line.Deviation from this phase boundary line represent states of superheated or wet steam or air & steam mixtures which may lead to sterilisation process failure...."
AND
"The maximum allowable wetness is 3%, which is equivalent to 97% dry, saturated steam"
I perceive this as being daft, & not a thing we can do about it, but need to be able to make a national standards body see reason... Obviously, we can do things to alter air removal (longer/deeper vac.pulses or purging etc) but the steam quality is not going to be alterable with the process / heating fixed.
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
My feeling is that you need to control the heating to back away from the sudden increase in temperature you are going to get when you move past the sat curve. How to guarantee this won't fall below x=97% would be a controller specification problem and some benchtesting/trial and error work to get it right. A fine tuning heater might be required.
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
I2I
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
I2I
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
Units work, this is more an argument about at what height they want to set the bar for proving the process.
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
http:
This is one example, other manufacturers make these as well.
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
1) A condensing steam "calirometer"
2) very exspensive high temp humidity sensor
the first one is reletivly cheap but has a very wide margen for operator error.
that said we have found the best solution in our hospital:
bring the highest pressure steam possible to the sterilizer location, ensure that line is properly trapped.
step down using a PRV as close to the point of use as possible
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
I2I
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
You can calibrate to be 'on the curve' only using present apparatus.
For this unit to be able to calibrated - you first need something to adjust. The independant has to be separately adjustable from the controlled variable. ie. if the temp is controlled on setpoint, you need to be able to fine-adjust the pressure to get this calibrated. and vice versa. Clearly adjusting a setpoint will put you anywhere from a sat liquid to a sat gas.
To calibrate the steriliser, Turn it on and let the unit stabilise at the temp setpoint at sat temp of your intended pressure. Trending the temp and pressure. Read tables to see if above or on/below sat curve. Adjust pressure until you are below the curve letting stabilise, then fine-adjust upwards until you see a temp spike. back and forth over this point until you are sure that you are on.
Better to control temp to setpoint and adjust pressure. Faster to calibrate, more accurate.
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
I2I
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
I2I
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
I think i've been pretty clear about the dependancy issue.
Sure they'll both vary, but you can adjust one and let the other one control to the setpoint. The point is, calibrate to the sat vapour line not to the region within.
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
I2I
RE: Steam "Wetness" in Benchtop Steriliser- Measure?
The setpoint is built into the software. We can only really calibrate temp & pressure to their measured values. The aim here is really to define if given a 20L vessel of steam (no feed pipe) is there a practical way to measure the quality?
We cant really do much about the superheat or saturation I feel as much of it is inherent in any particular design. Some are heated by element wrapped around (1500W), some are heated internally (2000W coil) & a few are fed via a tiny boiler(1L) or even a hot metal brick with a maze pathway which water is injected into. (cold water into brick at about 160C) . As you can see, there is a huge disparity in the way it is made, & the resultant properties. BUT Standards australia are, I believe asking for the impossible....