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1.1.1.1 How can I test a dial gauge?




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This article is from the Food Preserving FAQ, by Eric Decker ericnospam@getcomputing.com with numerous contributions by others.

1.1.1.1 How can I test a dial gauge?

USE OF A MAXIMUM THERMOMETER TO TEST A DIAL GAUGE

Pressure canning equipment needs to be checked yearly to be
sure it is proper operation condition. Canners with dial gauges
can get out of adjustment and no longer give an accurate reading
of the pressure inside of the canner. This happens as the gauge
gets old; with heavy use the gauge which results in expansion
and contraction of the metals parts causing them to become
brittle.

The gauge of a dial gauge canner SHOULD BE CHECKED EVERY
YEAR to be sure it is accurate. If it is inaccurate, you may not
be processing low-acid foods at a high enough temperature to make
them safe to consume.

We use a MAXIMUM THERMOMETER to check the temperature
(which reflects the pressure) inside a pressure canner. A
maximum thermometer works like a fever thermometer.

--------------------------

Procedure #1:

Get a .1C Checktemp and use that to bench mark your maximum thermometer.
You may find it seriously in error. Temperature lags pressure so processing
can be out of whack by 10% or more.

------------------------------

Procedure:

1. Before each use, shake down the maximum thermometer to 200 F
or lower.

2. Place the thermometer inside an unsealed pint jar and add 2
inches of water to the jar. Place jar in the pressure
canner.

3. Place 1 inch of water in the canner.

4. Adjust canner lid, lock and exhaust canner for 10 minutes.
Close vent.

5. Allow pressure to build to desired level (10 PSIG or 11
PSIG).

6. Once pressure is built up and holding stable, time for 10
minutes.

7. Turn off heat. Allow canner to cool until gauge reaches
"0". Before opening lid, tough safety vent to be sure no
pressure (live steam) escapes. Remove lid.

10. Check the temperature on the maximum thermometer. The
maximum thermometer will stay at the highest temperature
which was achieved in the canner until you shake it down.

11. A gauge which is inaccurate by 1 pound, high or low, should
be replaced. For every 1 pound of pressure the gauge is
"off", it will be off by 2-3 F. If the maximum temperature
thermometer reads 2 F high or low (at the corresponding
pressure), replace the gauge.


Desired temperatures:

          Pressure          Accurate      Acceptable Range 
           5 pounds      228 F (109 C)     226 to 230 F
          10 pounds      240 F (115 C)     238 to 242 F
          15 pounds      250 F (121 C)     248 to 252 F


           Vapor Pressure of Water Above 100 C                 
  Temp., C       Pounds per Square Inch          Temp., F    
 
      100              14.696 (atmospheric)         212.0
      105              17.521                       221.0
      110              20.779                       230.0
      116              25.330                       240.8
 
      120              28.795                       248.0
      125              33.664                       257.0
      127              35.789                       260.6 

(Table adapted from the Handbook of Chemistry and Physics.)


The difference in pressure between atmospheric and 10
pounds pressure (25.330 - 14.696 ) results in a temperature
(degrees F) difference of 28.8 F (240.8 - 212) or about 2.9
degrees F for every pound of pressure.

The difference in pressure between 10 pounds and 20 pounds
(35.789 - 25.330) results in a temperature (degrees F) difference
of 19.8 (260.6 - 240.8) or about 2 degrees F for every pound of
pressure.

What this tells us is that the change in the temperature is
not the same over the entire range of pressures. To be on the
safe side, we will use the lesser value--if the temperature on
the maximum temperature thermometer is high or low with respect
to the expected temperature at that pressure by 2 degrees
Farenheit or more the gauge should be replaced as it is
inaccurate by at least one pound pressure.

If the temperature is high or low by less than 2 degrees F,
we can adjust the pressure at which we process in order to
achieve the correct temperature. For example, if at 10 pounds
pressure, the maximum temperature thermometer reads 238 F, our
gauge is reading high by 1 pound--we are actually processing at 9
pounds pressure. To get the temperature up to 240 F, we need 1
more pound of pressure, so we adjust our processing pressure to
11 pounds (10 lb present + 1 lb needed = 11 lb).

In Illinois we are above sea level by about 1,000 feet, so
we will need to process at 11 pounds per square inch by gauge
(PSIG) to get the internal temperature of the canner up to 240 F.
If we find that at 11 pounds pressure the maximum temperature
thermometer reads 238 F, our gauge is reading high by 1 pound--we
are actually processing at 10 pounds pressure. To get the
temperature up to 240 F, we need 1 more pound of pressure, so we
adjust our processing pressure up to 12 pounds (11 lb present + 1
lb needed = 12.


                   Pressure-Temperature Chart                    
            Pounds Pressure (gauge)     Temperature, F            
                    0                        212
                    1                        215
                    2                        218
                    3                        222
                    4                        224
                    5                        228
                    6                        230
                    7                        232
                    8                        235
                    9                        238
                   10                        240
                   11                        242
                   12                        244
                   13                        246
                   14                        249
                   15                        250
                   16                        252
                   17                        253
                   18                        255
                   19                        257
                   20                        259



Take the temperature inside your canner at several
pressures, compare the temperatures with the temperatures in the
table above to see how close to accurate your canner is. If you
have questions about testing a dial gauge, call your local
Cooperative Extension Service office.



Prepared by Susan Brewer
Foods and Nutrition Specialist
July, 1990
EHE-68

 

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