The Pressure Gauge Shows More Than Pressure

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About Kim Blagborne

Born and raised on the family farm in B.C.'s Okanagan, agricultural engineer Kim Blagborne was the owner and president of Slimline Manufacturing (makers of the Turbomist airblast sprayer) for more than 20 years. A sought-after presenter known for his straight delivery, Kim's goal is to provide growers with simple, effective tools to improve the ag. industry.

See all posts by Kim Blagborne.

Kim Blagborne (formally with Slimline Manufacturing) has long said that the pressure gauge on an airblast sprayer indicates more than just pressure. It can be used to diagnose a number of pump and plumbing issues… if you know what to look for. Here’s Kim’s troubleshooting guide to reading into what your gauge is REALLY telling you:

Scenario One

“As the tank empties, the pressure drops”

First, try adjusting the pressure regulator (assuming a positive displacement pump). If you can maintain the pressure up until the tank empties, your intake line may be loose and it’s sucking the bottom of the tank. Check the fitting between the suction filter and the pump. Apply a light coating of grease to the O-rings on the elbows and filter to ensure a complete seal.

Second, try stopping mid-tank (that is, turn off the tractor PTO and let the sprayer sit for a few minutes). Does the pressure gauge return to the original set pressure? If so, then the intake line inside sprayer has likely come loose entirely. Open the lid, and using a straightened-out coat hanger, hook the intake line and give a few gentle tugs – it should not be able to move. If it does, you’ll have to re-fasten the intake line so it’s not sucking the bottom of the tank.

The humble coat hanger. It opens our cars and now fixes our sprayers. Remarkable!
The humble coat hanger. It opens our cars and now fixes our sprayers. Remarkable!

Scenario Two

“When I first start the sprayer, the pressure drops or fails to maintain constant pressure as the tank empties”

This might indicate improper mixing practices because the filter is probably plugging with product. Alternately, your PTO speed may be too slow to drive sufficient mechanical agitation. Check the suction filter as soon as the problem occurs (don’t finish spraying). If you wait to check when the tank is empty, the evidence of a plugged filter could be washed away before you can confirm it. This problem often happens when spraying nutrients, or when products aren’t compatible.

If that’s not it, it could be a collapsed suction valve. The pump will sound like it’s “missing” (like an misfiring engine). The suction valve might need to be replaced.

Or, perhaps you notice that you can compensate for the pressure drop by adjusting the regulator on the first tank. But it has to be dropped back down again for the second tank. In this case, the regulator might be sticking or jamming. Disassemble it and look for grit in the barrel of the regulator, then lubricate the parts.

Scenario Three

“I lose pressure when I turn my boom(s) on or off”

In this scenario, the pressure is fine as you approach the end of the row. You turn off the outside boom (or both) and finish the turn. But, when you re-engage both booms, the pressure drops. Even when you adjust the pressure regulator to compensate (assuming a positive displacement pump), the unit only gains the lost pressure slowly. In this case, the regulator might be sticking or jamming. Disassemble it and look for grit in the barrel of the regulator, then lubricate the parts.

Scenario Four

“The pressure gauge spikes when I turn off the boom(s)”

If you run a Turbomist, it could be the bypass balance. To solve this issue, head over to this article and pan down to see the step-by-step. If it isn’t the balance, then it’s likely the regulator. The issue of a spiking gauge and how to correct for it is covered thoroughly in this article by Ag mechanic extraordinaire Murray Thiessen.

Scenario Five (a positive displacement pump issue)

“My gauge pulses”

Is it more than a 20 psi range? Have you noticed that the deviation gets less as the PTO speed increases? Well, the pump pressure check-valve may have collapsed. Check the pressure check valves in the pump for broken springs on the suction valve plate.

Does the needle move rapidly through a 5 to 10 psi¬†range? The accumulator pressure might be low. Try¬†adjusting system pressure via the regulator and if that changes how the needle is responding, then set an air compressor to 90 psi (or manufacturer’s recommended pressure) and charge the accumulator.

Perhaps the needle movement is not affected by system pressure changes or the PTO speed. In this case the accumulator may have failed entirely and the diaphragm will need replacement.

Scenario Six

“My calibration is going farther than expected”

Sure, that sounds pretty good at first, but it may be that the gauge is stuck. With the PTO off and the spray boom on, the gauge must read “ZERO”. If it doesn’t, pony up the $40.00 and get a new one.