Tuning fork liquid level switch is a type of liquid level control switch. The tuning fork is excited by the crystal to generate vibration. When the tuning fork is immersed in liquid, the vibration frequency changes, and the frequency change is detected by the electronic circuit and outputs a switch value.
The vibration element (tuning fork) vibrates at a certain mechanical resonance frequency under the drive of piezoelectric ceramics. Piezoceramics are mechanically fixed and therefore not limited by temperature shocks. Once the vibrating element comes into contact with the measured medium, the vibration frequency changes. Integral electronics detect this change in vibration frequency and translate it into switching commands. Typical applications are overfill protection and prevention of idling. Due to the simple and robust structure of this instrument, the measurement is hardly affected by the physical and chemical properties of the measured medium. Even if the external vibration is strong or the measured medium is changed, the measurement will not be affected.
According to the connection form, the tuning fork level switch can be divided into: flange connection and threaded connection. It is mainly installed on storage tanks, containers, and silos by means of threads and flanges, and measures high, medium and low levels of solid powder or liquid, and achieves the purpose of signal alarm or material level control through the change of vibration frequency.
The tuning fork level switch is an indispensable measuring instrument in the raw material storage tank, conveying pipeline and liquid container of the factory. It can be said that as long as a highly reliable tuning fork level switch is installed, the position of the liquid level can be controlled intelligently. In the case of saving a lot of manpower and material resources, it can greatly improve the work efficiency of the factory.
Single machine test of tuning fork level switch
The tuning fork level switch is generally composed of a set of power lines and a set of relay feedback lines. As for connecting normally open contacts or normally closed contacts, it is generally determined by the instrument designer.
Each tuning fork level switch is equipped with a set of relay contacts, which are single-pole, double-throw. The relay is only used as a switch and cannot directly drive the alarm or other high-power equipment to act. All control signals are output from the terminal block at the top of the electronic unit. When the green light is on, it indicates that the relay is working normally, and when the light is off, the relay is in an alarm state (when the power is off or the instrument fails, the relay is also in an alarm state).
Since the tuning fork level gauge is on/off, there is no need to verify whether it is accurate, just test whether it can work normally. The method is: after the power is turned on, you can hear the vibration of the tuning fork, touch the fork end with your hand, and use the on-off position of the multimeter to measure the switching signal output by the tuning fork corresponding to the touch of your hand.
If the medium to be measured is a non-toxic and harmless liquid or solid, you can use a container to sample, then stick the vibrating fork into the medium, use the medium instead of the hand to pinch, and use the on-off gear of the multimeter to measure the on-off signal output by the tuning fork and The touch of the medium corresponds to that.
Nuonuo is an excellent instrument worker. Today, I received a notice to repair a tuning fork level switch. It is said that this watch is always falsely alarming and cannot accurately detect the level of the solid silo. Nuonuo reported to me: This watch has been debugged many times, the vibration fork body has been cleaned and put back, and it can be used for a few days. After a few days it failed again, and I don't know how to fix it.
Based on the above description, I make the following conclusion:
1. This tuning fork level switch is used to measure solid particles.
2. After the vibrating fork body is wiped clean, it can be put into operation for a period of time.
3. There is a lot of dust in the solid particle silo.
Nuonuo and I took the relevant test tools, climbed the ladder to the position of the tuning fork level switch, and tested that its voltage was 23.5V, which was within the allowable error range. When measuring the relay contacts, it will be on and off for a while, which is obviously inconsistent with the actual material level. After removing the vibrating fork body with electricity, it was found that it was covered with a thin layer of solid dust. After wiping the solid dust clean, and then measuring the relay contacts, it is in line with the actual situation.
However, doing so can only solve the problem of temporary use, and cannot guarantee that it will be permanently free of problems. This is obviously due to the fact that the measurement accuracy is too high and the sensing element is too sensitive.
So I decided to re-tune it, I asked Nuonuo to take some samples and put them next to me, while turning the sensitivity of the tuning fork level switch down, I tested it in the samples. until it is fully adapted to the material.
One day Nuonuo told me that the tuning fork level switch of the V-307 residual oil storage tank had been broken for a long time, and I never had a chance to repair it. This time there is no material in the V-307 tank, we can take it down for repair or replacement.
I came to this tuning fork liquid level switch with Nuonuo with relevant test tools and tested its voltage at 24.5V, which is within the allowable error range. The contact of the measuring relay is always in a disconnected state, which is obviously inconsistent with the actual material level.
Nuonuo immediately took out the watch, and found a dirt similar to coke between the vibrating forks, which prevented the fork from vibrating, and could not be pulled out with tools, so he could only take it to the maintenance room and use steam Heat it to dissolve. After the test is completely normal, it is back to the job.
Nuonuo came to the office in the morning and told me that a two-wire tuning fork liquid level switch was newly installed on the V-208 storage tank, and now it is not working properly. The no-load voltage measured on site is 24V, and the on-load measurement voltage is only 9.5V.
Together with Nuonuo, I came to this tuning fork liquid level switch with relevant test tools, and took out a multimeter to measure the voltage, as Nuonuo said. I went to the cabinet room to check that the power supply of this meter is not from the safety barrier, but from the DCS system module. After discussing with the design institute later, this power supply is not suitable, so the construction party is required to add a safety barrier and supply power directly from the safety barrier to this meter.