Pyrometer Answers Question—How Hot Is the Farmer’s Hay?

Pyrometer Answers Question—How Hot Is the Farmer’s Hay?

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Once the haying season gets under way, fire departments in Fond du Lac County, Wis., get calls for “hot hay.” The farmer himself or a farmer passing by who has experienced the final result of hot hay (a fire) smells the distinct odor and calls the fire department.

You cannot cry “wolf” every time a farmer thinks he has hot hay and will lose his barn, nor can you ignore his concern. Curing hay does have an odor similar to the smell of hay ready for spontaneous combustion.

Basic questions

Our departments make a rational response. We send a couple of officers, men and our “hay tester” and ask the following questions:

  1. Where is the hot spot? Normally the farmer knows about where he unloaded that “not quite dry” load. We want to know exactly where so we can answer the next question.
  2. How hot is the hay? The authorities have given us the following cook book guidelines: (A) 140 to 150°F, the hay should be watched carefully, (B) 180 to 190° F indicates fire is quite certain, (C) 230 to 300°F the hay is about to go into the combustion stage, (D) 300 to 400°F indicates combustion is in progress. After determining the temperature, we ask the next question.
  3. Should the hay be removed from the mow, moved, or just monitored for the next couple of days?
  4. Should the fire department set up for the inevitable fire?
  5. Is there any problem?

Old hay tester

Our old hay tester was very primitive. It consisted of a 1-inch copper pipe fitted with a heavy copper point and a candy thermometer. This pipe was 15 feet long. The procedure for testing the hay was to place the thermometer in the tube carefully. Force the tube thru the hay where you think the problem exists and guess at a depth. Wait for ten minutes for the thermometer to stabilize. Then withdraw the tube, remove the thermometer and read the apparent temperature. Then try again at some other depth or location.

This old hay tester had several problems. It was hard to find the hot spot, readings were not accurate because of the time lag, and the procedure was slow. Also, a probe could not be made horizontally, it was hard to push the pipe thru the incrusted hay associated with hot hay, thermometers broke, and it was difficult to find working room for the probe atop hay stacked to the roof.

By proper implementation of instrumentation techniques, we were able to eliminate these problems. We now got 20 feet of total coverage in the vertical and horizontal positions. It is even possible to probe from underneath the mow, and we get instant temperature readings.

New testing device

The new hay tester consists of two 10-foot sections of %-inch thick-wall pipe with a matrix of holes drilled so that a 2-degree gradient can be detected in any 6 inches of depth. A pipe-gripping T-Bar handle, 26 feet of thermocouple wire, and a pyrometer with a protective case complete the tester. We considered using a digital readout in place of the pyrometer, but discarded it because of the battery power supply required. (None is required for a pyrometer.) It is no chore to push the smaller diameter tube thru a stack of hay, especially with the T-Bar handle. The tube can be pushed through in 10-foot sections, eliminating the need for high ceiling clearance. The tester can be carried in a family-size car. It is so simple to use that we can let the farmers do the testing themselves.

When working in a “hot hay,” carbon monoxide is present, so don’t stay in the mow for long periods of time.

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